Homebrew Electronics UFO fix

[alange5]

Anyone have one of these?  A friend gave me one to fix.  There was a burnt resistor on the board between the 9V supply and the LED.  I'm guessing he had a power surge. His power supply is center negative, which is what the UFO calls for.  Anyways, now the pedal powers on, but there's a continuous volume swell, and it doesn't sound quite right.  It's fuzzy, but it's muddy and the knobs only affect subtle changes.  It's as if there's a big old wet blanket over the sound.  When the octave switch is engaged, the sound almost cuts out completely.  Only when the fuzz knob is maxed and with a hard string attack are the notes audible in the octave setting.  Assuming a surge took out the resistor, what other components should I be looking at?  There's no schematic online, and the transistors and diodes are all covered in epoxy.  All I know is it's supposedly based on the Foxx Tone Machine.  Any ideas would be much appreciated.

[mth5044]

Are the caps uncovered? Check too see if any of them have any 'growths' or look abnormal. Can you compare the circuit to a Foxx Tone Machine and check the voltages? With what value did you replace the scorched resistor?

[alange5]

Here's the Foxx schematic:
http://www.generalguitargadgets.com/pdf/ggg_ftm_sc.pdf

here are recommended voltages from GGG:
http://www.generalguitargadgets.com/pdf/ggg_ftm_sc.pdf

My voltages change when the octave switch is engaged.  Here's what I have without and with the octave:

Q1 C: 1.83   Octave: C: 1.73
     B: .60                 B: .56
     E: .07                 E: .03

Q2 C: 4.1     Octave: C: 3.02
     B: 1.83               B: 1.73
     E: 1.27               E: 1.18

Q3 C: 5.24   Octave: C: 4.21
     B: 1.83               B: .38
     E: .014               E: 0

Q4 C: 4.95   Octave: C: 4.04
     B: .47                 B: .37
     E: .017               E: 0

The fried resistor was replaced with a 1K.  I'm pretty sure it's the correct value. It connects directly to the power supply...9.6v on one side of the resistor and 5.4v on the other, which then goes to two more 1k resistors - 1 for each LED, for about 1.7v on each LED. 

In my "de-gooping" effort, I damaged the two GE diodes in the octave circuit.  I replaced them with 1N34's.  Without the octave engaged, I get 2.67v at the anode of D2 and 2.56v at the cathode.  With octave engaged, I get 1.44v at D1 anode and 1.36v at the cathode.

None of the other caps look damaged.  I thought about replacing all of the electrolytic caps but a few of them are all gooped up. 

[alange5]

FYI, it pretty much sounded the same with the original diodes.... volume swell and a big volume drop when the octave is engaged.

[mth5044]

I think you accidentally copied the same link, here are the GGG voltages

http://www.generalguitargadgets.com/pdf/ggg_ftm_instruct.pdf

Looks like your collector voltages are all too low. Is your 1k resistor feeding the rest of the circuit? If so, that seems too high. Usually those resistors are 68/47/33 ohm resistors.

[alange5]

That did the trick!  Right now i have 100 ohms in place... pedal functions as it should but still sounds slightly muddy.  Voltages are in the ballpark - maybe 10% lower than the GGG values.  I'll throw a 68 ohm in there and see what happens.  I appreciate the help.  Didn't realize that was feeding the whole circuit.  What would explain the fried resistor?  Power surge?  Reverse polarity? Daisy chain?  Or just a failing resistor?

[mth5044]

Lots of things can cause the resistor to burn. Accidental AC adapter being plugged in, too much current/voltage being pulled across it. Possibly something downstream of the resistor got shorted and pulled more current/voltage than the 1/4W resistor could handle when you had it open or the circuit wasn't re-installed correctly.

Doubt it was a power surge unless your power adapter also no longer works? I'd imagine something in the adapter would bite the bullet before it gets to the pedal. Maybe, anyway.

[alange5]

I appreciate your help.  According to my friend, he was using a boss adapter to power it without issue.  He switched to a Snark power supply for a gig, and accidentally left the pedal turned on overnight. Next morning, he smelled burning. Who knows.

As far as that resistor goes, I took it down to 68 ohms and now there's an absurd amount of volume.  Unity gain is achieved by barely moving the volume knob.  I'll probably hike it back up to 100 ohms.  If anyone owns one and would care to share the actual resistor value, I would appreciate it.  It's the first resistor on the top left of the board, directly next to the red wire.  Thanks again for your help.

[Fp-www.Tonepad.com]

I own one of those bad boys, Let me know if you still need me to peek at the value of that resistor, It's up in my 2nd backup pedalboard.

[alange5]

I own one of those bad boys, Let me know if you still need me to peek at the value of that resistor, It's up in my 2nd backup pedalboard.

If it's not too much trouble, I'd really appreciate it.  Currently have 100 ohms installed.

I got my voltages on par with the GGG foxx tone machine, but I still have one problem.  It is ABSURDLY loud.  The pedal sounds good - more or less like the various demo videos online, but unity volume is just a hair above zero on the volume knob.  By 10:00 it's shaking the floorboards.