Can anyone point me to what may be causing a channel switching relay to pop upon switching in a tube amp?
Schematics: http://www.diyguitarist.com/PDF_Files/HRDV-Sch.pdf
It happens when switching between "normal" and "drive" channels.
I haven't used the drive channel on the amp in literally years. Could it just be a case of corroded terminals? If so, how to fix that? Try squirting contact cleaner inside of the relays?
Any leakage of DC onto the contacts will do it. If it's not that, it's capacitive coupling of the relay coil signal into the signal lines. That last can sometimes be stopped by slowing down the voltage change on the coil. Try a BFC across the coil, maybe.
Quote from: R.G. on November 12, 2012, 12:17:03 PM
Try a BFC across the coil, maybe.
OK. What is a "BFC"?
big @#$%ing capacitor
you could also try a reversed diode, if the pop is coming from coil discharging, ie when the thing is turned off.
Quote from: Kesh on November 12, 2012, 12:55:58 PM
big @#$%ing capacitor
Oh! :icon_lol:
Quote from: Kesh on November 12, 2012, 12:55:58 PM
you could also try a reversed diode, if the pop is coming from coil discharging, ie when the thing is turned off.
Interesting idea but would be difficult to do with the PCB such as it is. It's a big pain in the rear getting the main PCB out of the chassis, got to practically dismantle the whole chassis. :icon_mad:
Put a sensitive DC voltmeter on the NC and NO pins of the relays (4 place). _ANY_ DCV is bad. Looks like 10mV in most of those poles will be real annoying. Likely cause of DC: leaky coupling caps.
> a big pain in the rear
Well, it's a heck of an amp with a lot of features all at an affordable price.... something's gotta give.
I hate relays in the signal path and I hate PCBs for what should be an easy tag-board.... but that's just me.
> try a reversed diode
It's already in there (Paul posted link to schematic). In fact I had to take off my shoes to count all the silicon in the switching circuit.... looks like 18 diodes, opamps, and transistors. Can't say they didn't try. And I hear a lot about HRD faults but not simple popping.
Quote from: PRR on November 13, 2012, 01:42:37 AM
Put a sensitive DC voltmeter on the NC and NO pins of the relays (4 place). _ANY_ DCV is bad. Looks like 10mV in most of those poles will be real annoying. Likely cause of DC: leaky coupling caps.
OK, leaky coupling caps in general, or in specific locations? I didn't try it with the footswitch yet, but it will pop when I use the switch on the chassis. I have done some mods to the amp, but I don't remember exactly what I did. I have to go back and figure it out real quick and see if maybe something I did has caused this problem. Although it was working fine for a long time and then I kinda just quit using the dirt channels at all for probably like close to four years now. I don't know what may have happened in between, but it has been kept in an air conditioned environment since I bought the amp in the late 90s.
EDIT: All of the mods I did were messing with tone control caps, a couple caps on the input, a cap in the presence control and two of the coupling caps on the power tubes (a little more capacitance than stock).
Quote from: PRR on November 13, 2012, 01:42:37 AM
> try a reversed diode
It's already in there (Paul posted link to schematic). In fact I had to take off my shoes to count all the silicon in the switching circuit.... looks like 18 diodes, opamps, and transistors. Can't say they didn't try. And I hear a lot about HRD faults but not simple popping.
I looked for diodes, it looks like most of them are in the footswitch. I see only three that would be on the amp's PCB.
Quote from: Paul Marossy on November 13, 2012, 10:05:02 AM
OK, leaky coupling caps in general, or in specific locations? I didn't try it with the footswitch yet, but it will pop when I use the switch on the chassis. I have done some mods to the amp, but I don't remember exactly what I did. I have to go back and figure it out real quick and see if maybe something I did has caused this problem.
Any capacitors that connect to the relay contacts. In the schemo you show, I'd worry about C3, C4, C18 and C23. All of these **should** be OK, as they all have pulldown resistors to ground. However, if any of the caps are especially leaky and can overwhelm the pulldowns, or if one of the pulldowns has a broken/cracked solder joint or is damaged and open, that would give you popping too. In this case, check R9, R27, R43, R52, R102, and the volume and drive pots. They should all show the proper resistance to ground when metered from the cap terminal to ground.
Quote from: R.G. on November 13, 2012, 12:08:56 PM
In the schemo you show, I'd worry about C3, C4, C18 and C23. All of these **should** be OK, as they all have pulldown resistors to ground. However, if any of the caps are especially leaky and can overwhelm the pulldowns, or if one of the pulldowns has a broken/cracked solder joint or is damaged and open, that would give you popping too. In this case, check R9, R27, R43, R52, R102, and the volume and drive pots. They should all show the proper resistance to ground when metered from the cap terminal to ground.
Thanks for the help RG, I appreciate it!
It so happens that I've changed the value of C23, but I think the soldering on it is good. Will have to double check.
I have a hunch that this might be happening because I changed the extremely touchy master volume control from a 100KB to a 250KA pot. That's probably it. I surmise that when I did this mod back in 2002 that it didn't mind, but with the passage of time some of these caps may have gotten a little leaky.
Anyway, this gives me a game plan! I will report back with my findings. :icon_razz:
While you're in there be sure and check for cracked solders, I can't even begin to explain how much warranty work I used to have to do on the whole Hot Rod series when I worked in repair and it was almost always cracked solders, mostly on the PCB where the tubes mount, but sometimes other places. They're great amps, but I'm not exactly sure why Fender thought it was a great idea to mount heat sources under solder joints with no other mechanical reinforcement in a high vibration environment.
Quote from: wavley on November 13, 2012, 01:02:44 PM
While you're in there be sure and check for cracked solders, I can't even begin to explain how much warranty work I used to have to do on the whole Hot Rod series when I worked in repair and it was almost always cracked solders, mostly on the PCB where the tubes mount, but sometimes other places. They're great amps, but I'm not exactly sure why Fender thought it was a great idea to mount heat sources under solder joints with no other mechanical reinforcement in a high vibration environment.
