Boss OC-2 Repair

[DrT]

I picked up a Boss OC-2 that isn't passing signal, either by-passed or affected. I've tried doing a quick sound probe to test the input and output buffers, my initial guess at the location of the issue.  For all tests, I grounded my external jack at Pin 2 on the PCB.  I had the following results:
Effect disengaged:
Pin 1 on the PCB - clean signal
Junction of Q1 and R2 - clean signal
Junction of Q4 and C8 - clean signal
Pin 5 on the PCB - no signal

Effect engaged - same results

I did some similar, but not as systemic testing last night and could only get effected signal some of the time off Pin 5. However, I haven't gotten any signal from the output jack directly.  It seems like there could be an issue with both the output buffer and the transistors controlling the signal path.  Any thoughts on other diagnostics I should run before unsoldering transistors and the like?

Schematic for reference:
https://www.hobby-hour.com/electronics/s/oc2-octave.php

[DrT]

One update, replacing Q2 (2SC732TM-GR) with a 2SC945 now results in the pedal passing signal.  The signal seems to be always lightly effected with no change resulting from turning the pedal on or off or from twiddling the knobs from what I can tell.  Any long term detriment from the transistor change?

[drummer4gc]

Do you get the octave down at IC4 pin 7 and 1?

[Rob Strand]

One update, replacing Q2 (2SC732TM-GR) with a 2SC945 now results in the pedal passing signal. 

The 2SC732TM-GR has a higher gain and is considered "low noise" (probably for high impedance sources).  That's why Boss chose it.  The 2SC945 has a lower gain and is general purpose.   If your C945 transistor is low gain then it *might* be bit noisy.  This stuff can tread a fine line and depends on the specific transistor.  When you use the high gain transistor you don't have to think about it as you can't do better.

[DrT]

Thanks Rob. I will see if I can order some of the GRs then. 

Drummer, I will have to check that tomorrow.

[DrT]

Alright, I've tried tapping in on the signal path going to and from the sub-board with the level pots.  I've included an image of my wiring for reference because the results seem to contradict the schematic I am using. Here are the results:

Black wire: Expected clean signal.  Result, clean signal with the guitar volume down, very loud octave down with the guitar volume up.

Green wire: Expected one octave down.  Result, two octaves down, only with the guitar turned up and digging in.  Hitting the strings very hard or soft gives a distorted direct signal.

Blue wire: Expected two octaves down.  Result, distorted direct signal.  Signal seems stronger with all the pots turned all the way down.

Orange wire: Expected mixed signal returned from pots.  Result, distorted direct signal that comes through clearly. 

[Rob Strand]

I agree with your expected signals.

range wire: Expected mixed signal returned from pots.  Result, distorted direct signal that comes through clearly. 

This is  a sign of weirdness.

Black wire: Expected clean signal.  Result, clean signal with the guitar volume down, very loud octave down with the guitar volume up.

Green wire: Expected one octave down.  Result, two octaves down, only with the guitar turned up and digging in.  Hitting the strings very hard or soft gives a distorted direct signal.

Blue wire: Expected two octaves down.  Result, distorted direct signal.  Signal seems stronger with all the pots turned all the way down.

I'm assuming these tap points don't improve when *all* the pots are turned down?


I think you need to check the DC voltages on the outputs of IC1a, IC1b, IC4a, IC4b, IC5a, IC5b.   Check with all pots off and all pots on.  Also check the DC voltage at the 4.5V point, the point where all the pots join when they are off - I think that's next to clean signal/black wire.

(I'm thinking there's something wrong with the 4.5V point)

[DrT]

I'll see if I can get to this tonight. My initial messing around with the input/output jacks made me thing that there was an overall grounding issue.  I am hoping that it turns out to be something simple in the power section and I don't need to replace half the ICs.   

[DrT]

It took me a bit to find time to get back to this.  Rob, you are dead on with the biased voltage, which I think is designed to be ~4.9v.  With the pedal on, the junction of R39-R46 reads 0 volts.  The opposite side of R49 reads 8.6 with a battery. I tried testing both R39 and R46 but couldn't get a reading for either.  When I switch to the closed circuit tester, they both test as an open circuit one direction and closed the other. I think this means that R46 is bad and acting like an open circuit so that I am really only testing D1 when I try to measure the resistors.  However, I cannot convince myself that this is correct since then R39 would be separated from D1 and I should be able to measure a resistance.

IC Voltages, pot position did not seem to have an effect.
IC1 - Pin1: 4.9
IC1 - Pin7: 6.1
IC4 - Pin1: 7.1
IC4 - Pin7: 7.1
IC5 - Pin1: 6.8
IC5 - Pin7: 6.8

[Rob Strand]

Some extremely puzzling measurements.  Some contradict others.  I'm having trouble trying to match them up to a fault!

which I think is designed to be ~4.9v.

Yes it is actually higher than 4.5V.

With the pedal on, the junction of R39-R46 reads 0 volts.  The opposite side of R49 reads 8.6 with a battery.

Very odd to get 0V.  As IC1 pin 1 and IC1 pin 7 could not get to the measured voltages with 0V on the R39-R46 junction.   I'd perhaps try to remeasure that voltage.

I tried testing both R39 and R46 but couldn't get a reading for either.  When I switch to the closed circuit tester, they both test as an open circuit one direction and closed the other. I think this means that R46 is bad and acting like an open circuit so that I am really only testing D1 when I try to measure the resistors.  However, I cannot convince myself that this is correct since then R39 would be separated from D1 and I should be able to measure a resistance.

You have to take in-circuit measurements with a grain of salt.   If you get crazy measurements it's a good idea to reverse the multimeter leads to see if you get the same crazy measurement, or a different crazy measurement.    Often the caps take a while to charge up or they hold charge from being previously powered.  (You have got the power off, no battery and jacks removed?)

Having said that there could still be a problem in that area.  Maybe measure across R39 and across R46 when the device is powered.  In both clean and effects modes.

Starting from the most basic measurement IC1 pin1 looks OK.   However IC1 pin 7 should follow IC1 pin 1 but it is way off.     I can't see how that can be the case unless IC1 is blown or you have some sort of short or cut-track on the PCB. 

When you take your measurements, what ground point are you using?   Note the DC jack -ve connection is not ground there's a resistor and diode (R17 and D3) between DC jack -ve and ground.  So the ground point must be the circuit side ie. the ground on the jacks or PCB.

If you ground is correct maybe measure along R8, R9 and R5.   Maybe also pins 2 and 3 of IC1.

[DrT]

Rob, after your last comment, I disconnected everything to let everything discharge for a day or two (or a week at this point). I can now get a resistance measured across 39 and 46 of 4K and 3.5k respectively.  The measurement is the same either for both polarizations and it takes a bit for the measurements to level out, which I am assuming is related to the caps charging.  I am reasonably confident at this point that IC1 is a bad apple and will see if I have something on hand that can serve as a replacement before ordering the TL022s used on this pedal.  I will let you know how it goes, when I get around to it.  :-)

[DrT]

I removed IC1 and replaced it with a socket so I can test with a JCR4558.  I also replaced the output transistor with the 2SC945 discussed earlier in the thread.  I now get the expected signal from the sound probes going to an from the pot board.  The pedal passes clean signal fine but when engaged I just get a quieter version of the clean signal.  I guess this means there is something wrong with Q3 and/or Q4 such that the flip/flop isn't flapping correctly.  I will try replacing both of those with my next bit of free time.