Boss OD--1. Need help with repair. no effected output.

[Rob Strand]

Actually if one of the JFETs is stuck on you could get low levels.

What happens to the output of the pedal in effects mode when the LEVEL pot is on full, at mid position, near minimum, at minimum.

If it sounds clean even down to near minimum then perhaps nearly cuts out on minimum that could mean Q1 is stuck on.

Measure the collectors of Q3 and Q4 in effects mode an bypass mode.
That will check the switching signals going to the JFETs is OK

We expect something like,

               Q3.c        Q4.c
Effect     6.5V         1.0V
Bypass  1.0V          6.5V

[Mr.Grim]

Level between C7 and R11 are the same as pin 8. But..level does drop a bit after R11 to the level pot lug 3. Not sure if that's normal but R11 does test properly. And pot seems to function properly. (Can they operate properly but degrade over time resulting in different audio quality like other components?. It's supposed to be 10k and I'm getting 8.7k really. Is that difference enough to cause this? I wouldn't think so)

And I did replace all electrolytic caps right away, it was the only thing I did before poking around. And it had made no difference to the original issue.

I also at a later time replaced all the transistors Incase any of them were the issue, also no change.

I did originally post my voltages, but I'll post new ones in a couple min.

[Mr.Grim]

Second thought on the pot reading. I'm sure it don't matter, my range may be slightly off but full on is still full on.

[Mr.Grim]

           Q3c.      Q4c
On.      0.58.     6.6
ByP.    6.59.      0.58

Seems fine

[Rob Strand]

Level between C7 and R11 are the same as pin 8. But..level does drop a bit after R11 to the level pot lug 3. Not sure if that's normal but R11 does test properly. And pot seems to function properly. (Can they operate properly but degrade over time resulting in different audio quality like other components?. It's supposed to be 10k and I'm getting 8.7k really. Is that difference enough to cause this? I wouldn't think so)

That all looks very normal to me.  R11 intentionally drops the level - that's why before I mentioned you can mod the pedal for a bit more level.  However from what you described you clearly have a fault.    Pots have a wide tolerance so the 8.7k is nothing to be overly concerned with.   You could try measuring the pot with the LEVEL pot turned to minimum.    It may show a better value.

Q3c.      Q4c
On.      0.58.     6.6
ByP.    6.59.      0.58

Seems fine

Yep, looks spot on.  Switching both ways as well.

So far the only un-addressed issue is the problem itself!

Definitely not a personal dislike of the output level, it's definitely not working.

When I try to use the pedal and turn it on it's more of a volume pedal and knocks 1/3 of my total volume down, not up (when both knobs jacked up).  And no OD at all.

So what we have done is reduced the problem and put a very tight box around the problem area:  The switching JFETS  Q1 and Q2.   

So there's two possibilities
- Q1 stuck on, or a short around Q1
- Q2 not switching on or, an open circuit or cracked solder on the pins of Q2 or tracks from Q2 to the surrounding components.

You could measure the voltages on the drain and source of Q1 and Q2 in both effect and bypass mode to see if you get any weird voltages.  You should just see
- in effects mode 4.5V on Q2, and output side of Q1, a bit less on the input side of Q1
- in bypass mode 4.5V on input side of Q2 and a bit less on both sides of Q1
(lower voltages due to the multimeter loading down the 470k R5).

My feeling is it's Q1.   Look for shorts around Q1.   A good idea would be to pull Q1 then see if you can get overdrive passing through Q2 in effects mode.   Bypass mode should produce no signal.

(It is possible to check Q1 on the bench.)

Edit: clarified voltages on Q1, Q2

[Rob Strand]

From your old measurements,

Q3 2SC536
E- 0.53
C- 0.56
B- 1.12

Q4 2SC536
E- 0.53
C- 6.11
B- 0.19

Q4 collector at 6.11V means effects mode.  (Confirmed by new measurements.)

Q1 2SK30A
S- 4.45
G- 0.02
S- 4.45

Q2 2SK30A
S- 4.25
G- 0
S- 4.45

These voltages show the right pattern for effects mode but it looks like the names of Q1 and Q2 are swapped.
See how one side of Q2 is a lower voltage (but we expect one side lower on Q1).

*** If you pull a JFET you want to pull the JFET for the clean signal.   Make sure it connects to the clean signal parts.

[Mr.Grim]

 . Ok so I checked everything and all you mentioned checks out. I even pulled Q1 and I get no change on the effected signal. ( I did swap the names of Q1/2 mistakenly). And all solder joints and traces are fine.

I honestly think it's about time I give up and work on sum thing else. 

[Rob Strand]

. Ok so I checked everything and all you mentioned checks out. I even pulled Q1 and I get no change on the effected signal. ( I did swap the names of Q1/2 mistakenly). And all solder joints and traces are fine.

I honestly think it's about time I give up and work on sum thing else. 

Problems like this can be a bit frustrating.   Like most repairs you start off delicately and try not to mess with much of the circuit.   As you progress you find you need to be more heavy handed.

