problems with modded SD-1

[SM]

I’m having problems with my SD-1.
It’s new and I performed the mods described as Robert Keeley mods by Phillip at Fuzzcentral, (Excellent description and schematics by the way). It worked pretty well after performing the RK mod, although I wasn’t very impressed with the sound, I like my modded Blues driver much better.
I wanted to try different diodes and tried a small red LED and some germanium diodes.
Sometime around her the unit starts to crack like hell, in both OFF/ON position. I couldn’t locate where the problem was and removed the diodes altogether ‘cause the damn thing had started to oscillate as well…. I can control the frequency of the oscillation with the “Drive”-pot. The sound is louder in the OFF position, some signal from the guitar passes through, but not much. I changed the OpAmp, tried to resolder everything, swapped some caps back to original values but I’m clueless concerning the source for the oscillation.
My CV concerning effects is a homebuilt RM (RG Keen model) and a modded Boss BD and a modded CE2. So I’m obviously no expert.

Is there anyone that could help me here? Where to start?


:-)
Bjorn

[smoguzbenjamin]

Well if it's giving trouble with bypass operation, maybe you screwed up the FETs that do the switching. Try replacing all the original parts, then the switching FETs and see what happens.

[SM]

Well if it's giving trouble with bypass operation, maybe you screwed up the FETs that do the switching. Try replacing all the original parts, then the switching FETs and see what happens.

Hmm, I'm not sure I follow. I can perform the ON/OFF oparation without problem. There is a clear difference in sound when the pedal is on or off.
It was cracking quite a lot in both positions and when I tried to fix it it started to oscillate. I tried to unfix all my fixes but I don't fix it, if you understand what I mean  :oops:

:-)
Bjorn

[smoguzbenjamin]

No, you're not understanding right.
What BOSS does to switch the effect is turn one FET off (asin high resistance) and one FET on (low or no resistance). One FET is in the wet path and one is in the dry path. If you fry these and they're leaky and not working right, you might be experiencing problems with the sound like you describe.

Sometime around here the unit starts to crack like hell, in both OFF/ON position

The actual flip-flop circuitry doesn't have to be damaged. Find the FETs in the signal path that do switching, preferably with a schematic, there must be one around somewhere. Replace those FETs, and your problem could be (partially) solved

[SM]

OK - I understand. I know where the FET's are located.

Thanks,
Bjorn

[smoguzbenjamin]

Good luck!

[claydavis]

check the input and output jacks, as well. occasionally these get mangled when boss pedals are removed from their cases, and can cause all sorts of interesting problems.

[SM]

Good luck!

Yeah, I really could use some of that!  :lol:

I removed the two FET's - no difference.
I replaced every transistor - same effect.
I removed the big elyts and replaced them with new ones - same.
I removed and resinstalled every modded cap - same.
I changed the opamp - same.
I removed the distorsion diodes altogether - same

The oscillation has changed character somewhat - from a high frequenzy buss to motorboating.
I can effect the frequence with the drive knob.
Connecting/shorting point 2 and 3 increases Hz (where the cables joins the board)
Removing the little elyt (c11) lowers Hz. shorting C2 increases Hz.
Shortin the circuit or the elyts will make the oscillation start from scratch upp to motorboating Hz again.

I'm pretty stubborn but this may be hard to fix.

:-)
Bjorn

[bobbletrox]

Maybe you've accidently heat damaged a trace?

[petemoore]

Just close eyeing and inspection...
 I'm thinking it's something simple.
 They only way the Fet switch could get fried is if you overheated it [I think] or spiked it with too much juice somehow [unlikely], or broke something like a wire loose.

[smoguzbenjamin]

My money's on a short.

[SM]

My money's on a short.

I agree, but WTF could that be... I'm getting more grey hairs from this  :?

I’ll get back if/when I find what’s wrong â€" probably something simple, as mentioned above.

:-)
Bjorn

[smoguzbenjamin]

Well, look for solder bridges, preferably with a magnifying glass. Sometimes, solder goes places where it shouldn't be. And be systematic and careful. The last thing you want to do is get confused