Help with squeal with SD1 with modded diodes!

[jimlam]

Hi guys,

I've a problem with the Boss sd1 GE 5 star mod which I did myself.  I kinda need your help because I'm stumped with this one.

I replaced D6 with a switch which selects either a 1N60 or a 1n60-red LED (in series) combination. Shops didn't have 1N34 so I bought the 1N60 which is supposed to also be a germanium diode.

I'll try to describe them as thoroughly as possible.

1. With everything plugged in except the guitars input jack, the pedal squeals.
2. The squeal is louder for the 1n60-red LED combination than the 1n60 alone.
3. It starts after the drive is turned past 12 o'clock.
4. It happens whether the pedal is engaged or not.
5. If the guitar is plugged in, the squeal disappears. And I know the switching works because the volume is boosted when switching from the 1n60 to the 1n60-red led combination. I have no problems when playing.
6. As an experiment, I took out the switch and just soldered a red led in D6 and there was no squeal whatsoever.

I desoldered it and resoldered it several times and different things would happen. But there is always some squeal.

What could be the problem with this? Did I burn a diode while soldering it? Is the 1n60 inappropriate for pedal use? Is it because I didnt do the bypass bleed mod?

Any help would be appreciated.

Thanks!

JL

[Jobet]

It has happenned to me too. See if my solution will fit your situation.

I suspected the grease inside the switch giving off stray capacitances, so what I did was to get a little denatured alcohol, and with a syringe, applied it to the inside of the switch and rocked the bugger back and forth. There are many ways to do this, and in one extreme you can just dunk the thing in a small glass of alcohol and then rock the switch back and forth hopefully to clear out the grease inside.

Then again, you can try swapping out the switch.

At other times, I have resorted to using shielded wire to PTP the switch to the board. It works sometimes, but not always.

I do hope you're not living beside a radio station though. These remedies will not help you a bit.

[Jobet]

Scratch what I said.

Seems that you don't have a problem after all.

Many pedals don't like an open input line.

[jimlam]

An update. I think I now have a bigger problem. I installed the switch into the pedal (previously it was just hanging loose while I was testing it) and put back the bottom plate an all.

Now the squeal also happens with the guitar plugged in! Turning the drive knob past 12o'clock causes the squeal to come on. It's loud and unplayable.

This is the second switch I used so I dont think its the switch. The first i tried was a on-off-on switch. When the red LED was selected(middle position off position), it squeals. With both switching system, its always any position where there is an LED that causes it to squeal. But like I mentioned in my original post, I took out the switching and just soldered a red LED in D6 and there was no problem. Very strange

Anyone else have such a problem before? Why is it that the squeal is dependent on the amount of drive? Did I burn some component that causes it?

Thanks.

[ironman28]

You are experiencing local feedback from the connections to your switch. Try moving the leads to the switch while the feedback is occurring. You may be able to move the leads to where the FB stops. This is known as " lead dress" and is important in high gain circuits. Also try twisting the leads going to the switch to keep the loop isolated. As a last resort use shielded cable for the switch leads. Be sure to ground the shields at one end. Keep the component leads of the LED/diode on the switch as short as possible.

If you can hear the FB with the pedal bypassed it may indicate you have some bleedthrough in the off condition. You might want to fix that after your FB issue is fixed.

Good luck

[Jobet]

Okay, put back my recommendations on the table.

There is also a possibility that we're barking up the wrong tree.

Feedback is usually caused by stray capacitances. Check the underside of the board, see if you can find micro-bridges. Sometimes even the solder flux from the lead can also contribute to stray capacitances. So with some contact cleaner and a toothbrush, clean the underside of the board.

This is just an SD1, I've been having these problems in MT2's and HM100's. So certainly if there's something wrong, it should be easier to find than on a higher gain pedal, which by its nature is prone to squealing.

[wampcat1]

First problem:
don't use your pedal without the guitar plugged in.

Second problem:
whatever you did between the time that it you didn't have the problem and now, undo that change. sd-1's aren't usually going to oscillate unless you changed the cap/resistor feedback @ the first opamp to something crazy. Diode changes in it, the same thing. It's not there. I'm hesitant to say it's the wiring in the sd1 too... I've had the wires all twisted up in a nasty mess and never had the problem.. again, sd-1 = not a ton of gain.

There's something else going on... you have a solder bridge SOMEWHERE I bet.

bw