Yeah, since I have to pull the whole amp apart you can bet I'm going to go over it with a fine tooth comb! :icon_wink:
Did you tweak the drive channel on yours? I've never been that fond of nearly every dirty channel Fender has ever made, they're great with clean, but... Somebody told me once that the engineers at Fender in the 70's when they were playing with drive channels and master volumes said "Look at the scope, it's a square wave, that's what the kids want. Put it into production." I'm sure it was just a funny anecdote, but sometimes it really does seem like that's what they did. Maybe it's just me, a lot of people use the dirty channel on the hot rods so I'm assuming that they like them.
Point is, I bet there is some love in those hot rod dirty channels, but I've never taken the time to mod one. I'm curious to see how one would turn out.
Quote from: wavley on November 13, 2012, 01:28:41 PM
Did you tweak the drive channel on yours? I've never been that fond of nearly every dirty channel Fender has ever made, they're great with clean, but... Somebody told me once that the engineers at Fender in the 70's when they were playing with drive channels and master volumes said "Look at the scope, it's a square wave, that's what the kids want. Put it into production." I'm sure it was just a funny anecdote, but sometimes it really does seem like that's what they did. Maybe it's just me, a lot of people use the dirty channel on the hot rods so I'm assuming that they like them.
Point is, I bet there is some love in those hot rod dirty channels, but I've never taken the time to mod one. I'm curious to see how one would turn out.
Here's what I did to mine:
1. Changed C1 from 47uF electrolytic to 10uF/50V electrolytic
2. Changed C23 from 1.5nF to .022uF/400V cap of same type
3. Changed C7 from 250pF ceramic to a 330pF/1KV ceramic cap
4. Changed R12 from 130K to 82K 1/2 watt resistor
5. Changed C26 and C27 from 0.1uF to .033uF/630V cap of same type, then later to 0.047uF
6. Removed C29 and replaced with a jumper wire
7. Kept R45 a 47K and instead changed the pot from the stock 100K linear pot to a 250K audio pot
8. Put Celestion G12S-50 speakers in it and then later put the originals back in
Most of this was tweaking the EQ, input caps and the power tube suppressor caps. I think the drive channel sounds alright, but I've never really liked amp distortions much. I prefer to use various pedals. But for certain things it sounds pretty good.
Quote from: wavley on November 13, 2012, 01:02:44 PM
While you're in there be sure and check for cracked solders, I can't even begin to explain how much warranty work I used to have to do on the whole Hot Rod series when I worked in repair and it was almost always cracked solders, mostly on the PCB where the tubes mount, but sometimes other places. They're great amps, but I'm not exactly sure why Fender thought it was a great idea to mount heat sources under solder joints with no other mechanical reinforcement in a high vibration environment.
It probably comes from the changeover from lead-bearing to lead-free solders. That was a period of pure hell for manufacturers all over the world.
Quote from: Paul Marossy on November 13, 2012, 01:36:07 PM
Quote from: wavley on November 13, 2012, 01:28:41 PM
Did you tweak the drive channel on yours? I've never been that fond of nearly every dirty channel Fender has ever made, they're great with clean, but... Somebody told me once that the engineers at Fender in the 70's when they were playing with drive channels and master volumes said "Look at the scope, it's a square wave, that's what the kids want. Put it into production." I'm sure it was just a funny anecdote, but sometimes it really does seem like that's what they did. Maybe it's just me, a lot of people use the dirty channel on the hot rods so I'm assuming that they like them.
Point is, I bet there is some love in those hot rod dirty channels, but I've never taken the time to mod one. I'm curious to see how one would turn out.
Here's what I did to mine:
1. Changed C1 from 47uF electrolytic to 10uF/50V electrolytic
2. Changed C23 from 1.5nF to .022uF/400V cap of same type
3. Changed C7 from 250pF ceramic to a 330pF/1KV ceramic cap
4. Changed R12 from 130K to 82K 1/2 watt resistor
5. Changed C26 and C27 from 0.1uF to .033uF/630V cap of same type, then later to 0.047uF
6. Removed C29 and replaced with a jumper wire
7. Kept R45 a 47K and instead changed the pot from the stock 100K linear pot to a 250K audio pot
8. Put Celestion G12S-50 speakers in it and then later put the originals back in
Most of this was tweaking the EQ, input caps and the power tube suppressor caps. I think the drive channel sounds alright, but I've never really liked amp distortions much. I prefer to use various pedals. But for certain things it sounds pretty good.
Very nice, I'm with you, I don't tend to ever really like amp distortions and don't own a single amp with a master volume. I do however love single channel JCM800s, Hiwatts, and old Oranges, but have never committed to one because a good clean is more important to me than a good dirty
You and me are on the same "wavelength" in this area wavely. :icon_wink:
I started a reply this morning then got hung-up, so if I say stuff already said, sorry.
> caps in general, or in specific locations?
Feeding the relays.
I'd look first at what's leaking into the NO/NC contacts, mostly because in tube systems the relay output is a tube grid which "usually" is innocent. BUT swap tubes around first-- a gassy tube could be grid-leaky. If in storage, leave it on for 10-100 hours, the tube may clean-up.
> messing with tone control caps
Classic Fender tone-stack, the "tone caps" also serve as DC-block caps. (They "couple" treble and bass separately then mix to taste.)
(http://i.imgur.com/IbOwg.gif)
> I looked for diodes, it looks like most of them are in the footswitch
The PCB is filthy with them. Lower-left is what happens behind the pedal jack (and pedalless switching). Big red line circles the relay coils, where we often find a diode, it's there.
Fascinating, but probably irrelevant, is how many other diodes are in that system. Long way from a 1959 amp, just two shorting switches.
-- {dupe}
OK, so I changed the Master pot back to a 100K and I fired up the amp and it played for a minute and then I got a very loud pop and then basically no sound. With the volume all the way up it's barely audible. The Volume, Drive and Master controls are all functional. I also get a scratchy noise when turning the Volume and Drive pots. So my guess is that the relay itself has crapped out and now I have DC on the input as a result.
