Magnetron Delay Problem

[Jovi Bon Kenobi]

Here's the problem. When I test the board out of the enclosure I get two possible outcomes:
a.) I only get one repeat turning the full range of feedback knob with self oscillation at the very end, the dry signal is quieter than the repeats, there is faint RF, and my overall guitar signal is quieter when the pedal is engaged than when it is bypassed.
Or b.) It works perfectly with none of the above issues so I put it back into the enclosure and the problems come back.

Upon wiggling connections and stuff I can't seem to force replicate a vs b. I thought I may have solved it when I noticed that the OPA2134 was slightly lifted from it's socket and it cleared when I pushed it back in. However, once back in the enclosure the problems came back.

Link to schematic here: Magnetron Delay

I built the PCB with a couple mods added:
1. Momentary feedback footswitch
2. Internal gain trimpot moved to external control

I drew this up to make it easier to understand:


Photos incoming as soon as my housemate is out of the shared music room/my work area. ?

 

[moonlightpedalbuilds]

Try checking the continuity of the IC legs to the underside of the pcb where the sockets are soldered. Those kind of sockets are less reliable than the rounded hole type. Any bend on the pcb(when being mounted) may affect the connection of the IC to socket.

 

[Jovi Bon Kenobi]

Try checking the continuity of the IC legs to the underside of the pcb where the sockets are soldered. Those kind of sockets are less reliable than the rounded hole type. Any bend on the pcb(when being mounted) may affect the connection of the IC to socket.

will do, first thing tomorrow. appreciate it!

 

[Jovi Bon Kenobi]

I only know the basics but being able to read schematics, even just a bit, is a huge help when troubleshooting.

Take a look here if you get a chance https://learn.sparkfun.com/tutorials/how-to-read-a-schematic/all

Thanks, I am liking it so far! No time like the present. May as well try to get something productive out of 2020.

 

[zgrav]

and you can build the other board and set this aside to come back to later if you prefer. odds are pretty good it is one thing to find and fix and then it will be OK.

 

[Mcknib]

Late to the party as usual!

With the clues you've got e.g. 1 repeat, lower volume I'd look at what controls those two things, it may well be the same thing

If you look at the schematic the feedback path runs out from the PT2399 pin 14 and back in via pin 16 and obviously goes through the components in that path so it is very much like join the dots

The 1 repeat suggests too high a resistance on the repeats wiper could be a cold joint, the trimmer, the pot or a resistance off it.

The feedback volume is attenuated by the two diodes D1 and 2, I think it's a clamping arrangement whereby once the clipping threshold is reached it'll dump some signal to ground stopping it from getting too loud and clipping/distorting your guitar signal it should still get louder / distort a bit as you turn the pot and decrease the resistance between 2 and 3, like feedback generally does

For fun seeing as they're socketed remove those and see how loud it is or use schottky diodes with a lower vf, I've never built a PT2399 delay with those diodes it'd be interesting to know If it gets a bit loud, mental and chaotic

They may well be causing the problem idk

Zooming into your pic I don't really notice anything other than left side of R8 doesn't look as shiny as the other joints.

Did you test it before the mods it wouldn't be that difficult to remove the feedback mod and see if it's ok that would at least narrow it down a bit ie the problems in circuit or with the mod

 

[Jovi Bon Kenobi]

Late to the party as usual!

With the clues you've got e.g. 1 repeat, lower volume I'd look at what controls those two things, it may well be the same thing

If you look at the schematic the feedback path runs out from the PT2399 pin 14 and back in via pin 16 and obviously goes through the components in that path so it is very much like join the dots

The 1 repeat suggests too high a resistance on the repeats wiper could be a cold joint, the trimmer, the pot or a resistance off it.

The feedback volume is attenuated by the two diodes D1 and 2, I think it's a clamping arrangement whereby once the clipping threshold is reached it'll dump some signal to ground stopping it from getting too loud and clipping/distorting your guitar signal it should still get louder / distort a bit as you turn the pot and decrease the resistance between 2 and 3, like feedback generally does

For fun seeing as they're socketed remove those and see how loud it is or use schottky diodes with a lower vf, I've never built a PT2399 delay with those diodes it'd be interesting to know If it gets a bit loud, mental and chaotic

They may well be causing the problem idk

Zooming into your pic I don't really notice anything other than left side of R8 doesn't look as shiny as the other joints.

Did you test it before the mods it wouldn't be that difficult to remove the feedback mod and see if it's ok that would at least narrow it down a bit ie the problems in circuit or with the mod

This is great. I will go through each and report back. I really appreciate you taking the time to help.

 

[Mcknib]

This is great. I will go through each and report back. I really appreciate you taking the time to help.

I'd probably start with removing the mod, intermittent faults are a mo'fo could be just about anything

But if you remove the mod and it's all singing all dancing you'll know the mods the bad boy that's misbehaving

 

[Jovi Bon Kenobi]

Try checking the continuity of the IC legs to the underside of the pcb where the sockets are soldered. Those kind of sockets are less reliable than the rounded hole type. Any bend on the pcb(when being mounted) may affect the connection of the IC to socket.

All points tested have continuity.

