Help needed troubleshooting BOSS CH-1 Super Chorus

[danymal_X]

I just recently acquired a non-working BOSS CH-1 Super Chorus for a steal and have begun troubleshooting it.
I read a few threads on this and similar pedals (IE: CE-2) with similar problems, but unfortunately the fixes therein have helped but not solved the problem.

Problem:
Bypass mode works fine, but when pedal is activated there is no signal coming from the effect circuit, just the 'stereo' clean signal... IE: when using a single output (output A only) in mono mode, only the clean signal comes through output A, and when using dual outputs, the clean signal comes through output B, and A is silent.

Steps:
I measured voltages at all of the Q's and IC's pins, and resistance between R50 which corresponds to R22 in the CE-2 schematic and pin 6 of IC5, which corresponds to pin 2 of IC1 as indicated by Mark H in a post from last year:

http://www.diystompboxes.com/smfforum/index.php?topic=37792.0

Value = 382Ω so it should be ok, according to what Mark said...

Mark's post also references a trimpot, which my board does have but the schematic I have does not show... it lists a fixed 1K resistor inplace of the 10K pot mine has. The VR in the CE-2 schem is in a different location than in CH-1 I'm not even sure the trimpot Mark mentions performs the same function... mine is between pin 3 of IC2 (MN3007) and ground, and unfortunately, I went and tweaked it while initially troubleshooting the pedal without marking it's initial position or measuring its value... duhh... so now I have no idea what the original value is.

Here are the voltage values:

2.Name of the circuit

   BOSS CH-1 Super CHorus

3.Source of the circuit (URL of schematic or project)
   
   Free Information Society Schematic Archive:
   http://www.freeinfosociety.com/electronics/schemview.php?id=2290

Battery voltage = 8.55v
Voltage at the circuit board end of the red battery lead = 8.46
Voltage at the circuit board end of the black battery lead = 0.00

Q1= 2SK30ATM-Y
S = 4.28
G = 0.00
D = 4.28

Q2= 2SK30ATM-Y
S= 4.28
G= 0.00
D= 4.29

Q3= 2SC2240
E = 3.55
C = 8.57
B = 3.67

Q4= 2SC1685R
C= 5.05
B= 7.79
E= 5.23

Q5= 2SC1685R
E = cycle 2.98-3.13
C = 7.44
B = 2.91-3.04

Q6= 2SC1685R
E= 0.00
C= 6.60
B= 0.41

Q7= 2SC1685R
E = 7.66
C = 8.82
B = 8.09

Q8= 2SC1685R
E= 0.00
C= 0.05
B= 0.64

Q9= 2SC1685R
E = 0.00
C = 5.69
B = 0.02

Q10= 2SC2240
E= 3.42
C= 8.78
B= 3.02

IC1= M5218L
P1= 4.26
P2= 4.27
P3= 4.20
P4= 0.00
P5= 2.14
P6= 4.33
P7= 4.33
P8= 8.54

IC2= MN3007
P1= 7.41
P2= 3.85
P3= 3.99
P4= 0.50
P5= 0.00
P6= 3.80
P7= 4.54
P8= 4.54

IC3= MN3101
P1= 7.43
P2= 3.82
P3= 0.00
P4= 3.87
P5= 0.37
P6= 6.40
P7= 2.39
P8= 0.50

IC4= M5223L
P1= cycle 1.67-6.06
P2= 3.784-3.91
P3= 3.72-3.89
P4= 0.00
P5= 1.54-6.02
P6= 3.36-3.54
P7= 0.34-7.18
P8= 8.48

IC5= M5218L
P1= 4.28
P2= 4.28
P3= 2.10
P4= 0.00
P5= 4.22
P6= 4.24
P7= 4.23
P8= 8.48

... and that's is where I'm at now.

Anyways, any help would be greatly appreciated...

Thanks

Dan

[Mark Hammer]

From the sounds of it, the first place to check is to verify that the FET switching system is doing its job properly.  Wouldn't be the first time a FET went kaplooey.  Q2 is the one to concentrate on.

If the switching system is working properly, then the voltage applied to the gate of Q2 to turn it on and off should change when the momentary switch is stepped on.  If you have a meter, set it to DC and measure the voltage from ground at the junction of D8 and Q2 (the stripe end of that diode).  It should change when the status LED changes.

If that is confirmed, then the next thing to check is that the FET is behaving as it should.  That FET acts as a high and low value resistor in series with the signal.  Set your meter to several megohm range and measure the resistance between the two other pins (drain and source are interchangeable here) as the status liht changes.  When the light is on, that FET should read a low resistance, probably something less than 2k at most.

Let's start there and see where that takes us.

[danymal_X]

OK, Thanks or the quick reply, Mark
I measured the voltage between Q2/D8 and ground, and got a reading of around 3v with the circuit on, and 0.01 with it off.
the resistance between S and D of Q2 is 386Ω with the circuit on and anywhere between .5MΩ and 15MΩ with it off.
You said the cathode of D8 at the junction between D8 and Q2, but the cathode of the diode is away from Q2, is this correct? I took the measurements from the cathode, not the actual junction.

Dan

[Mark Hammer]

Well, those measurements suggest that nothing is amiss with the switching FET.

You noted that the fixed-resistor bias circuit normally found on the CH-1 has been supplanted by a trimpot.  Does this appear to be  an add-on or original?  You will note that the stock divider (R15/16/17) shown in the linked-to schem functions like an 8k2/11k divider, producing a bias voltage well over V/2.  Possible that bias source is incorrect?  You should have a DC voltage of at least +5v on pin 3 with no signal applied.

[danymal_X]

5.03 volts at pin 3 with no signal... anfd the trimpot is not an add-on, it is definitely an original part of this circuit.

[Mark Hammer]

Well, that seems right.

Okay, now the ugly part....finding out if there is actual audio signal going as far as the BBD and beyond.

Plug your guitar in, power up, and measure the AC voltage at the emitter of Q3.  You should get at least something around 60-80mv when you strum the open striongs with the guitar volume pot up all the way.  If that checks out OK, now take your AC reading from the emitter of Q10,  If you don't get anything there, then there is a decent chance the BBD is fried.

[danymal_X]

OK... but unfortunately I'll have to do this a little later.
I really appreciate the help, Mark...

Thanks

Dan