OC-2 Repair help

[sirlemonhead]

Hi,

I've just got a broken OC-2 (bought as broken) that I'd like to fix. It's a 2001 year.

Powers from PSa adapter and battery. When I run a clean guitar into it, I just get a very very quiet clean tone out, with some hiss on the signal. This is actually inaudible with my clean channel on the amp, I need to go to a high gain preset to hear it - Sounds like this https://www.dropbox.com/s/jnv8be6zof3v1 ... hannel.mp3

This is the same with the pedal both turned off and on.

The PCB looks in great condition - no bad solder joins, clean board with traces looking intact. The only thing odd is IC1 must have been removed as it now has a socket in place, with an RC4558P IC in place. The other 8pin ICs are all soldered-to-board IR9002s.

Wiring all looks OK and I've got continuity through the ribbon cables to the control pots.

I've got a multimeter and audio probe and I've been poking about the circuit. I've been following this schematic https://www.hobby-hour.com/electronics/s/oc2-octave.php but I'm not overly knowledgeable at this sort of stuff!

I tested the IC pins first - the INs and OUTs


IC1
----------
Pin 1 - Perfectly clean sound
Pin 2 - clean
Pin 3 - clean
Pin 7 - clean (but somewhat muffled, like the high end is rolled off)

IC2
--------
pin 1 - heavy clipping with almost flangey
pin 2 - very quiet signal
pin 3 - clean (but somewhat muffled, like the high end is rolled off)
pin 7 - heavy clipping with almost flangey

IC3
-------
Pin 1 - heavy clipping with almost flangey
Pin 2 - clean (but somewhat muffled, like the high end is rolled off)
Pin 3 - quiet
Pin 7 - heavy clipping with almost flangey

IC4
-------
Pin 1 - I hear the effect - a low octave thats the same volume as input signal
Pin 2 - I hear the effect - a low octave thats the same volume as input signal
Pin 3 - I hear the effect - a low octave thats LOWER in volume to input signal
Pin 7 - I hear the effect - a low octave thats the same volume as input signal

IC5
----------
Scratchy and flangey sounding low octave on pins 1, 2, 3. Quieter than input level
Flangey low octave on pin 7. same volume as input

I then started checking other components with the audio probe and noticed some differences between the pedal being off or on.

-clean on centre pin of Q1 and pin below (if you consider "top" is where the DC in jack is on the board)- Is same with pedal on or off. No signal on the other pin

-clean on either outer pins of Q4 with pedal off. Low rumble on either with pedal on.

-clean on one side of C8, nothing on the other with pedal off. Low rumble on one side of C8, nothing on the other with pedal on.

-capacitor c24. Clean on + and - sides

-clean on 'band' marker side of D10

- R44 clean on one side, flangey and staticy on the other. Reads as 1.4k in circuit with my multimeter but schematic shows its 47k

- Clean on both sides of C19
- nothing on either side of R16
- nothing on c11
- nothing on any of 3 legs of q2

I get signal on one side of C8 but not the other - these are those square, whiteish semi transparent capacitors. Is this normal? Based on measuring the other capacitors I would assume signal on both sides but I'm not experienced enough to say with certainty.

So maybe the signal isn't getting beyond C8?

Is there anything that sounds off from above or what specifically should I be testing? It seems like the switching section isn't working based on no clean tone going through with the pedal off - how would I troubleshoot that?

Thanks!

[laurie]

Signal on one side of C8 but not the other, and nothing on any of the legs of Q2 feels like the most obvious thing to look at. Is Q2 working? What is the DC voltage (no signal) on the side of R15 connected to the emitter of Q2?

[sirlemonhead]

Hi Laurie,

I'm getting 8.3v on the side of R15 connected to Q2.

Then the inner leg of Q2 gives me 8.95 and the 3rd leg 8.3v

This is the same with both the pedal switched on and off.

Thanks!

[laurie]

Well... this is definitely not right

What is the DC voltage across C25 (no signal).

[sirlemonhead]

Across as in black probe on negative of cap and red on positive? I get 4.86v

[laurie]

Yep. That's all good.

Now. It gets tricky - diagnosing this from descriptions online is difficult.

From what you have given:
1) The base of Q2 should be close to 4.86V.
2) The base of Q2 is actually 8.3V

The only way that can happen is if Q2 has failed, or if there is something weird going on around it - e.g. a solder-dag shorting a couple of tracks together. Don't discount possible weirdness... it happens more often than you imagine. But. If it was on my bench I'd carefully check for solder dags or broken circuit-board traces, then swap Q2 as the first step.

Q2 can be replaced with almost any modern NPN small signal transistor (it truly doesn't matter). I'd use a 2N5088 but only because that's what I have laying around. NOTE - the pins of the existing transistor and the pins of a new transistor will almost certainly not be the same. You will need to ensure the correct pin goes into the correct hole.

If you have an old dead appliance laying around you could always cannibalize a transistor from it. You can often find them inside compact fluorescent light bulbs, old transistor radios, etc.

[sirlemonhead]

Hi,

I replaced Q2 with this, https://ie.farnell.com/multicomp/2n3904 ... ?st=2n3904

turning it backwards and bending two of the legs to swap position so it matched this pinout https://www.datasheets360.com/pdf/8018894242523559341

I'm getting nothing through the pedal now. With the audio probe and pedal off, I get my input signal intact on the emitter side of Q2. Nothing on the base or collector.
Turning on the pedal, with the knobs all at 12 o'clock, I get the input signal mixed with the working effect (I hear an octave at least!).

If i measure voltages on Q2 -

Pedal Off - Emitter 4.4v.
Collector 8.9v
Base 0v

Pedal On:
Same voltages as off.

I think I put the transistor in correctly...

[sirlemonhead]

OK! so looking at the transistor I actually removed from the OC-2, it appears to be this part number - http://www.farnell.com/datasheets/661741.pdf

which looks like exactly the same leg config as my replacement parts! so.. bending the leads as I did last night won't work. Doh I'll redo that and get back to you..

[Pepe]

Those transistors can be really tricky! One of my analogue polyphonic synths sounded crazy after I replaced a transistor incorrectly. Good luck!

[laurie]

Hi. Am out of town with flakey Internet. .. will get back to helping next week