SOLVED Magnetron Delay: Signal until R18

[malcolm]

Hi all,

3rd build, first two worked fine but this one is causing me to learn a lot.

I get bypass signal, but silence when effect engaged. I can see the Rate Indicator LED pulsing and the speed of that pulse changes with the speed pot.

So after a bit of inspection and forum reading, I built an audio probe. I followed the audio signal across the top of the schematic, and can hear the effected signal correctly until after C16. Going into r18 I can still hear it, but coming out it’s silence. I checked on the actual resistor leg coming in (to rule out my soldering) and I can hear it fine, but on the resistor leg coming out, silence.



Am I correct in assuming that this means the problem is here? My first time with an audio probe so I’m learning as I go.

Anyway, I desoldered R18 and checked with a LCR meter, but it says its fine and the correct resistance (1k). I swapped it anyway but no change.

Any suggestions on what to do next?

Cheers
Malcolm

FYI: Build doc is here https://docs.pedalpcb.com/project/Magnetron-PedalPCB.pdf

 

[Cybercow]

If you have signal on the left (positive) side of C16, but no signal on the right (negative) side of C16, the culprit is likely C16.

Got any good clear photos of the both sides of the PCB?

 

[Coda]

If you have signal on the left (positive) side of C16, but no signal on the right (negative) side of C16, the culprit is likely C16.

Got any good clear photos of the both sides of the PCB?

He does have signal on the negative side.OP: is your value for R19 correct?

 

[malcolm]

If you have signal on the left (positive) side of C16, but no signal on the right (negative) side of C16, the culprit is likely C16.

Got any good clear photos of the both sides of the PCB?

Sorry about that, re-reading my post I could have been clearer. As Coda mentioned, Signal is good until it enters R18.

 

[malcolm]

He does have signal on the negative side.OP: is your value for R19 correct?

Thanks Coda,

It is, but to be safe, I pulled it out and tested it and it seem ok.

I'm at work now, but wondering should I be checking voltage at these points as well as audio signal?

Cheers
Malcolm

 

[Coda]

Thanks Coda,

It is, but to be safe, I pulled it out and tested it and it seem ok.

I'm at work now, but wondering should I be checking voltage at these points as well as audio signal?

Cheers
Malcolm

When you were using the audio probe, what’s what you heard (before the signal died) effected? Was there delay?…

Edit: I learned how to read real quick and reread the OP. Disregard my question…

 

[jimilee]

The problem would then be after r18. Let’s have a look at that wiring and see what’s grounding out. Most likely, something simple you’ve overlooked.

 

[Cybercow]

Sorry about that, re-reading my post I could have been clearer. As Coda mentioned, Signal is good until it enters R18.

So there is signal between C16 & R18? If that's the case, then the values of R18 and/or R19 would be in question.

And as the risk of redundancy, got any good clear photos of the both sides of the built out PCB?

 

[malcolm]

Sorry all, work and life kept me away from this until last night. I'll try and get some good photos today, but I did some poking around with the multimeter after your comments about grounding and I found something that (maybe) seems odd.

I had removed R19 to check it's value, so while it was out I was surprised that I still had continuity between it's through holes. I can go from the input hole for R19 straight to ground, with no resistor in place. I can't see any obvious bridges or shorts on these holes, but is it feasible that it's somewhere else?

I'll try and get some photos today.

Cheers
Malcolm

 

[KR Sound]

That makes it sound like there’s a short to ground at or around the output jack. Have any pics?

 

[malcolm]

That makes it sound like there’s a short to ground at or around the output jack. Have any pics?

Aha! That was the final tip I needed.

The out wire running from the board to the switch daughterboard, where it joined the duaghterboard, had one single strand splayed out and shorting. One strand, bloody thing.

I've learnt more chasing down this one fault than on the prior two pedals that worked first go.

Thank you all for your help. I NEVER would have found this without your tips and suggestions.

Cheers
Malcolm