A new problem with my Dr. Boogey has arisen

[ayayay!]

The problem:  High, glass-scraping sound squealing when gain or volume knob is past about 60% (1 O'clock.) 

It goes away, even with gain or volume or both up full blast, when a buffered pedal is before it in the chain.  (Buffered pedal doesn't even have to be on, just in the chain)

We've all seen these types before:  High gain pedals, needing to follow some general guidelines.
What I've done so far:  -Moved Gain & Volume knobs away from jacks.  -Shortened lengths of cables from jack to 3PDT, and 3PDT to board.  -Moved in's & out's to "outsides" of 3PDT, placing LED Neg cabling in the center.   -Reseated all transistors in sockets.  -Battery &/or ?DC supply produces same results.  -Measured drains to trimpots, all are fine. 

Only thing I haven't tried that I can think of so far is shielded cabling, which I would really like to try.  None handy. 

All that being said, my gut feeling is the buffer in the preceding pedal is leading towards the cure.  I guess I could place an input buffer in the pedal, but is there a simpler route?  What would say a 220uf cap across the DC jack do?   

It's pretty much from this schem, (but I also added a 150pf in parallel w/ the 1M to ground from the input.) http://geocities.com/electrictabs/dr.boogey.png but

[John Lyons]

Welcome to the DR boogie!
I hate to say it but this is why the gaussmarkov PCB was made and refined to what it is now.
What you are experiencing is Oscillation...
There are a few causes and I don't know why buffering is helping but basically
the layout shielded cable (although not needed with the gaussmarkov layout) and clean wiring has all to do with the cure.
There is too much to go over and re write here.
You would be better off searching and then making a pot of coffee

john

[zyxwyvu]

The oscillation is caused by feedback from the output of the circuit to the input. The input of the board sees two signals: the feedback, and the real input signal. With a buffer before the pedal, the real input signal overpowers the feedback signal because it is much lower impedance. Without a buffer though, the two signals become mixed, and the feedback signal grows. This is why a buffer solves this problem. I would recommend trying shielded wire for the input jack to board connection. This will increase the effective impedance of the feedback signal. You can also try the buffer, but make sure it is away from the main board.

[ayayay!]

Thanks guys.  I figured it was some combination of all those things you replied with, but I needed to air it out and see why the buffers from the previous pedals helped.   Plus like I've said before, I like all this because I love troubleshooting.  It doesn't bother me one bit to learn about things this way.  Rest assured it won't happen twice with me.   

So let me dissect this a little more:  Do you think it's more because of the tonestack not being scaled down, my board layout, or non-shielded cable?  I'm personally leaning towards the cable first, layout second, and full scale tonestack last. 

...And I need suggestions for a good type of shielded cable please.   

[ayayay!]

I can't believe I left out one of the most important parts:  This only happens when bypassed.  (Sorry I left that out John!!!!)

I know the solutions will still all be the same, but when the pedal is on this problem doesn't occur, even when all the knobs are maxxed out.  Actually it's surprisingly quiet.    I would gander that zyxwyvu's explanation of the input signal overpowering the feedback signal is the reason it sounds fine when on.  I'm seriously *thinking* shielded cable would solve this, especially since this only happens when bypassed. 

Thoughts? 

[frokost]

Have you seen this recent thread? http://www.diystompboxes.com/smfforum/index.php?topic=70610.msg568719

[ayayay!]

Have you seen this recent thread? http://www.diystompboxes.com/smfforum/index.php?topic=70610.msg568719

I had read all but the very last part, which is may be just what I'm looking for.  Thank you!

[Morocotopo]

Does your bypass switch wiring grounds the circuit input in bypass mode? If the input is left floating in bypass in high gain circuits they can oscillate. Check tonepad´s wiring schemes.

[John Lyons]

Thanks for the clarity Josh. I can see how the buffer affects the circuit's input now.

Jono, try the grounded input bypass switch at tonepad or at GGG.
That should cure it.
Do you have a pic of the guts of your build?
That would help..and I'm curious to see it anyway.

john

[ayayay!]

Yep, I'll be jumping lugs "A & F" on the 3PDT tonight.  Then the shielded cable if that doesn't do the trick.  I'll bet grounding that circuit input when bypassed takes care of it. 

Do I have to show the ugly side pic of the gut shot too?  Ha!  I'll entertain that thought but no promises.  I'm really eager to record a soundclip because this thing sounds great.  But child #4 will be here any moment (hopefully this weekend) so it may be a few days. 

[John Lyons]

Congratulations on the baby!! Almost...

Don't worry about the uglyness of the board.
A lot of times things are easier to troubleshoot with a pic.
That way we can see what you are looking at in there 


john

[ayayay!]

Sorry it's been awhile.  I did infact have the baby.  Well... the wife did, I didn't.  I've got a thread going in the Lounge so you can dig it there. 

Last week I did ground the input on the 3PDT when bypassed and that did the trick 100% for eliminating oscilattion while bypassed.  Thank you!!!

Today I went downtown and picked up some RG-174 coax.  That stuff is perfect for sheilded cabling.  I used it on the Input to the switch and Input to the board, and also the output to the Volume pot.  I figured that was a good start and if I needed to keep going, and go from the volume to output(s) I could. 

No need, that thing is now DEAD silent.  No oscillation, and hardly any hiss.  Sounds pristine. 

I will try to post clips this weekend if at all possible, but this is my 4th kid so I'll have to do it long after they're in bed which means I might be knocked out too.     Thanks for all the help guys.  This one is cured! 

[ayayay!]

Ooops, posted in the wrong thread.  I meant to let this thread die...