Help Needed - Troubleshooting Fuzz Factory Copy

[fuzz guy]

I just finished building a Filthy Fack Vertical kit form Fuzz Dog. Here a link to the build doc,

http://pedalparts.co.uk/docs/FilthyFack-Vertical.pdf

I did not change or substitute anything and used AC128 for Q1 and 2. I did install the fat switch and I also used their switch daughterboard, found here,

http://pedalparts.co.uk/docs/3PDTDC.pdf

There is sound from the guitar coming through but it's quiet. There is lots of hiss. All the knobs and the switch seem to be working, or at least have a noticeable effect when turned. The Volume works, turning the Drive all the way up causes a loud squeal and turning the Comp all the way up causes no signal.

I have 9.06v coming in at the DC jack and the same at the board. The voltages for the transistors are as follows,

Q1 E 9.06 / B 8.91 / C 7.76
Q2 E 7.85 / B 7.76 / C 3.68
Q3 E 0.00 / B 0.58 / C 1.27

If anyone could suggest where to start it would be greatly appreciated as I'm fairly new to this stuff. The number at the Q3 emitter seems strange, but I don't really know. If you need any more information from me please let me know.

Thanks.

[PRR]

Voltages look OK. Most should change as you diddle the too-many knobs.

Q3 emitter is hard-grounded. If it was anything other than 0.00V, that would be wrong.

[fuzz guy]

Thanks for the reply Paul, I appreciate the help. I have soldering skills and can read a schematic and follow instructions, but that's about as far as my knowledge goes.

I contacted the guy at Fuzz Dog and he thought the voltages on Q1 were off and asked me to check that I'd put some of the resistors in the proper spot. They all were, but when I put a DMM across them all sitting in the circuit R5 measured at 24k rather than 47k as it's labelled. So I'm thinking it's a mislabeled resistor?

On another note, I see you're from Maine. I grew up in Mass and spent lots of time up there, fishing in the Belgrade lakes and along the coast, skiing and hiking in the mountains, canoeing down the Saco. My Mom's parents had a place on Linekin Bay and my other Grandfather had a camp at Tripp Lake near Poland Springs. Such a beautiful place, I miss it.

Thanks again for the help!

[fuzz guy]

I pulled out R5 and tested it, it read 47k as it should. Soldered a new one in and now the board isn't passing any signal. 

I had to de-solder one of the pots to get to the resistor, so I must have bodged something up in the process. 

[jmasciswannabe]

Well the good thing is it is a small circuit, sometimes they can still be real buggers though. This is what I do when it doesn't work:

1. Check orientation of electros (i am notorious for putting these in backwards) and diodes, double check transistor pinouts and that they are in the right sockets. Recheck part values includeing pot values. If you practice long enough you'll be able to read resistor codes without looking at a cheat sheet. Looks like the pdf has a nifty chart to use.
2. Inspect the board front and back for solder bridges. I use a magnifying glass.
3. Reflow all solder joints.
4. Test again.
5. Use continuity tester to make sure everything is connected right. Occasionally a trace will lift by a pad and you can catch it, or something will have continuity that shouldnt and you find the solder trace. More importantly it insures your wiring is correct. I usually start with the power lines, then move to ground plane and then run through the circuit. Be sure to trace through the 3pdt to insure it is working properly.
6. The audio probe is a huge help in debugging. I would encourage you to make one if you are going to be building more projects. It can help you isolate the problem when a faulty component may be the issue. In my experience, it is rare that a new build has a faulty resistor or capacitor. Pots, switches, transistors and ICs are typically more suspect.

Be patient and thorough. You'll find it. Good luck!

[PRR]

> R5 measured at 24k rather than 47k

In-circuit, R5 is paralleled by Q1(b-e) plus Comp and Drive pots. So there is a sneak path "around R5" in the circuit. The internal resistance of Q1(b-e) depends how hard your ohmmeter leans on it. But ~~30k is not unreasonable, which would give about the reading you got.