I'd probably start with removing the mod, intermittent faults are a mo'fo could be just about anything

But if you remove the mod and it's all singing all dancing you'll know the mods the bad boy that's misbehaving

Removed. Problem still persists

The feedback volume is attenuated by the two diodes D1 and 2, I think it's a clamping arrangement whereby once the clipping threshold is reached it'll dump some signal to ground stopping it from getting too loud and clipping/distorting your guitar signal it should still get louder / distort a bit as you turn the pot and decrease the resistance between 2 and 3, like feedback generally does

For fun seeing as they're socketed remove those and see how loud it is or use schottky diodes with a lower vf, I've never built a PT2399 delay with those diodes it'd be interesting to know If it gets a bit loud, mental and chaotic

They may well be causing the problem idk

Removed em and the only noticeable difference was that oscillation started sooner. The only Schottkey's I have on hand are 5817's. would those be suitable? I have lots of gen purpose, zeners, germies, etc.

Zooming into your pic I don't really notice anything other than left side of R8 doesn't look as shiny as the other joints

R8 is shiny and clean top to bottom. In fact, I usually have at least one or two resistor pads that don't go all the way through and make a perfect shiny wee peak on the business side, but in this case every resistor looks picture perfect.

After doing the above all problems still persist in and out of the enclosure. Aside from the previously stated problems the main thing that I'm noticing is the indicator if it is intermittently working or not is that there is RF whenever I touch the modded external gain pot. If I touch it and get no RF, it temporarily works. If I touch it and cumbia comes through the amp, it's a no-go.

 

[Mcknib]

So its not the mod it's in the circuit

With dry signal low

I'd check resistance between pins 1 and 2 with no power to the circuit and the op amp removed from the socket you should expect R3 value + whatever the pots at and check pin 4 ground

What's your op amp voltages

Have you tried another dual op amp

 

[Jovi Bon Kenobi]

I'd check resistance between pins 1 and 2 with no power to the circuit and the op amp removed from the socket you should expect R3 value + whatever the pots at and check pin 4 ground

Resistance between 1 & 2 looks ok: around 47k to 142k.
Pin 4 is grounded

What's your op amp voltages

Forgive my ignorance but I need a refresher on this. Been a while. Black lead anchored to ground point, then red lead to pins 1 through 8... but what do I set my meter to again?

Have you tried another dual op amp

Just did. Same prob.

 

[zgrav]

I think you would set your dial on Auto V, which should measure DC voltage.

 

[Jovi Bon Kenobi]

OK thanks.
1: 4.2v
2: 4.2v
3: 4.2v
4: ----
5: 4.2v
6: 4.2v
7: 4.2v
8: 8.4v

 

[Danbieranowski]

Do you have a little DIY audio probe handy?

 

[Jovi Bon Kenobi]

Do you have a little DIY audio probe handy?

Yes, but no looper pedal for signal generation. Also, I'd need to know exactly where to follow on the schematic and that would be difficult as I don't know what I'm doing. Why did I build an audio probe then? lol

 

[Danbieranowski]

I could probably help by highlighting the audio path for you although I’m not great at reading a schem either. I’ll try to sort that out later today.

 

[Mcknib]

Your op amp voltages look good

You could probe it see if it gives any clues ie where it's quite, I'm not sure with the nature of the problem if it'll be any good

I'd probe your IN pad see how loud it is then IC1.1 in's and out and so on

Here's a quick audio path not sure what kind of audio goes through R4 and C2 perhaps you can verify how accurate it is? I've not done a lot of tracing audio paths recently! But I think it's fairly accurate, good enough to blast through it anyway and that's all you need innit?

Red part obviously LFO so no audio

 

[Jovi Bon Kenobi]

Your op amp voltages look good

You could probe it see if it gives any clues ie where it's quite, I'm not sure with the nature of the problem if it'll be any good

I'd probe your IN pad see how loud it is then IC1.1 in's and out and so on

Here's a quick audio path not sure what kind of audio goes through R4 and C2 perhaps you can verify how accurate it is? I've not done a lot of tracing audio paths recently! But I think it's fairly accurate, good enough to blast through it anyway and that's all you need innit?

Red part obviously LFO so no audio

View attachment 7047

You're a gd champ!

 

[Chas Grant]

Mcknib's drawing will get you through the audio path. The audio signal going through C2 and R4 should be the same as the clean signal. C2 will block the DC from the output IC1.1 to the inputs the PT2399, and R4 is a load resistor doing the same thing as R6, both 10K. Following the green path should be a lot of fun, that's where all the delay is created!

 

[Mcknib]

Mcknib's drawing will get you through the audio path. The audio signal going through C2 and R4 should be the same as the clean signal. C2 will block the DC from the output IC1.1 to the inputs the PT2399, and R4 is a load resistor doing the same thing as R6, both 10K. Following the green path should be a lot of fun, that's where all the delay is created!

Thanks Chas it's always threw me that bit on a PT2399 delay I was thinking clean with it being way before the level / mix pot, but thanks to your explanation I now know that wee bit more

The thing that always throws me is what direction it's flows in! Can never suss it!

Cheers

 

[Jovi Bon Kenobi]

I'm taking a break from this for a bit. I will definitely come back to trace that schematic with my probe but I need to walk away for a spell. Haha. I just finished populating a second board. This time I ran a standard 3PDT and just ran the external gain pot mod. For the momentary oscillation pot mod I just wired one up and put it off to the side ready to test when needed. I could just solder it in after knowing the second one works. So, I fire it up out of the enclosure and it's perfect! Then I add the momentary oscillation mod by jumpering it in and it still works! Then I hardwire the mod in and box it up...and it has the same problems as the first one! ?
I'm starting to wonder if the initial drawing I made is the culprit.