Relays And Popping

[Paul Marossy]

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?

[R.G.]

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.

[Paul Marossy]

Try a BFC across the coil, maybe.

OK. What is a "BFC"?

[Kesh]

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.

[Paul Marossy]

big @#$%ing capacitor

Oh! 

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. 

[PRR]

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.

[Paul Marossy]

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).

> 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.

[R.G.]

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.

[Paul Marossy]

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. 

[wavley]

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. 

[Paul Marossy]

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! 

[wavley]

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.

[Paul Marossy]

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.

[R.G.]

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.

[wavley]

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

[Paul Marossy]

You and me are on the same "wavelength" in this area wavely. 

[PRR]

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.)



> 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.

[PRR]

-- {dupe}

[Paul Marossy]

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.

[R.G.]

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.

I 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.

So 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.