I'm falling back on the Top #1 Most Popular trouble: solder joints. The fact that your DC voltages seem right show you "usually" get good joints. But there's AC/audio connections that don't show in DC readings. And there is the clue that you messed with it and it works different.

I recently tore-out a lot of bad phone wiring because of erratic joints. (Hasn't helped yet because the phone co obviously has problems after that lightning strike.) Always suspect your joints!

[fuzz guy]

Thanks Paul. Unfortunately with this build there's five board mounted pots that block much of the back, so doing a visual inspection is difficult. I think my next step is to build an audio probe and use that to find the possible bad joint(s).

[PRR]

> I see you're from Maine.

We hit 114 deg F today. (In a bottle in the sun... maybe 87F in open shade.)

That's far up-coast (what sailors call Down East).

In the 1970s we never needed air-conditioning in this area. We been whipping the freon pretty hard this weekend. (Not so much the heat but the coastal humidity.)

[soggybag]

Build an audio probe! All you need is a cap, any size, and a 1/4" jack. Search the forum or google. This is best way to debug. Just start at the input and work your way into the circuit until there is a problem.

[fuzz guy]

> I see you're from Maine.

We hit 114 deg F today. (In a bottle in the sun... maybe 87F in open shade.)

That's far up-coast (what sailors call Down East).

In the 1970s we never needed air-conditioning in this area. We been whipping the freon pretty hard this weekend. (Not so much the heat but the coastal humidity.)

Wow that's quite hot for up there. One thing I don't miss about living there is that damn humidity. It gets much hotter down here on average, but it's almost always dry.

Build an audio probe! All you need is a cap, any size, and a 1/4" jack. Search the forum or google. This is best way to debug. Just start at the input and work your way into the circuit until there is a problem.

Yep, I bought the stuff for one today. 

[fuzz guy]

So I built my audio probe today and started working through the circuit. I was getting sound through all the way to the collector of Q2 and on that side of R3, but on the other side of R3 and lug 3 on the volume pot it's gone.

Does that mean that R3 is bad, does that happen? In order to change it I have to desolder the gate pot, so I want to be sure.

Also, any tips for desoldering? I've got one of those spring loaded pumps but it's still a difficult job. Is that just the way it is?

Thanks

[Slowpoke101]

R3 could be faulty (open circuit ) but I suggest checking its resistance before declaring it as faulty. According to the construction info it is supposed to be 5K1. So test it without removing it (no power to the board ) and even though it is still "in circuit" it should show as being about 5K.
What is most likely is pin 3 of the volume pot is shorting to ground (0V ).

De-soldering can be a pain without the correct tools. The spring powered vacuum toy is best used for damaging the circuit board and the component that you are trying to remove (yes, I do not like them at all ). The next thing I would recommend is "De-soldering Braid". I see that you are in Adelaide so Jaycar may be close enough to you. Here is a link to a suitable size braid;

https://www.jaycar.com.au/2-0mm-goot-desolder-braid/p/NS3027

It's fairly simple to use but be aware that you can overheat the circuit board and damage it - pads can lift off and the board can be burnt if you apply too much heat for too long. I think that there are plenty of You-Tube videos covering how to use it.

The braid is fairly effective, easy to use and far cheaper than the professional de-soldering stations (Hakko, Pace, Weller, etc ).

[fuzz guy]

Thanks Slowpoke, I'll double check R3 and if the volume pot is shorting out.

I have a Jaycar nearby where I get most of my stuff, so I'll grab some of that braid and learn how to use it.

Thanks again!

[fuzz guy]

Got it working! A bad solder join at R3 and another on the footswitch. That audio probe is a great tool, super easy to track down issues.

On a side note, it's a good thing I knew what to expect from this circuit or I might have got it working and still thought something was seriously wrong. There's some wild sounds that come out of this thing!

Thanks again so much to everyone who gave advice. I learned a lot on this build, and hopefully I can someday help someone else.