Any second opinions?
EDIT: I ordered a couple of relays from Mouser. We'll see if it's the relay that crapped out. I've had it happen on appliances and other places, so it wouldn't surprise me at all if that is what happened.
Quote from: Paul Marossy on November 13, 2012, 11:55:32 PM
OK, so I changed the Master pot back to a 100K and I fired up the amp and it played for a minute and then I got a very loud pop and then basically no sound.
My first guess would be collateral damage from the pot changeout.
QuoteI also get a scratchy noise when turning the Volume and Drive pots.
Scratch when turning a pot is a Strong Indicator that there's DC across the pot, and in this amp (well, OK, almost all amps) there should not be DC across an audio signal pot. If it were mine, I'd use my meter to see if I could measure DC across these controls - and the master.
QuoteQuoteSo my guess is that the relay itself has crapped out and now I have DC on the input as a result.
My guess is that's not the same problem. Not that it could not be another problem. But the symptoms you've quoted don't say that to me.
Quote from: R.G. on November 14, 2012, 11:29:39 AM
My first guess would be collateral damage from the pot changeout.
I've changed the master pot before without incident. All I did was made it the original value again (from my 250K to the factory 100K). I checked all the solder joints, checked for continuity etc. before putting the PCB back in. If it were the master pot, then I expect that the drive channel only would be having a problem since the master has no affect on anything until the drive channel is engaged.
Obviously, there shouldn't be DC on those pots, so something is definitely awry. If there was an internal failure of the relay coil, couldn't that leak DC into the audio path since it's a DC operated coil? (I have no idea LOL) It still switches, and there is still a pop that you can hear in the reverb circuit but the signal level is almost non-existent.
I'll check to see if maybe the preamp tube on the input may be the cause of some of this when I get home from work tonight. But I am still feeling that the relay coil just failed somehow. I've had relay coils fail on me in other things, one time it was in a dishwasher. Somehow the contacts inside get messed up and I was able to take it apart and fix it. In this case, that's not possible since it's so small and it's potted anyway.
EDIT: It's hard to say exactly what is going on since these relays are a little hard to follow. I might have to measure voltages at these test points on the schematic to find the problem. I prefer the shotgun technique however. :icon_lol:
Quote from: Paul Marossy on November 14, 2012, 11:43:05 AM
I've changed the master pot before without incident. All I did was made it the original value again (from my 250K to the factory 100K). I checked all the solder joints, checked for continuity etc. before putting the PCB back in. If it were the master pot, then I expect that the drive channel only would be having a problem since the master has no affect on anything until the drive channel is engaged.
Side effects have a very wide range of applicability. I know this from experience. :icon_eek:
QuoteObviously, there shouldn't be DC on those pots, so something is definitely awry. If there was an internal failure of the relay coil, couldn't that leak DC into the audio path since it's a DC operated coil?
It's possible, since there is both DC and signal inside that one part. However, relays are mostly designed specifically to prevent that. Most relays have insulation between coil and contact in the thousands of volts, for safety reasons in many of their common applications. My gut feeling is that it's more likely to be board contamination, soldering issues, broken/shorting wires or leaking caps than a relay suddenly leaking coil into contact, unless you know of the relay being tinkered with or damaged. But it's possible.
QuoteI prefer the shotgun technique however.
I do too, in general, as it makes a hit more likely. But just like with firearms, the ammunition is more expensive per round with shotguns.
It occurs to me to wonder if anyone has ever made a Gatling-style shotgun. :icon_lol:
They make full auto assualt shotguns. :icon_twisted:
Well, I'll pull it apart again tonight and see if maybe a via on the PCB got screwed up or something.
You shorted C29. Does the Presence-pot crackle OK now?
Quote from: Lurco on November 14, 2012, 05:45:35 PM
You shorted C29. Does the Presence-pot crackle OK now?
I actually made that a switch a while ago. It doesn't affect any part of how it operates in either clean or drive mode except that I can't switch it while the amp is on or it makes a big pop. The presence control makes no scratchy noises when that cap is switched out of the circuit, it just gets defeated.
Quote from: R.G. on November 13, 2012, 01:51:42 PM
Quote from: wavley on November 13, 2012, 01:02:44 PM
While you're in there be sure and check for cracked solders, I can't even begin to explain how much warranty work I used to have to do on the whole Hot Rod series when I worked in repair and it was almost always cracked solders, mostly on the PCB where the tubes mount, but sometimes other places. They're great amps, but I'm not exactly sure why Fender thought it was a great idea to mount heat sources under solder joints with no other mechanical reinforcement in a high vibration environment.
It probably comes from the changeover from lead-bearing to lead-free solders. That was a period of pure hell for manufacturers all over the world.
My experience exactly. Our guitarist has two HR deluxes, which started to get noisy quickly after a recap. I recalled having a bit of a hard time soldering in the new caps - really had to clean the pads with wick, reflow, clean again and only then it would take nicely - so I checked for poor ROHS solder joints - bingo. ALL power tube connections had circular hairline cracks - practically invisible, but responded to the poke-it-with-a-chopstick-test, sometimes with pretty sparks. Yikes. Same solder routine as with the caps solved it.
Be careful though, these PCB's aren't the best quality, and a lot of soldering might lift traces. Take care, and manage your iron's heat (hot enough to flow *quickly* but no hotter) and make damn sure the powercaps are discharged properly.
Quote from: Mike Burgundy on November 15, 2012, 08:29:59 AM
I recalled having a bit of a hard time soldering in the new caps - really had to clean the pads with wick, reflow, clean again and only then it would take nicely - so I checked for poor ROHS solder joints - bingo. ALL power tube connections had circular hairline cracks - practically invisible, but responded to the poke-it-with-a-chopstick-test, sometimes with pretty sparks. Yikes. Same solder routine as with the caps solved it.
I've looked for things like that but haven't found anything conclusive yet.
