FF clone problem.

[jjmantele]

I’ve posted this problem on the BYOC and TGP forums and I’m hoping that something might ring a bell with someone here who can help. I have the FF clone kit which has worked great for about a year. The clone has a few extra components but the basics are here. (Flip the +'s and -'s since the kit uses NPNs and is negative-grounded.




It has always had more than enough output and fuzz. The other day it started working way too quiet when engaged and there is almost no fuzz. I have changed and/or checked every wire, trace and components possible. Also re-touched every solder connection. The Output and Fuzz controls “work.” The included tiny bias pot (which is a 10k in place of the FF’s 8.2k resistor) is currently NOT changing the sound/volume when rotated. It used to adjust the compression/fuzz nicely. I verified it is “functioning” properly on the board with my DMM. LED functions properly. Bypass mode works fine. I only use regulated 9VDC power supplies and they are ok. I jumped the switch’s related contacts, when engaged, to eliminated it as a cause.

I found a chart of someone’s transistor voltages and they are close to what I’m seeing on mine:

Q1 Q2
c -0.48 -4.65
b -0.09 -0.48
e 0.0 -0.37

(Ignore the negatives since this is NPN.)

I’ve removed the “optional” diode and filter cap to eliminate them as possible causes of the problem. I disconnected the LED with no change. I tried removing the 1meg input-to-ground resistor with no change, then tried removing the 1-meg “output” to ground also with no improvement. I tried a 9.7v battery to no avail. I moved 4-5 wires off the board and connected them PTP where possible to eliminate some hypothetical bad traces as culprits, with no change. I’ve checked the transistor sockets.

This problem is actually very similar to one I always had whenever I tried to engage BOTH the FF and my old wah pedal. Major volume reduction when both were engaged. I had never figure that out…. (I did receive feedback on that issue which can be resolved by adding a buffer to the wah.)

I injected a 40mv ~800hz sine wave on the input and discovered some strange readings. In bypass mode the input signal is fine but when I switch the pedal on, the signal goes down to about 7mv. I figured some reduction might be appropriate from the 1meg-to-ground input resistor and/or thru the 101k path to ground thru the feedback resistor thru the fuzz pot. But I checked and those paths are not the culprit. So I checked the Q1 collector and is shows about 1mv AC. (I’m using my fluke and not the scope.) That’s lower than the signal on the base/input so something is wrong with that.

I figure the Q1 transistor could be leaking some of the signal from it’s base to it’s emitter which is grounded. So I left the pedal switched on but pulled the power supply out. I then confirmed that merely inserting the Q1 into it’s socket takes the input signal from 40mv to 7mv. I tried a silicon transistor and it also reduces the signal but not as much. I have no idea if this phenomenon and it’s strength are normal.

I also noticed that the DC voltage on the collector of Q1 goes from the “normal” 0.5v down to almost zero when I turn the signal generator from off to on. So I changed the 33k resistor since it feeds that node from the power supply. It was reading 33k before I removed it and the replacement did not solve the problem. The power side of the 33k always reads 9.04vdc. Should the DC on the collector go almost to zero when there is an input signal?

So the signal comes in, gets eaten by the Q1 to ground, and the DC voltage on the Q1 collector goes way down. It sounds like all my transistors are horribly leaky but the silicon one measures 4megaohms+ from base to emitter. And no leakage from collector to emitter. I also tried a few more germaniums to no avail.

This is the only distortion pedal that sounded good thru my SFDR of 4 I have so I really need to get it back up and running. Any hints or ideas would be greatly appreciated. I know tubes fairly well but these newfangled transistors are still somewhat foreign to me.