If you rip out both Q1 and Q2:

1) You should get no output from the Output Jack.  If you did there would be a short somewhere or someone resoldered some fo the wires and screwed something up.

2) You should be  able to trace the overdrive signal on these points
   - IC pin 8
   - R11/Wire Connection 6/Clockwise pin of LEVEL pot;   some reduction of level
   - Turn the LEVEL pot on full.    Listen to the wiper of LEVEL pot/Wire Connection 7.   
     Level should be *exactly* the same as previous point

So if you get that far you should be confident the Overdrive is working.  If it poops out on the last step I'd be thinking the LEVEL pot has a problem.  They do get old and have bad connections.

So what next?

Solder a wire across Q2 drain and source.   If you can put on long wires so you can disconnect it easily.   That will make it a permanent overdrive.

Go back and listen to the LEVEL pot wiper/Wire Connection 7.   Make sure the overdrive signal does not change or stuff up when the added Q2 replacement wire is connected or disconnected.   If you hear a difference then maybe there's a short or low impedance looking into C18.   That could be a problem on the board, or, it could be a short or low impedance looking into pin 12 of the IC.    If all is good you should get the same level signal on the LEVEL pot wiper, pin 12 and pin 14 and also at the output socket.   If you get this far you should have a permanently wire overdrive from the input socket to the output socket.

That's checking a lot of circuit, basically the whole signal overdrive path. 

[Mr.Grim]

All is check. Everything is fine that you mentioned. Level pot is fine.  I do get the same level signal at all points you mentioned. But it's all the same bad signal Ive had the whole time

[Mr.Grim]

Oh and I ran the wires mentioned, there was no I'll effect when attached or disconnected. Or nothing odd with the level pot

[Rob Strand]

All is check. Everything is fine that you mentioned. Level pot is fine.  I do get the same level signal at all points you mentioned. But it's all the same bad signal Ive had the whole time

I thought you got a *good* overdrive signal out of, IC pin 7, IC pin 8, junction between C7 and R11.   Then a good overdrive signal on the clockwise terminal of the LEVEL pot except the level is lower than at IC pin8.

Is that right?

I thought we got all the way to the JFETs and then it stuffs up?

[Mr.Grim]

Not sure where my confusion started here. But I originally stated that when engaged and knobs full up, it acts like a volume cut  with no overdrive.

And this whole time I've never heard any overdrive anywhere in the circuit.

I've been confirming getting "A" signal throughout your guidance, but it's always been one of 2 sounds. Either bypass sound, or the lower volume sound with no OD.    I've never got an actual OD.

Can it be the IC?  Can I sub a modern quad op? Just to test temporarily.

[Mr.Grim]

When you would ask me to check things I was confirming continuity, or that, that path or components are passing the signal without alteration, but I had never found a proper effected signal at all.

I don't know how our wires got crossed like this, this bad.

 

[Rob Strand]

I've been confirming getting "A" signal throughout your guidance, but it's always been one of 2 sounds. Either bypass sound, or the lower volume sound with no OD.    I've never got an actual OD.

Can it be the IC?  Can I sub a modern quad op? Just to test temporarily.

When you would ask me to check things I was confirming continuity, or that, that path or components are passing the signal without alteration, but I had never found a proper effected signal at all.

I don't know how our wires got crossed like this, this bad.

 

I don't know what happened either.  We'll just file it in the shit happens folder 

It certainly puts a different spin on things.   Everything following IC pin 7 seems to be working because your descriptions and all the repeat checks seem OK.  That's what was puzzling.   No overdrive in the first place puts the problem before R8. The problems has to be around opamp pins 4, 5, 7. 

All you opamp voltages seem OK so I'm thinking an open or a short in this area.

So here's the steps:

1) Does signal make it to IC pin 5:   Is the signal on IC pin 1 pretty much identical to the signal on pin 5?

2) Is the signal on IC pin 7 pretty much identical to IC pin 5? or is it much louder?
    Does the OVERDRIVE pot do anything to change that?

If the OVERDRIVE does nothing and the signal on pin 7 is the same as pin 5.   
- Look for open circuit along the parts IC pin 6, resistor R6 (4k7),  Cap C5 (47n).
  If you want to try something solder another 47n cap across C5.  If it comes to life C5 is open.
- Look for shorts along any of the parts between IC pin 6 and IC pin 7.
   If you have a diode test on your DMM, set the OVERDRIVE pot to maximum and
   check the diodes D5, D6 and D7.   A shorted D7 will pretty much take out the overdrive and the gain.

[m4268588]

I'm not convinced that there is a problem with the op-amp.
However, the manufacturer changed to a different chip because the RC3403 generate problems frequently.

[Rob Strand]

I'm not convinced that there is a problem with the op-amp.
However, the manufacturer changed to a different chip because the RC3403 generate problems frequently.

At this point I'm not thinking opamp either.  The voltages look too good.