Sparkle Boost has negative gain???

[whannah1]

Hey Y'all,

I accidentally posted this in the beginner section so I'm re-posting it here.

I'm a beginner with these pedal circuits. I built the sparkle boost circuit and then instead of putting it into a pedal housing I built it into the faceplate of my rhodes piano. I wired it in with a bypass switch after the "tone" pot and before the volume. At one point during the process, the thing worked great! but since then I wired in an LED and the bypass switch and moved things around to make it look nicer and now I have a problem that I have no clue how to troubleshoot.

The issue is that when the gain is all the way up, there's no increase in volume. Turning the gain down lowers the volume, but when I flip switch to bypass the circuit the signal is much louder. How did this happen? What can happen to make a simple boost pedal go from working perfectly to doing the opposite of what it should do?

Could this be a bad transistor? I'm using an MPF102. I've messed with the bias for the transistor, which doesn't seem to fix it. I've also just tried wiggling the wires and components to see if anything is loose. The pedal seems to work fine except for this huge drop in volume. I've also gone through with a continuity tester and all the connections seem to be right. I haven't checked the voltages at the different leads of the transistor (not sure what they should be).

I should also mention that I omitted the volume knob in the schematic, (the circuit only has the gain knob) so leaving the volume turned down can't be the problem.

I didn't change anything on the circuit itself since the previous tests when it was working (that I know of). Any suggestions on things to try would really help.

[Steben]

Check for loss connections or wrong impednace values. Mistake in the range 10 fold is easy to make.
A volume pot to ground of 10k will easily draw more volume away instead of a 100k one. etc...
Check the 100k resistor to ground the same way. Putting a faulty 10k there will draw volume too!

beware : MPF102 is one of the low gain fets. J201 is high gain. 2N5457 medium

[Brossman]

check solder connections, try checking if it works w/o the bypass switch, etc.

Check the "DeBugging" sticky on the main forum here for more info...

[twabelljr]

but since then I wired in an LED and the bypass switch and moved things around to make it look nicer and now I have a problem that I have no clue how to troubleshoot

If the problem started here, undo it and see if it works like it did. If so, find a good true bypass with led diagram to follow like this:
http://gaussmarkov.net/wordpress/thoughts/wiring-up-a-1590b/

I haven't checked the voltages at the different leads of the transistor (not sure what they should be).

You don't really need to know what they "should be". If you follow the debugging sticky suggested, there are people here that do know what your voltages should be at or near. It would probably be easier to post your voltages before undoing anything but triple check everything you did since it last worked. (If you can remember what you did exactly, I need to keep notes.) 

[whannah1]

Thanks for the suggestions.

I've gone over the connections over and over. I think I can rule them out around the bypass switch. I unsoldered one of the led leads and the problem didn't change, so i think i can rule that out as well. The piano sounds normal when the effect is bypasses, and the gain knob doesn't have any effect.

I'll read over the debugging sticky again and check the voltages next.

Since the "character" of the boost circuit appears to be coming through when the effect is on (not bypassed), it would make sense that it is an impedance issue as Steban mentioned. This is what I was mainly asking about, because I would suspect that without a change in the impedance, I shouldn't get a quieter signal when the effect is on. I'm not that familiar with transistors, so I wanted to rule out whether or not a bad transistor could cause this problem.