Boss OS-2 no sound when pedal on

[pihalla]

Hi everybody, newbie here with a problem...

My friend gave me a Boss OS-2 pedal which had a typical issue, I guess. No sound when the pedal is on, LED illuminates etc. When it's off, sound can be heard.

So I googled all over the internet and started troubleshooting. I found the schematics from https://www.experimentalistsanonymous.com/diy/Schematics/Distortion%20Boost%20and%20Overdrive/BOSS%20OS-2.gif

Once I opened the pedal, found that D1 was "exploded" so I replaced it with 1N4007 and also C3 with a new similar cap. Well, no luck, pedal did not work. Next I replaced IC1 and IC2, did not help either. Then it was time to do some debugging with a voltmeter and an audio probe... I thought the problem had something to do with Q1, Q6 and Q7 (2SK118Y). So I replaced them with 2N5485 and the pedal started working, but did not sound as it should sound. Only a really really weak distortion, Pots didn't affect the sound... So basically as a total noob, it seems I was trying to fix something that I didn't even understand what I was trying to fix. Naturally next thing to do was to replace some other transistors... . I replaced Q3, Q4 and Q5 just for fun. Unexpectedly it didn't help...

Well, did I check the pinout on the replacement transistors (Q1, Q6, Q7)? Turns out, it was different than the originals so I re-soldered them to match. Unfortunately, back to the square one... No sound from the pedal when it's on. I even replaced Q7 with the original 2SK118Y. Now I'm completely lost... Please help me!

I gathered some info below as the debugging page suggests:

Everything was measured while the pedal was switched on.

5.Any parts substitutions?
D1 1N4007
C3 100uf/16V
Q1 2N5485 (2SK118Y)
Q2 2SC2240GR (2SC3378GR)
Q3 2SC2240GR (2SC3378GR)
Q4 2SK170 (2SK184GR)
Q5 2SC2240GR (2SC3378GR)
Q6 2N5485 (2SK118Y)
Q7 back to original 2SK118Y

7. What is the out of circuit battery voltage = 8.91
Voltage at the circuit board end of the red battery lead = 8.63
Voltage at the circuit board end of the black battery lead = 0.00

Q1
D = 3.91
S = 3.91
G = 4.28

Q2
B = 2.99
C = 8.57
E = 2.61

Q3
B = 3.55
C = 8.56
E = 3.08

Q4
D = 8.56
G = 3.89
S = 4.48

Q5
E = 3.87
C = 8.56
B = 4.46

Q6
D = 4.24
S = 4.24
G = 3.89

Q7
S = 3.91
G = 0.02
D = 3.91

Q8
E = 0.00
C = 5.76
B = 0.01

Q9
E = 0.00
C = 0.03
B = 0.62

IC1
P1 4.25
P2 4.29
P3 4.29
P4 0.00
P5 4.25
P6 4.25
P7 4.23
P8 8.63

IC2
P1 8.04
P2 6.20
P3 4.20
P4 0.00
P5 4.25
P6 7.60
P7 7.40
P8 8.65

D1
A = 0.00
K = 8.64

D2
A = 3.81
K = 3.61

D3
A = 0.00
K = 0.00

D4
A = 0.00
K = 0.00

D5
A = 0.07
K = 5.63

D6
A = 4.24
K = 5.20

D7
A = 7.50
K = 7.30

D8
A = 7.30
K = 7.06

D9
A = 7.05
K = 7.30

D10
A = 0.02
K = 0.07

Pretty long post.. D3 and D4 looks weird, huh?
I appreciate your help!

[antonis]

D3 and D4 looks weird, huh?

Weird..?? Dunno..
Open..?? For sure..!!

D3/D4 pair consist the hard clipping part of the pedal..
With these diodes open, only the soft clipping part works..
(the one with diodes inside NFB loop..)

P.S.
Did you measure them across their legs or across respective Anode/Cathode & GND..??
(for the later, it could be a burned out trace issue..)

[pihalla]

P.S.
Did you measure them across their legs or across respective Anode/Cathode & GND..??
(for the later, it could be a burned out trace issue..)

Thank you for your answer!
Measured them across Anode/Cathode and GND. Now measured also across their legs, 0.00.

[DIY Bass]

I may be wrong but I would not expect to see any voltage across D3 and D4 unless there was a signal going through the pedal.  The coupling capacitor just before should be removing any DC offset, so you would only see anything there if there was an AC audio signal.

[willienillie]

No DC on D3, D4.  That seems correct.  IC2 has problems.

[antonis]

I would not expect to see any voltage across D3 and D4 unless there was a signal going through the pedal.

Which "signal" should come out of DMM on Diode test mode.. Which mode should be the only one for testing diodes good working state.. Which state should be revealed by a 500-700mV measurement for a small signal, back-to-back, healthy Si diode pair, despite test leads polarity..

[pihalla]

No DC on D3, D4.  That seems correct.  IC2 has problems.

I tried to replace IC2 with NJM4558D, RC4558P and NJM5532D but still the pedal is mute..

[GibsonGM]

Pin 1 should NOT have 8V (supply voltage) on it.  Find out why that is (solder bridge, short....) and correct....

[pihalla]

Pin 1 should NOT have 8V (supply voltage) on it.  Find out why that is (solder bridge, short....) and correct....

I measured it again, 6.58 on Pin 1. Most likely 8.04 was a user error...

[willienillie]

Pins 2, 6 and 7 are too high too.

[pihalla]

Pins 2, 6 and 7 are too high too.

Ok, I'll try to find out why.

[GibsonGM]

Pin 1 2 6 7 should be more like 4.5V. Even 6.58 is too high.   Something is wrong in the area around the IC or before it, or it is damaged....

[antonis]

A shorted C1 results into DC gain higher than unity..

[pihalla]

Thank you for your replies.

Prior replacing the C1 I measured IC2 P1, 2, 6 and 7 again. P1 was approx. 6.50, the others 3.9-4.24.
Once C1 was replaced, P1 started from 5.7ish and while measuring, it slowly climbed over 7.00...

[antonis]

Someone is a bit confused..

According to diagram posted, C1 has nothing to do with IC2..

[pihalla]

Someone is a bit confused..

According to diagram posted, C1 has nothing to do with IC2..

I have no idea what I'm doing.

[antonis]

I have no idea what I'm doing.

You definately are on the right point for debugging..!!! 

[pihalla]

One year later!
Finally had the time to tackle this pedal and now it works!
I've had acquired an oscilloscope and a transistor tester, so troubleshooting started again.
Turned out that a couple of transistors were not working/installed properly... oops.

I tested all the old transistors and they were functional, so I installed them back.
As some of you said earlier, the original IC2 was bad, so I replaced that. Then I was able to pinpoint the issue to IC1, it needed to be replaced as well.. And voilà, the pedal works.

There is one issue though...
When turning the drive knob beyond midpoint, it starts to squeal. Tone/frequency of the squeal changes as I turn the knob.

As you've already noticed, my knowledge of these things is less than minimal. So, would you be able to guide me how to get rid of the squeeeaaallll... thanks!