Boss CH-1 Help Troubleshooting - Very low wet signal!!

[aliceinchains88]

Hi! I'm new to the forum, I got to this place when I was looking for a solution to my problem, and I found a topic with a very similar issue, but it was unresolved. I see there is a lot of experienced people here!

The thing is that I bought a broken analog CH-1 to repair it myself, the problem was (at first sight) that there is no effect when the pedal is triggered on, only dry signal all the time.

After a little investigation, I found out that there is actually a wet signal at the output, but it is so low in volume that it cannot be noticed in the mix. I realized this by connecting both outputs of the pedal to two different amplifiers. When connected in stereo, out B is OK, always dry signal, and out A has no perceptible output.....except when I turn the master of the amp all the way up, the wet signal appears!! but it is so weak that it can be barely heard.
This wet signal is affected by the action of the pots, I can change the depth and EQ by turning the knobs.

I have checked the most common issues:

1- I measured the Q2 (bypass fet) drain-source impedance to see if the bypass is working, seems to be ok (about 360ohm when on and more than 1Mohm when off)
2- I replaced the MN3007 IC with another I had from an older project, and problem is still the same. Considering this plus the fact that I do get some wet signal, I think the LFO and the delay parts of the circuit are working fine.
3- I tweaked the bias trimpot (R16 I believe) and it doesn't produce any substantial variation (it does distort the wet signal when it's all the way up).

I have also read something about the bias resistors for the LFO (R42 and R47) to be swapped and with wrong values, but when I swapped them, I killed the weak wet signal I had, so that's not the problem.

Any ideas? I would REALLY appreciate your help/comments/suggestions, maybe this will help others with the same issue.

This is the related post I found with almost the same problem, but it seems to be abandoned:
http://music-electronics-forum.com/t14908/


Thank you in advance!

James

[Mark Hammer]

Your instincts to check out the switching FET, and swapping the BBD, are good. Knowing it behaves appropriately helps in narrowing down the issues.

The CH-1 does not have a bias trimpot for the BBD, so we know it can't be that.

What I wonder is if there is something amiss with the output jacks. Specifically, it appears that the B jack impacts on what the A jack gets.

Finally, the EQ and Effect Level pots need to provide electrical continuity or else no delay signal will pass. Just for the hell of it, bridge the two wires leading to the EQ pot and the other two wires leading to the Level pot. If your wet signal is restored, then there is likely something amiss with one of those pots, and hopefully something that is rectified with a simple cleaning. If not, then your problem lies elsewhere.

[cjlectronics]

Unless I read your post so fast I overlooked it, you mentioned nothing about the DC voltage. Do you get 9v at the ICs and 4.5v at Q7?

[aliceinchains88]

Thank you guys for your replies!! Sorry I haven't answered earlier, I've been with a lot of work these days...

Mark, just to be sure, I dismantled the whole thing out of the enclosure, and checked the input and output jacks, they seem to be ok. There is actually something weird with Q1, I measured the D-S impedance of Q1 and Q2 again and this is what I got:

Both outputs connected:

Effect ON
Q1: slowly rises from 100 ohm to more than 1Mohm
Q2: 360ohm

Effect OFF
Q1: 323ohm
Q2: 1M

Only out A conneted:

Effect ON
Q1: 350ohm
Q2: 350ohm

Effect OFF
Q1: 350ohm
Q2: 1Mohm

It sounds crazy, but the D-S impedance of Q1 when the pedal is on, gradually increases its value until it reaches >1M...I did measure this several times and it behaves the same way every time. I don't know if there's something wrong with my multimeter or the jfet is actually not working well....
Since Q1 seems to only let the wet signal through output A when out B is plugged, and I do have wet signal on out A, I don't think this could be the main cause of the problem.

Also, I shorted the leads of all the pots without success, nothing changes. The only interesting thing is that the EQ, RATE and DEPTH pots affect the delayed signal at out A (a vibrato-like signal), but the E.LEVEL pot does NOTHING to this signal when I turn it. I measured its impedance variation and it is ok. I believe this should at least vary the volume of the wet signal.

cjlectronics, you're right, I didn't write down the voltages I took, so here they go:

Opamps:
IC1
1- 4.25
2- 4.25
3- 4.18
4- 0
5- 2.11
6- 4.27
7- 4.27
8- 8.49

IC4
1- varies between 3.3 and 4.2
2- 3.8
3- 3.8
4- 0
5- varies between 3.3 and 4.2
6- 3.4
7- varies between 3.0 and 4.8
8- 8.37

IC5
1- 4.23
2- 4.23
3- 2.07
4- 0
5- 4.17
6- 4.19
7- 4.19
8- 8.42

BBD and Driver:
IC2
1- 7.3
2- 3.84
3- 3.95
4- 0.49
5- 0
6- 3.76
7- 4.6
8- 4.6

IC3
1- 7.28
2- 3.75
3- 0
4- 3.8
5- 0.4
6- 6
7- 2
8- 0.48

Again, I have varying voltages at some pins of IC4. I don't know if this is normal or I'm just going crazy. Anyway they seem pretty normal to me. I used a 9V battery measuring 8.67V connected to the battery clip of the pedal and with both outputs connected.

In conclusion, I'm not an expert interpreting electronic circuits but I think something must be "blocking" the way of the wet signal to fully reaching output A, and we get a weak but still operational delayed signal.

I hope this helps you help me interpret the results and finding out where the issue lies.

Thank you so much for your help!!


James

[aliceinchains88]

Hi people, good news, it works!!!!

I have improvised an audio probe and tested diferent spots to see if I have a good wet signal. I tested the outs of the EQ and LEVEL pots (pcb pads 15 and 16), and I had a nice full-volume wet signal!! Also checked the emitter of Q10 with the same great result!!
So I kept checking, IC5 was OK at pins 1, 2 and 3, pots were OK, then I checked C40....I had no signal AFTER C40, so just changed it for a brand new 1u/50v capacitor, plugged in and that was it, full chorus effect on output A!!

Then I kept testing with the probe, and I see that R50 (connected after C40 and into Q2) with the effect ON I have no signal after it (actually it is the same weak signal heard only with vol all the way up), and I do have full wet signal after R50 when the effect is OFF.
That is something I don't fully understand, since I should always have wet signal after R50 and it should be "muted" by the large impedance of Q2 when the pedal is OFF.
Also checked the voltage at pin 3 of IC5 and it's still +-2V, which seemed to be too low voltage. Beside this, the effect works fine...for now.

Considering all this, is there anything else I should check, just in case?

Anyway, I'm very happy because I finally figured it out! Thank you people so much for your support!!


Kind regards from Buenos Aires, Argentina!

James