Quote from: Mike Burgundy on November 15, 2012, 08:29:59 AM
Be careful though, these PCB's aren't the best quality, and a lot of soldering might lift traces. Take care, and manage your iron's heat (hot enough to flow *quickly* but no hotter) and make damn sure the powercaps are discharged properly.
Yes, I am well aware of all that stuff. These are some of the cheapest PCBs I've ever had to work on. I would expect that the tube PCBs would be very sturdy, but they are in fact some of the flimsiest PCBs I have ever had to work with. Way too thin and fragile for the application IMO.
Try this
http://relays.te.com/kilovac/appnotes/fig48.asp
So I got my relays yesterday. Switched them out and used some 16-pin IC sockets for them.
(http://sphotos-b.xx.fbcdn.net/hphotos-ash3/31028_4975410428509_2029525390_n.jpg)
And while I was at it, I also socketed the opamps. I dissected one of the old relays just for my own edification to see how it's constructed and it look like it's pretty much not possible for it to fail in such a way that DC could appear in the signal path.
The relay(s) appears to have been part of the problem, as now I have sound out of the normal channel at a normal volume level. It appears that the DC on the pots is gone now (at least I can't hear anything). The drive channel however is still malfunctioning. It's only like 10% as loud as it should be and when I switch out of the drive channel it pops, even when volume controls are all turned down completely. So that tells me that the problem is with the drive channel somewhere. Now I am back to where I was when I started this topic. At this point I am kinda stumped. I've checked the master volume control many times, it works and there is continuity in all the right places. I've checked the B+ voltages, relay coil voltages, the voltages on the ICs, reflowed several solder joints, etc. and I can't find anything that's obviously wrong. I tried it with the footswitch which overrides the onboard switches, nothing. I did a tube dance, can't find a problem there either. So what the heck is it?!?!?!?!
The only active components I haven't monkeyed with are the transistors in the switching section. What if one of those went bad? Would it still switch channels but maybe somehow kill most of the signal on the drive side? It does seem like the gain/more gain relay isn't working as I don't hear it click when I switch it like I do the other relay (but it does change the sound like it's supposed to). I'm thinking it's got to be maybe the 2N4401 or the 2N4403. (or a bad zener diode?) The LED switches as it's supposed to, however. I can't really measure the voltages on the transistors because they are stuffed so far into the PCB that I can't even access the pins with a DMM. I am convinced that Fender wanted to make the Hot Rod DeVille the hardest amp ever to work on. Geez. :icon_rolleyes:
Any ideas?
Well it's not Q3 or Q4, tried something else in their place, same end result. I don't have any J111s to try, so i can't rule them out just yet, although I swapped them with no chsnge in behavior (all active components are in sockets now).
I'm not measuring any voltages on any of the pins other than 1 & 8 on the opamp in the preamp out / power amp in section. I would think that if this is were the problem is that both channels would be affected, but I guess not necessarily?
I'm beginning to wonder if a Zener diode somewhere went bad. Is there an easy way to check those?
What are the DC voltages on the gate, source, and drain of the J111s in both states, and also the voltages on both sides of CR16?
Quote from: R.G. on November 21, 2012, 02:42:40 PM
What are the DC voltages on the gate, source, and drain of the J111s in both states, and also the voltages on both sides of CR16?
Q1 -- G= -13.45, S=0, D=0
Q2 -- G= -13.6, S=0, D=0
The voltages stay the same whether in normal or drive mode.
CR-16 has -13.7V on the banded end and -13.4V on the other end.
"More drv" signal not getting to the JFET gates. Check pin 1/u3 for what happens there with both conditions. If pin 1 is changing and JFET gates are not driven, problem is the series resistor between them. If pin 1/u3 is not changing, it's with the opamp or circuit around it.
Quote from: R.G. on November 21, 2012, 05:19:18 PM
"More drv" signal not getting to the JFET gates. Check pin 1/u3 for what happens there with both conditions. If pin 1 is changing and JFET gates are not driven, problem is the series resistor between them. If pin 1/u3 is not changing, it's with the opamp or circuit around it.
Yes, I follow, this would make sense.
I measured 1/U3 in both conditions, it's -15.4 volts either way.
Series resistor R87 measures 100K and it appears to be OK.
Different opamp yields same results.
Looks like I'll have to poke around some more, but this certainly helps to narrow it down!!!!! Thanks RG. :icon_razz:
Quote from: Paul Marossy on November 21, 2012, 05:34:30 PM
Looks like I'll have to poke around some more, but this certainly helps to narrow it down!!!!
At this point, measure the DC volts at test points 31 through 36 for the different switch positions. You've found that it's not likely to be the opamp bad. That means that the opamp that's in there is not being told to operate. The test point voltages should tell why.
And one suspects that's why they were made check points. The design is a bit too clever, running AC into a footswitch with both positive and negative rectifier diodes and then messing about with what's higher and lower than what else in a mess of diodes and zeners. That's the hardware equivalent of beginners writing FORTRAN - it's almost 100% side effects and bandaids.
Quote from: R.G. on November 21, 2012, 05:52:35 PM
At this point, measure the DC volts at test points 31 through 36 for the different switch positions. You've found that it's not likely to be the opamp bad. That means that the opamp that's in there is not being told to operate. The test point voltages should tell why.
Ha ha, that's exactly the next thing I did, measured all the test points having to do with U3. To my consternation, I can not find anything that looks like it's way out of spec. Everything is fairly close to what is specified on the schematic. I have just found that 1/U3 does change, but only when the "more drive" option is used, otherwise no change between normal and drive. But the signal level is still only like 10% of what it should be. Guess I'm going to have to look at the PCB for a bad solder joint or a cracked via in that area.
Quote from: R.G. on November 21, 2012, 05:52:35 PM
And one suspects that's why they were made check points. The design is a bit too clever, running AC into a footswitch with both positive and negative rectifier diodes and then messing about with what's higher and lower than what else in a mess of diodes and zeners. That's the hardware equivalent of beginners writing FORTRAN - it's almost 100% side effects and bandaids.
