SOLVED Modulation on Magnetron not working

[Falterthethomist]

Hi everyone,

this (Magnetron) is roughly my 30th pedal build, but now I am running out of options or solutions to my problem.

The delay on it works fine, however, the modulation isn't doing anything - neither the LED nor any kind of recognizable change in the sound when I engage the knobs. I have checked everything for continuity (fine). However, I have a feeling that my voltages on the LM358 are off:

Pin 1 0v
Pin 2 (goes up to 1.3 V and Led blinks, then immediately down to 0V)
PIN 3 (goes up to 0.8V and Led blinks, then down to 0V)
Pin 4 0V
Pin 5 0.1 V
Pin 6 0V
pin 7 0V
Pin 8 8.7V

If I follow the signal with an audio probe, the signal that leaves the pt2399 at pin 6 has a lower level than the other pins - it stays low until pin 6/7 on the LM358, and there is no signal on pin 5. Not sure what this means - this is only my second delay pedal, and the other one didn't need any troubleshooting.

I hope to take some pic soon from the board, but maybe someone ran into a similar issue and has a solution? Could it be a bad Opamp?

Thanks, Markus.

 

[chris.knudson]

From the looks of the schematic, Pin 1 of the 358 feeds pin 5. Pin 1 is the output of the first Op Amp. If you aren't getting any voltage from pin 1, your problem is probably somewhere in that part of the circuit.

 

[Falterthethomist]

Hi thanks for your response. I was thinking the same, but I can't really find the problem.

What is kind of awkward is that I have vcc on both sides of R20 (app. 8.7 V) but no voltage on r21 or pin 3 of IC 1. However, when I test it without power, I do have continuity between these points. Could the issue lie there? Maybe a ground short somewhere (I did look for it but couldn't find any)?

 

[chris.knudson]

Hi thanks for your response. I was thinking the same, but I can't really find the problem.

What is kind of awkward is that I have vcc on both sides of R20 (app. 8.7 V) but no voltage on r21 or pin 3 of IC 1. However, when I test it without power, I do have continuity between these points. Could the issue lie there? Maybe a ground short somewhere (I did look for it but couldn't find any)?

No, probably not, but that is interesting. R20 and R21 work in tandem to set the voltage on pin 3. If when you say "without power" do you mean in bypass? A lot of foot switch wiring will basically short the pedal to ground when it's bypassed so that current doesn't flow through the circuit, so it's possible that you have continuity through ground.

Can you post photos? It might be helpful in diagnosing the problem.

 

[Robert]

What is kind of awkward is that I have vcc on both sides of R20 (app. 8.7 V) but no voltage on r21 or pin 3 of IC 1.

The right end of R20 connects to the right end of R21 and pin 3 of IC2.

You should measure the same voltage at all of these points, and I'd expect something closer to 5.8V.

 

[Falterthethomist]

No, probably not, but that is interesting. R20 and R21 work in tandem to set the voltage on pin 3. If when you say "without power" do you mean in bypass? A lot of foot switch wiring will basically short the pedal to ground when it's bypassed so that current doesn't flow through the circuit, so it's possible that you have continuity through ground.

Can you post photos? It might be helpful in diagnosing the problem.

"Without power" was meant in the sense that I unplugged the power supply.

I'll post pics later today - work is just so crazy at the moment, but weekend is on the horizon

 

[Falterthethomist]

The right end of R20 connects to the right end of R21 and pin 3 of IC2.

You should measure the same voltage at all of these points, and I'd expect something closer to 5.8V.

That's what I thought to, r20 and r21 are a voltage divider, right? That is why I assumed that the connection from r20 to r21 is broken, but when I use the multimeter it shows continuity. i could try

Could it be the opamp? I ordered new ones and will try them out tonight.

 

[chris.knudson]

"Without power" was meant in the sense that I unplugged the power supply.

I'll post pics later today - work is just so crazy at the moment, but weekend is on the horizon

Ok, now I'm confused too. The only way unplugging power should change where you have continuity is if there is a switch in your power plug, but I need to see photos to know what's going on.

 

[Falterthethomist]

Hey guys, I found the culprit. Chris was right all along. the problem was between pin 1 and 5 of the lm358. The ground connection of c19 had a cold solder joint. It is working properly now. Thanks for your help!

 

[chris.knudson]

Hey guys, I found the culprit. Chris was right all along. the problem was between pin 1 and 5 of the lm358. The ground connection of c19 had a cold solder joint. It is working properly now. Thanks for your help!

Glad you got it working!