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. ?

 

[Chas Grant]

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

Not a problem at all Mcknib! The PT2399 is a beast on its own, and the delay path can be bit confusing since the in and outs share the same path to give you the effect. Electrosmash has a good article on it here https://www.electrosmash.com/pt2399-analysis . All the articles there are good and explain a lot.

 

[Mcknib]

Right I think you may have sussed it

As I said in my pm initially I looked at the wiring but am hopeless pressing an imaginary switch in my head and tracing it through the mod

Now that you mention it the way it's wired you're not bypassing the pot so you're adding resistance with the trimmer giving you less repeats so like the wf mod you want to use a 5 or 2K trimmer or a 1K resistor and take it from the left side of R9 to feedback lug 3

Bypassing the pot and pressing the footswitch introducing 1K resistance or whatever you set the trimmer to infinite or almost infinite repeats regardless of the feedback pot setting

Whereas before you'd need to max the feedback pot to have no resistance between 2 and 3 and just 1K from R9

Should work!

Your grounding issue could have been because the pot wasn't bypassed, the more you turn it down the more signal it dumps to ground via lug 1, At the same time increasing resistance between 2 and 3 and decreasing repeats and volume

Can't draw with my fingernail on my phone but this

 

[Jovi Bon Kenobi]

On it! Yeah, the WF mod seems just the ticket. After my tea though

 

[Jovi Bon Kenobi]

you want to use a 5 or 2K trimmer or a 1K resistor and take it from the left side of R9 to feedback lug 3

So, like this? Specifically, how the pink and green wires are connected to the trimmer.

 

[Mcknib]

That's what I'd try as long as that's the side of R9 that has continuity with D2 cathode

When I say left side of R9 I meant in the schematic

Let's see if the theory's bs or not

 

[Jovi Bon Kenobi]

When I say left side of R9 I meant in the schematic

Phew glad I asked. It's the right pad. ? Ok, going in.

 

[Jovi Bon Kenobi]

Ok, so I didn't try it inside the enclosure but I'm pretty certain it is the same problem as before because I get the same squealing radio interference upon touching the gain pot.

Good news is that rearranged momentary mod works though.

 

[Jovi Bon Kenobi]

I managed to dial the feedback knob to get more than one repeat, but it's like a hairs width turn away from self oscillation.

 

[Mcknib]

Have you already tried another PT2399 just to rule that out

I'd check the feedback pot alters resistance as you turn it that's strange it going from a few repeats to infinate that quickly

 

[Jovi Bon Kenobi]

Have you already tried another PT2399 just to rule that out

I've tried 2 chips. I have a few more. I'll give it a whirl.

I'd check the feedback pot alters resistance as you turn it that's strange it going from a few repeats to infinate that quickly

Ok so, upon further testing I was wrong. The feedback knob seems to work normally as far as repeats are concerned. Now I'm not sure if the one repeat thing was happening before! I swear it was though. Actually, the volume discrepancies are not appearing if I dial the gain in right.

Even though it still makes RF turning to a specific range of the gain pot I'm going to box it up once more.

 

[Jovi Bon Kenobi]

It all comes back when in the enclosure. Both boards, both with the same problem.
I will probably just revert them back to unmodded so it's not a complete loss except for the predrilled enclosure. Pretty sure they will both work fine once I take off the external gain pot mod as that's the area that seems to be causing all the issues.

One last check then I think I might be done trying to sort this out. I really do appreciate all the help though.

 

[Mcknib]

It hopefully will dissapear when it's completely shielded

You're just having one of those faults with many possible causes, rf, could be a cap not filtering properly near the op amp or in the power supply, the power supply itself, something acting as an antenna etc, and just to add to your woes its intermittent

Try putting the gain pot back to a trimmer may even be the long pot wires causing the rf problem

 

[Jovi Bon Kenobi]

It hopefully will dissapear when it's completely shielded

You're just having one of those faults with many possible causes, rf, could be a cap not filtering properly near the op amp or in the power supply, the power supply itself, something acting as an antenna etc, and just to add to your woes its intermittent

Try putting the gain pot back to a trimmer may even be the long pot wires causing the rf problem

Took the gain pot mod out of both and soldered the 100k trimpot back onto the PCB. both work fine now. Even with the momentary mod. Well, that was a fun adventure. Sorry for all the troubles.

 

[Jovi Bon Kenobi]

Hey, at least we learned something! And once I finish drilling and labeling a new enclosure I can add it to the mod thread with a note to not try both mods. Haha.

 

[Mcknib]

Well I never

Sometimes it's staring you right in the face and you did mention it more than once! Going with your gut is quite often a winner

The was the last shot haha
Glad it's sorted

Its all learning, nearly 10 years down the line I've still got my 'L' plates on

Good to meet a fellow Ashton Lane frequenter small world innit?

 

[Danbieranowski]

I was going to say you’re drawing for the momentary switch was at least accurate Bc that’s what I used when I did mine. Did not do the gain pot piece though. Glad you got it figured!

 

[Jovi Bon Kenobi]

Good to meet a fellow Ashton Lane frequenter small world innit?

Yeah man. I miss living there, it was a great experience.

So if I ever have troubleshooting again, like if my IC chips are good, I can say "gony geez a chip mate" and you'll know what's up.

 

[Mcknib]

Yeah man. I miss living there, it was a great experience.

So if I ever have troubleshooting again, like if my IC chips are good, I can say "gony geez a chip mate" and you'll know what's up.

Aye big man nae bother jizt geez a shout

 

[zgrav]

maybe use shielded wires to and from the gain pot ......

 

[Jovi Bon Kenobi]

maybe use shielded wires to and from the gain pot ......

That's so simple it might work. Pulling all but three wires out of some shielded cable (or whatever), connecting em up, then running the shield to ground points at each end. I just left home base and am on the road. Won't be back for a week, but that sounds promising