Please help me fix my Fulldrive 2 Mosfet

[ekamal]

Hi all, my first post here although I've been a casual visitor to this great forum.

I've emailed Fulltone and have also posted this on TGP but haven't had any concluding answers and still waiting for a reply from Fulltone, I was wondering if any of you guys can help pinpoint the problem.

I inherited a Fulldrive 2 Mosfet and it's not sounding the way it's meant to be, I can still hear different notes and all that but it sounds like a spitty sputtery blatty fuzz pedal - quite a cool sound if I needed a fuzz sound. I was suggested to change the 1uF bipolar cap after the opamp, did that, but didn't fix any problems.

The volume and the tone knobs still work although now the volume knob gives out an obscene amount of volume. The drive and the boost knob doesn't seem to change the sound at all - maybe it's way too distorted that it doesn't make any difference anymore? Flicking the mosfet/std and vintage/fm/comp cut switches doesn't seem to make any difference either.

Any ideas?

--
ekamal

[jimma]

If there's not a lot of gain to the sputtery sound and considering the clipping switch isn't working properly it seems the problem could be around the clipping section somewhere (could be only op-amp clipping your hearing, that would also explain the touchy volume situation- that would make it a lot louder). Maybe the clipping switch has developed issues. Just a thought, hard to say without actually looking at it.

[Jered]

  Post some voltages, makes it much easier to narrow it down.

[petemoore]

Any ideas? 
  Get a schematic.
  Read debugging page.
  Get Mosfet pinout.
  Start debugging and post voltages.
  GEO also has mosfet biasing information.

[ekamal]

Thanks for the comments so far guys.

@jimma: it's pretty gainy, not obnoxiously fuzzy though.

@jered: I will try to get voltages either tonight or tomorrow night.

@petemoore: Thanks for the tip. I've done the first two before posting here, I wasn't successful in getting a schematic, spent a lot of time looking for one. I keep stumbling upon the non-mosfet one, unless I was looking at wrong places. The debugging page was very informative but can't seem to use it at this stage.

The signal probe is very interesting though, I will try that when I get some free time this week.

[liquids]

Off the top of my head....

What kind of signal is the input seeing?  Is it getting a straight guitar signal or a hot signal from some other source; is it in your amps FX loop, etc?

Sounds to me like one way or another you are absolutely clipping the crap out of the input....which would probably mean youd get an obsene amount of volume and no matter how you set the clipping, the differences would be negligable....

Good luck...

[ekamal]

G'day liquids, it's just getting a straight guitar signal.

[Mike Burgundy]

As said, probing the circuit and getting voltages helps. A lot.
That said, if I understand you correctly and apply some simple reasoning, the volume and tone stages are behind the clipping stage - and they work. Reasonable to assume that stage is ok.
Gain and boost don't work properly, and there's way too much amplification in there. This suggests a fault in the clipping stage - I'm not saying this *is* the problem, but it makes me think the first opamp is attempting open-loop gain - as high as it will go, and it is totally swamping itself. If that is the case, the feedback loop containing the clipping diodes is interrupted somewhere. Might be as simple as a broken wire to a switch - check that bit of the circuit for correct connectivity etc. while you're in there checking voltages.
I think the MOSFets are used as clipping diodes, not as actives, by the way.
Happy debugging!

[petemoore]

I've done the first two before posting here, I wasn't successful in getting a schematic, spent a lot of time looking for one.
  I kinda figured, and just brazenly mentioned 'schematic'...
  There are other ways...pics of top and bottom of board..
  Drawing up a schematic...start at the ground, then mark every component with one of those, keep going...easy to miss a node-line though when there are 3 or 4 connections per node...the long way around to get a scheme going...but I've done it before on boards that hide stuff...gotta get the meter out...follow a lead that goes to..a resistor or cap...mark it...try to find any other connections from that node...continue doing the same from the other side of the component. 

[johngreene]

Sounds like it has been messed with. To me it would seem that it would take at least 2 things wrong for this to happen.
1. the drive pot is open
2. the vintage/FM/compcut switch is open/wrong

If both of those are true then you basically have an opamp running open loop. It explains the boost/drive pots not working, the loud volume, and the splattery sounds.

What color is the pedal?

--john

[ekamal]

Hi guys, thanks for the great pointers.

@johngreene: this is the newer sparkly blue pedal.

Okay, I finally had a bit of time to do some probing and quick pot testing - forgot to check the toggles but I think they're alright. The pots seem to be fine, resistance reading on the pots is as expected, 500k drive pot, 1M boost pot, within the +/- 10% tolerance. I haven't had the chance to do any voltage readings.