I was thinking that this design seems overly complex or something, but I wasn't exactly sure why. It's very hard to follow what is going on in this case.
Quote from: Paul Marossy on November 21, 2012, 06:10:16 PM
I was thinking that this design seems overly complex or something, but I wasn't exactly sure why. It's very hard to follow what is going on in this case.
The primary requirement seems to be stuffing three or four different states onto one signal wire line that goes to the foot switch. Notice it only has signal and ground, but controls channel switch on/off and more drive on/off. It seems to do that by selective loading of both positive and negative sides of the AC voltage that's fed to it, so you get different positive and negative voltage peaks back at the sense circuits made out of dual opamp U3 (ab)used as a dual comparator. The diodes, zeners, transistor switches, etc. are to let the comparators get binary up/down levels out of the analog stuff happening back at the footswitch input.
I'm pretty sure that's what it does. I instinctively shy away from the digging, supposition, error band evaluations and tolerance evaluations that would be needed to ensure that it worked properly. I guess I'm getting lazy as I get old. This situation fairly screams for a pair of $0.35 six-pin PICs with one-wire interfaces to run power and signal down to the footswitch and back up to the panel in a clean way.
Of course, then they probably would have programmed the PICs in FORTRAN. See http://www.eng.uwaterloo.ca/~comp03a/misc/humour/shootfoot.html (http://www.eng.uwaterloo.ca/~comp03a/misc/humour/shootfoot.html)
Yes, I did notice what they are doing with the signal path. It gets hard to follow through the relays though. :icon_confused:
OK, so I found something that definitely is not right. I measured the voltage at Pin 8 on V2B, it measures +5.4V instead of the +1.93V shown on the schematic. I think maybe this is where my problem is, but I am not sure why. I did notice that the B+ voltage marked "X" measures 355V vs. the specified 388V. I don't think that would cause my problem though. Pin 3 on V2A measures about what the schematic says. So it looks to me like there is some problem with V2B.
Quote from: Paul Marossy on November 21, 2012, 06:43:48 PM
OK, so I found something that definitely is not right. I measured the voltage at Pin 8 on V2B, it measures +5.4V instead of the +1.93V shown on the schematic. I think maybe this is where my problem is, but I am not sure why. ... So it looks to me like there is some problem with V2B.
Well, it's for sure not right. The plate voltage can vary all over the place, but a 12AX7 with 5+ volts on the cathode and its grid at 0V can't pass signal. I think you found a better lead on the issue than I did.
1. Have you replaced V2 with a known good 12AX7? I have to ask. Swapping with known good tubes is always the first thing to do in a tube circuit.
2. Do the voltages on pins 6, 7, and 8 change when you change the channel and drive switches?
3. What *are* those voltages, anyway? Particularly is pin 7 at ground and does pin 8 vary when you flip the switches?
> a bit too clever, running AC into a footswitch
I think they also have LEDs in the pedal.
27V AC via 820 ohms (2W!) is up-to 33mA available. More than a logic-switch needs. But you could run 20mA into an LED and still have >5V for clean logic.
Not that it uses any 5V logic. Looks like TP32 TP35 clamp for 10V up and 10V down. Hmmmm... 27V-10V in 820r looks a lot like 20mA. (It isn't, because of wave-shape, but WTF.)
I've seen much worse in pedal-jackery. This one starts clean U3B U3A. But then Q4 tacks-on some AND/OR function. MORE_DRV, FET versus relays, and the 3-way LED add more ad-hoc clutter.
Rip it all out, poke a screwdriver in the top to engage frills.
_______________________________
> beginners writing FORTRAN
Or old guys maintaining legacy code.
> screams for a pair of $0.35 six-pin PICs
Plan is dated 1995. While PIC was 20 years old, it had a troubled history and didn't become wildly popular until 1993. Post-Leo Fender has rarely been early-adopter.
OTOH, their new cheap Mustang has a mechanical latching switch, plus a many-patch interface exposed via USB and your laptop.
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> Pin 8 on V2B, it measures +5.4V instead of the +1.93V
12AX7 with 100K+1.5K, the plate will sit near 70% of B+.
If observed B+ at X is 355V, plate should lie near 248V. 355V-248V is 100V, in 100K is 1mA, so cathode should be 1.5V. 1.6V without rounding. 1.75V with nominal 388V supply.
Check V2B Grid. It should be super-close to Zero. Much less than 0.1V for good bias. Much-much-much less than 0.1V for quiet switching. 10mV may be tolerable.
Quote from: R.G. on November 21, 2012, 08:41:58 PM
Well, it's for sure not right. The plate voltage can vary all over the place, but a 12AX7 with 5+ volts on the cathode and its grid at 0V can't pass signal. I think you found a better lead on the issue than I did.
1. Have you replaced V2 with a known good 12AX7? I have to ask. Swapping with known good tubes is always the first thing to do in a tube circuit.
2. Do the voltages on pins 6, 7, and 8 change when you change the channel and drive switches?
3. What *are* those voltages, anyway? Particularly is pin 7 at ground and does pin 8 vary when you flip the switches?
I did swap all three preamp tubes around many times, and had more or less the same problem every time. So I never really thought the preamp tubes could be the problem, especially since they are pretty new. But when I switched V1 with V2, the +5.4V on the cathode problem followed the tube around. Then I stuck a preamp tube in there that I knew was good and the amp works like it should now. :icon_exclaim:
So now I feel like the biggest idiot in the world! Although, looking at this schematic, this has to be one of the screwiest channel switching schemes I have ever seen in an amp and I would have never suspected a bad preamp tube in this shell game-like schematic, ha ha. So I learned a few things I didn't know before in the process, I guess it's not all bad. Man, I think I'm sticking with the old Fenders from now on. I just use this at church and keep it there, I'm not really married to it for any reason. But it has been a decent & reliable amp for me since 1998.
