Modded SD-1 Squealing When Not Plugged In

[JVO]

My issue is very similar to this one but I have some more color: https://www.diystompboxes.com/smfforum/index.php?topic=63946.msg504750;topicseen#msg504750

A little background, I did the mod from here: https://www.diystompboxes.com/smfforum/index.php?topic=16164.msg91839#msg91839

But instead of a germanium transistor, I did a switch for D6 that switches between germanium (1n34a) and silicone (1N4148) diode that has a 47pf capacitor bridging the diode to smooth it out. I also removed C6.

Being curious, I wanted to change out for a switch that went between germanium (1n34a) and a 3mm red LED. After I did that the pedal, when the distortion and tone were all the way up, will squeal like a banshee if the cable is unplugged or if I hook up to my poorly-grounded Telecaster.

I undid the new switch and yet the squeal persisted. It goes away if drive is dialed below 12 o'clock and the tone lessens it. In fact, if the effect is switched off, you can still hear it albeit less so. This happens when powered by battery or power supply. If I pull the guts out of the case, the squeal goes away and, in fact, it gets worse the more I push it into the box. It does not do this on other pedals and I've switched out the instrument cables. If I use a patch cable in the input, it goes away. If I plug in my better-grounded Stratocaster, it goes away.

Looking at the schematic (https://www.hobby-hour.com/electronics/s/sd1-super-overdrive.php) and using the audio probe, the squeal appears when I probe the emitter of Q5. I tried switching out that transistor but it persisted.

Any ideas here? The cables aren't any different length than a stock SD-1 and the only big addition is the switch but before I swapped it out, there was no squeal. I've looked for bad solder joints, solder bridges, reflowed each point where wires come into the board, and, as I said, swapped out Q5 only to have it persist. There's a lot going on in the schematic after Q5 so I don't really know where to start looking for replacing components.

Any help would be appreciated; thank you in advance.

[antonis]

It goes away if drive is dialed below 12 o'clock

Place a 47pF (or so) cap across pins 6 & 7..
You may tweak its value for marginally not squealing when Drive pot set Full CW..

[duck_arse]

. If I pull the guts out of the case, the squeal goes away and, in fact, it gets worse the more I push it into the box.

you've highlighted a clue there. show us a photo of your build, that we may see the length of your connecting wires. outta the bocks, stretched out, they may be far enough apart for 100% tikkety-boo-ness, but when you push it into the box, proximity takes over, and the screaming starts.


[ edit : ] ALSO, and for the 9000th time, welcome to the forum.

[JVO]

Thank you for the suggestions, I really appreciate it.

It was something with the switch as I took it out and now it sounds normal.

I now have a hole in the case where the switch was but at least it is functional. I'll revisit this again, maybe with a new switch.

[JVO]

It goes away if drive is dialed below 12 o'clock

Place a 47pF (or so) cap across pins 6 & 7..
You may tweak its value for marginally not squealing when Drive pot set Full CW..

Antonis, just to be clear here, do you mean on the IC?

[JVO]

. If I pull the guts out of the case, the squeal goes away and, in fact, it gets worse the more I push it into the box.

you've highlighted a clue there. show us a photo of your build, that we may see the length of your connecting wires. outta the bocks, stretched out, they may be far enough apart for 100% tikkety-boo-ness, but when you push it into the box, proximity takes over, and the screaming starts.


[ edit : ] ALSO, and for the 9000th time, welcome to the forum.

Thank you for the welcome, duck_arse! As a posted above, it was something with the switch because as soon as I took it out, it sounded normal. I want to revisit the this though and might try with an entirely different switch and wiring set up.

I'll get a picture once I'm done with that. Thank you for the reply!

[antonis]

Antonis, just to be clear here, do you mean on the IC?

Yes..
(like in most high gain distortion circuits..)
It's effectively set in parallel with feedback resistor thus forming a LPF preventing high pitch oscillation (squealing)

[JVO]

Antonis,

I put the switch back in (still squealed) but put in 47pf between 6 and 7 and now it is gone!

Thank you very much for your help!

Why would adding a capacitor between those two spots get rid of the squeal? Is it filtering the high noise or is it getting rid of weird voltage coming in?

Again, thank you!

[antonis]

Capacitive reactance (resistance) goes down when frequency goes up..
Consider that cap in parallel with Gain/Drive pot which is the negative feedback resistance..
They form a Low Pass Filter, 'cause cap "leaks" high frequencies from passing through feedback resistor, hence lowers high frequencies signal gain thus preventing high pitch oscillation (squealing)..

It's weird enough for a signal flow series cap to be considered part of low pass filter but roles are reversed inside negative feedback loop..