I rigged up the signal probe last night, very useful, felt like a doctor with a stethoscope . Only managed to do a bit of that until real life takes over and had to go away and do stuff. I've managed to trace the signal from the input jack -> 3pdt -> opamp, and got stuck there. I think it's the opamp, but I can't be sure.

This is what I found, every pin of the opamp (except the input pin) spits out that massive overdrive/fuzz sound, probed all the pots and they all give out this splattering sound. Excuse my ignorance, but shouldn't the drive knob regulate how much drive the opamp gives out from the output pin? Does it mean I've got a broken opamp? I kinda stopped there because everywhere else I went I got that spitty sound.

More questions:

Does getting a signal out from the drive lugs mean that all the connections after the opamp is still intact?

Should I be getting any sound from the clipping diodes? I don't remember getting any sound there though, will need to check again.

If I didn't get any sound from the diodes where I should have, what does that mean?

I didn't have the chance to take photos last night, my camera seems to be missing, could well have been hidden in my wife's handbag

[johngreene]

I would check the boost footswitch first. Make sure the center lug is connected to R7 (near boost pot) in the boost position and connected to the center lug of the boost pot in the non-boost position. Check that the center lug of the middle switch is connected to the wiper of the drive pot. If these connections were broken this would cause the feedback loop to open up and cause sputtering. Not sure what the opamp would do with no DC feedback other than what comes through the diodes. There's always the possibility the opamp is bad.

[ekamal]

Okay, had a bit of time just now but sacrificing a bit of my much needed sleep.

John, as far as I can see the boost switch center lug is properly connected as you described, and the center lug of the switch is connected to the wiper of the drive pot. How can I test the fm/vintage/comp cut switch?

Quick post of the voltages, excuse my ignorance with which leg is base, emitter and collector, my last training in electronics was in high school about 10 years ago, will look them up properly tomorrow when I get the chance at work.

Opamp: JRC4558
pin 1: 1.79V
pin 2: 1.15V
pin 3: 0.85V
pin 4: 0V
pin 5: 1.70V
pin 6: 1.73V
pin 7: 1.75V
pin 8: 9.49V

Q1: C1815 GR6H
pin 1: 0.19V
pin 2: 9.49V
pin 3: 0.42V

Q2: 2N7000
pin 1: 1.38V
pin 2: 1.38V
pin 3: 1.78V

Q3: 2N7000
pin 1: 1.79V
pin 2: 1.70V
pin 3: 1.38V

Q4: C1815 GR6H
pin 1: 0.20V
pin 2: 9.49V
pin 3: 0.56V

Q5: 250KL F0624AA
pin 1: 6.53V (labelled S on PCB)
pin 2: 2.22V (labelled G)
pin 3: 6.52V (labelled D)

A shot of the guts:

[johngreene]

Looks like pin 3 of the opamp is blown and is pulling the 4.5V line down. Since it only has a 10K connected to 4.5V, it is going to have more effect on the 4.5V than either transistor would if one of them was shorted.
If you would like to verify this before taking out the chip you could try lift one end of R5 and check the voltage between R22 and R23 to see if it comes back up to 4.5V. The resistors have their leads really tightly bent to the body so it may be just as difficult to try this than it would to just change the IC. 

The transistors are ECB, btw. 

[ekamal]

Hi John, thanks for that. I'm not ignoring this thread at all, I just haven't had the chance to go into the pedal again. I need to get new soldering tip too, the pcb is really tricky to work with. And I've already got an IC socket and a TL072 with RC4558 to test out.

Will post result when I get the chance to do the fix.

[johngreene]

Hi John, thanks for that. I'm not ignoring this thread at all, I just haven't had the chance to go into the pedal again. I need to get new soldering tip too, the pcb is really tricky to work with. And I've already got an IC socket and a TL072 with RC4558 to test out.

Will post result when I get the chance to do the fix.

It is best to removed the PCB to work on it. Do not try to do it in place. To remove it you first need to unsolder the DC jack and then remove it. Then all you have to do is take the nuts off the input and output jacks, the pots, and the footswitches and the whole thing will come right out. Be careful to line up the LEDs when you put it back in.

Good luck.

--john

[ekamal]

Looks like this isn't gonna happen. The PCB is a machined type and too hard to work with, contacts are too small and fragile to hand desolder. Ended up ruining the contacts.

Maybe I should just build a tubescreamer and house it in this enclosure..with plenty of mods.