Only one problem... I still have a pop when switching between the normal and drive channels. :icon_mad:
Seems like as the amp warms up it starts to happen more. I must have a leaky cap somewhere I guess.
EDIT: How loud the pop is proportional to how strong the signal is when I hit the switch. With no signal at all it doesn't pop, so that is making some progress I guess compared to when I started.
Quote from: Paul Marossy on November 21, 2012, 10:40:09 PM
EDIT: How loud the pop is proportional to how strong the signal is when I hit the switch. With no signal at all it doesn't pop, so that is making some progress I guess compared to when I started.
I don't know that this is your problem, but it may be.
Imagine a sine wave, simple, rounded signal wobbling up and down. Now image that you have a metal-contact switch that can conduct that sine wave to some amplifier. If the switch happens to close when the signal is near zero, the signal seen by the output amplifier starts near zero and continues on, as you'd expect. But if the switch happens to make contact near the peak of the signal, the output sees the signal jump from zero to maximum instantly. This is a problem organ manufacturers have wrestled with for about a century. Simple metal-contact switching simply cannot be completely transient-free, by the nature of the beast.]
There is another possibility. It is possible that some DC level inside the circuit is being increased by bigger signal levels. This may be too slow to hear when it's continuously varying, but switching the signal may suddenly connect the DC level where it shouldn't go. This can be an issue in too-simply-designed compressors and other amplitude-sensitive circuits. Again, don't know if it's your problem, but it might be.
Maybe that could be it. I'm beginning to wonder if these preamp tubes are my problem. Going to have to mess around with that some more to rule that out.
I do notice that relay K1 makes a noise you can physically hear when it switches, whereas K2 you can't hear at all when it switches. Not sure why that would be. This whole thing is odd to me as my old 1984 Seymour Duncan Convertible never pops when switching, but it's also nowhere near as whacky circuit-wise as this Hot Rod DeVille of mine.
Quote from: Paul Marossy on November 22, 2012, 12:48:07 AM
I do notice that relay K1 makes a noise you can physically hear when it switches, whereas K2 you can't hear at all when it switches. Not sure why that would be.
Since you socketed them, swap the relay positions just like you would with a tube so you can find out whether it's the part or the socket it's in.
The schemo shows both relay coils in parallel, so they should be getting the same drive. However, I am distrustful of the soldering what would make "parallel" be strictly true. If it follows the relay, OK, different vintage of relays. If it follows the socket, there's an oddity about the connection to it on the PCB.
QuoteThis whole thing is odd to me as my old 1984 Seymour Duncan Convertible never pops when switching
Design finesse and completion. Maybe luck. There aren't many situations where a part does exactly what you want without you tinkering and compensating for its quirks.
Quote from: R.G. on November 22, 2012, 12:46:34 PM
Quote from: Paul Marossy on November 22, 2012, 12:48:07 AM
I do notice that relay K1 makes a noise you can physically hear when it switches, whereas K2 you can't hear at all when it switches. Not sure why that would be.
Since you socketed them, swap the relay positions just like you would with a tube so you can find out whether it's the part or the socket it's in.
That K1 relay always makes the clicking noise no matter what relay is that particular socket. I also went over the soldering on the sockets carefully to make sure all those solder joints were solid. It's like that particular relay gets slammed when it switches and the other one doesn't. Must be some quirk in the circuitry somewhere. I still need to try different preamp tubes, the other two in the amp are currently under suspicion now.
That is *really* odd. Aren't K1 and 2 coils hooked up in parallel? That suggests it's either some kind of physical resonance thing (they both make the same physical click but one gets dampened by the surroundings and one resonates freely) or a bad electrical contact slowing one down.
Quote from: Mike Burgundy on November 22, 2012, 01:33:02 PM
That is *really* odd. Aren't K1 and 2 coils hooked up in parallel? That suggests it's either some kind of physical resonance thing (they both make the same physical click but one gets dampened by the surroundings and one resonates freely) or a bad electrical contact slowing one down.
Yeah, they are in parallel. All I know is that when I push the "drive" and "more drive" button, I can not hear any audible noise coming from the relay itself. But when I switch between "normal" and "drive", it's a click you can definitely hear with everything turned down.
I would lean towards a bad electrical contact, but where the heck it is so far have been a mystery. I can not find the source of the problem to save my life. :icon_confused:
EDIT: It looks like maybe I was erroneously thinking that one relay was for the "drive" and "more drive", but I think that just happens with a switch and not a relay. So what I must be hearing is both relays clicking when switching between "normal" and "drive" channels. At first I thought one relay switched between "drive" and "more drive" and the other one switched between "normal" and "drive" channels. I'm not sure exactly.
*Both* relays switch at the same time when changing between normal and drive. more drive engages some solid state switching, nothing mechanical, so the behaviour is perfectly normal. I though you heard a distinct difference between the two relays, but it turns out it's two different circuits. I can't say anything about the hotrods at our studio (they're never switched, just used for poweramps and speakers) but I'll have a look how noisy/quiet their switching is.
Quote from: Mike Burgundy on November 22, 2012, 04:49:02 PM
*Both* relays switch at the same time when changing between normal and drive. more drive engages some solid state switching, nothing mechanical, so the behaviour is perfectly normal. I though you heard a distinct difference between the two relays, but it turns out it's two different circuits. I can't say anything about the hotrods at our studio (they're never switched, just used for poweramps and speakers) but I'll have a look how noisy/quiet their switching is.
Yeah, I eventually figured that out. :icon_redface:
Anyway, here is the inside of the relay just for kicks.
(http://diyguitarist.com/Images/HRD-Relay.jpg)
There's about 1/2 millimeter between contacts and the white assembly swings to the left or the right. So you would think that it would be pretty quiet with such a short distance for the contacts to travel, switching would be quite fast. I must either have a bad cap somewhere or some crappy preamp tubes. I'll have to do that preamp tube thing later today. It's probably these preamp tubes. I'm surprised, I thought J&J tubes were supposed to be pretty good. :icon_confused:
FWIW
Using sockets for the relays and ICs is a recipe for failure. An amp vibrates and the parts could/will fall out or have contact issues. I do not use sockets for anything in an amp or an effect except for tubes (I sometimes solder in tubes in microphone builds).
Did you changeV1 (V1b) and V2 (V2a, V2b)? If there is charge built up on the grids you might be getting pops from the triodes.
Quote from: Gus on November 23, 2012, 12:34:17 PM
FWIW
Using sockets for the relays and ICs is a recipe for failure. An amp vibrates and the parts could/will fall out or have contact issues. I do not use sockets for anything in an amp or an effect except for tubes (I sometimes solder in tubes in microphone builds).
Did you changeV1 (V1b) and V2 (V2a, V2b)? If there is charge built up on the grids you might be getting pops from the triodes.
I don't agree. This isn't the space shuttle, things aren't going to just fall out of the sockets from a little speaker vibration. I don't ever have volume knob up past aboout "4" anyway.I have to use needle nose pliers just to get the opamps out of the sockets. And the relays and transistors take considerable force to pull out too. And my Seymour Duncan Convertible built in 1984 never had any of its ICs fall out of their sockets. Nor have I ever had a tube fall out of any sockets in any of the several tube amps I own. :icon_wink:
Anyway, I need to get a couple of new preamp tubes. I'd be willing to bet it's these preamp tubes I have in the amp.
FWIW I had a quick peek at the ones we have - the channel switching is quiet when not playing, it might pop when audio is present but that's not uncommon. Tiny physical click audible at the chassis. There is a slight delay going from drive to clean which is a bit odd, but no real problems. This amp is developing (after a week or two of being quiet after a capjob and reflowing the output tube sockets) some more hum, so I'll have to hunt for more bad joints again. Maybe I'll just reflow the whole damn thing and be done with it. Hope you find your problem - wouldn't be surprised if it's also solder related.
Quote from: Mike Burgundy on November 28, 2012, 02:27:46 PM
FWIW I had a quick peek at the ones we have - the channel switching is quiet when not playing, it might pop when audio is present but that's not uncommon. Tiny physical click audible at the chassis. There is a slight delay going from drive to clean which is a bit odd, but no real problems. This amp is developing (after a week or two of being quiet after a capjob and reflowing the output tube sockets) some more hum, so I'll have to hunt for more bad joints again. Maybe I'll just reflow the whole damn thing and be done with it. Hope you find your problem - wouldn't be surprised if it's also solder related.
Thanks for getting back to me on that. :icon_razz:
So far so good on mine. The popping problem appears to have for the most part went away after changing out the faulty preamp tube. If there is anything audible in the speakers when switching, it happens only after the amp gets warmed up. Now I'm getting a smell like something is overheating, but so far nothing has failed. The power tubes seem to be running quite hot even though I've set the bias to factory specs. These hot rod Fenders sure do seem to be finicky. Mine's been pretty bulletproof since I bought it in 1998, but lately I'm beginning to wonder about it... :icon_confused:
To be sure - check the output tubes for solder cracks (ie poke it with a wooden stick when on). If there is no hum (which might indicate a severely misbehaving tube which might cause the other to take most of the current and fry), and the bias measures ok (60mV for 30mA across each tube) across the 1 Ohm cathode resitor you shouldn't have hot running tubes. 30mA isn't that hot for these voltages- although I can't remember what the plate voltage was (always use plate voltage, not B+ for bias calculation). If memory serves you're well below 50% plate dissipation. Make DAMN sure the tubes seat well and the bias voltage is reaching the tube - if it's off, or encounters a bad contact (increasing series resistance) the tubes might intermittently run very hot. If bias voltage fails, there's nothing stopping the tube from running wild. The stick poking bit (BE CAREFUL) helps find problems that occur not when you're measuring but only when playing/vibrating/the amp gets really hot etc.
That said: tubes get hot, really hot, but I don't think a hot-running tube causes enough difference in outside glass temperature to be noticable, let alone cause a smell. If that were the case, I think your plates are glowing and the tubes are write-offs.
If it's not just you being extra perceptive (and noticing little stuff that's been there before for the first time) because you're hunting problems, and there really is a distinctive smell of magic white smoke trying to escape, you'll find new clues soon. I don't want to talk about how often I've been suspicious about smells, sounds, etc that turned out to be modus operandi that I just hadn't noticed before, or dust getting hot that was disturbed by the previous repair ;P
Quote from: Mike Burgundy on November 28, 2012, 03:54:33 PM
To be sure - check the output tubes for solder cracks (ie poke it with a wooden stick when on). If there is no hum (which might indicate a severely misbehaving tube which might cause the other to take most of the current and fry), and the bias measures ok (60mV for 30mA across each tube) across the 1 Ohm cathode resitor you shouldn't have hot running tubes. 30mA isn't that hot for these voltages- although I can't remember what the plate voltage was (always use plate voltage, not B+ for bias calculation). If memory serves you're well below 50% plate dissipation. Make DAMN sure the tubes seat well and the bias voltage is reaching the tube - if it's off, or encounters a bad contact (increasing series resistance) the tubes might intermittently run very hot. If bias voltage fails, there's nothing stopping the tube from running wild. The stick poking bit (BE CAREFUL) helps find problems that occur not when you're measuring but only when playing/vibrating/the amp gets really hot etc.
That said: tubes get hot, really hot, but I don't think a hot-running tube causes enough difference in outside glass temperature to be noticable, let alone cause a smell. If that were the case, I think your plates are glowing and the tubes are write-offs.
If it's not just you being extra perceptive (and noticing little stuff that's been there before for the first time) because you're hunting problems, and there really is a distinctive smell of magic white smoke trying to escape, you'll find new clues soon. I don't want to talk about how often I've been suspicious about smells, sounds, etc that turned out to be modus operandi that I just hadn't noticed before, or dust getting hot that was disturbed by the previous repair ;P
Yeah, it's probably me just being extra perceptive about everything now. There is no hum and the power tubes are not "red plating". I did put some leather cleaner stuff on the whole amp exterior, so maybe part of it is that it is getting "burned off" the backplate thing on the chassis (that was my suspicion right off the bat). Anyway, I know that the bias at 30mA is running the tubes relatively "cool", I just don't remember the power tubes getting quite so hot before. I'm sure it's nothing to worry about.
The glass is not a good indicator of running "heat" - but it does get hot. Very hot. Like up to 200 degrees Centigrade.
Quote from: Mike Burgundy on November 29, 2012, 04:24:42 AM
The glass is not a good indicator of running "heat" - but it does get hot. Very hot. Like up to 200 degrees Centigrade.
Yeah, I've noticed that. :icon_wink:
Even more interesting - even a mild airflow over the glass will make the glass much cooler, only slightly above ambient. But this changes the plate temp hardly at all because of the extremely high insulation value of the vacuum between plate and glass. It's an almost exact analog of a voltage divider, where the top resistor is huge. Lowering the bottom resistor changes the current (heat) flow almost not at all.
Power tubes, at any decent dissipation, will run very close to their glass-limit. That's around 415 degrees F for the usual guitar-amp tubes.
They tend to run near max temp because heat from plate to glass is all Radiation, and (without fan) a considerable part of glass to ambient heatflow is also radiation. Radiation goes as the cube of temperature difference. A large change of Watts is a small change of temperature.
12AX7 and kin run far cooler, less surface but far less heat. 200 deg F IIRC.
While I'm typing: _I_ do not like the one cathode resistor nor the stupid diode. _I_ would want to convert to two 1 ohm resistors and no diode. With one resistor, if one tube is dead you can't tell (except by extreme grid voltage and redplate in the other tube). With two resistors you can verify each tube is at a safe dissipation. However I sure know that PCB construction can foil such mods.
Quote from: PRR on November 29, 2012, 06:01:28 PM
12AX7 and kin run far cooler, less surface but far less heat. 200 deg F IIRC.
Yeah, I can touch those and hold onto them without getting burned. Power tubes OTOH will burn me (duh). I've accidentally touched some EL84s with the back of my hand once, that was not a pleasant experience. :icon_wink:
Quote from: PRR on November 29, 2012, 06:01:28 PM
While I'm typing: _I_ do not like the one cathode resistor nor the stupid diode. _I_ would want to convert to two 1 ohm resistors and no diode. With one resistor, if one tube is dead you can't tell (except by extreme grid voltage and redplate in the other tube). With two resistors you can verify each tube is at a safe dissipation. However I sure know that PCB construction can foil such mods.
It could be modified to take two cathode resistors easily enough. I'm not sure what the purpose of that diode is. Never seen anything like it on any other amp schematics I have had to use for repairs, mods, etc.
R66 1 ohm resistor is marked fp1 ohm I am going to guess it is a fuse resistor and the diode is to keep it from opening (by clamping the max voltage to the diode drop) for short term output tube over current events.
Is the fuse a blue color? or does it look different?
Quote from: Gus on November 29, 2012, 06:56:12 PM
R66 1 ohm resistor is marked fp1 ohm I am going to guess it is a fuse resistor and the diode is to keep it from opening (by clamping the max voltage to the diode drop) for short term output tube over current events.
Is the fuse a blue color? or does it look different?
I assumed that R66 was a flameproof resistor. I've never heard of a fuse resistor before. There is such a thing?
Anyway, it looks much like the blue flameproof resistors I have used in the past. Maybe you're right about that. But it sure seems like kind of a roundabout way of doing things (kind of like a lot of the rest of the amp).
google bing etc "fusible resistor" and "flameproof resistor"
Quote from: Gus on November 30, 2012, 06:57:54 AM
google bing etc "fusible resistor" and "flameproof resistor"
I did last night. You learn something new every day!
> accidentally touched some EL84s with the back of my hand
I once rested the back of my hand on a 6550 idling at FULL 42 Watts dissipation for several seconds.
Yup, it is as hot as a cookie-oven.
I took a picture of the coke-bottle shape 1st degree burn on my skin (actually a small 2nd-deg blister but too small to show in photo). Hurt for a week.
> I'm not sure what the purpose of that diode is.
My guess is some green engineer "protecting" a cheap part which "can't" ever blow-up without some expensive part (tubes) failing first.
0.6V in 1r is 600 milliAmps. I forget what amplifier this is, but only the crazy-large amp tubes can flow 600mA. Unless they short plate or G2 to catode, and then they will blow the 1N4001 and then the 1r resistor. So it does not improve reliability, just increases MTTR (repair takes longer for finding and swapping-out the silly diode).
For more fun: unless available current is large, diodes can fail *short*. (They do tend to short, but in higher current circuits they soon blow open.) Now the 1r is moot, also useless. When the amp runs again, a bias-check shows ZERO tube current (because cathode resistor is shorted-out) and the tech turns the G1 voltage more and more trying to find some life in the tube; meanwhile maybe not noticing the dull red glow of broiling plates.
Diodes are REAL CHEAP at the factory, and warranty costs are high, so you do see them in places a DIY builder would never think to worry about. But this one seems to me to be a dud.
That's pretty much the analysis that Kevin O'Connor went through in his "The Ultimate Tone" book on the 1R and the diode. It went into my thinking on an experimental device I made for protecting tubes and the PT.
I put a 1R to ground in the cathode of each of the output tubes for measuring current/bias, then a high voltage MOSFET in series with the resistor to the cathode. Some simple comparator and logic circuitry sensed max current on each tube, then latched off all the MOSFETs (and hence all tubes) if the current was over X. The logic messed with things like soft on, standby, LED for "over current on this tube", and so forth. The logic was all MML CMOS. Today, I'd use one PIC and do it all. Some PICs have internal comparators for just such emergencies. I'd probably put a thermistor over the tubes and one on the PT, just because it's easy once you're over the hump of deciding to do the design.