Morley Tel-Ray Fuzz Wah help

[Ben79]

Had a few of these enormous chrome spaceship fuzz wahs in the past. Got one here that's giving out a very gated fuzz tone. My partially-educated guess is that the voltage is too low on the collector of one or more of the trannies and needs a different resistor. Not sure which trannies do the fuzzing, here's the schematic. ...

http://i22.photobucket.com/albums/b323/momsazombie/PWF.jpeg

Here's the board...



Any help much appreciated.

[Gone Fission]

Collector voltage would be my guess, as well. I think the wah transistors are the two to the left-hand side of the board. Handy resistor color-code chart here for helping match your trace to the schematic: http://www.google.com/imgres?imgurl=htt ... Q&dur=1518

[eatyourguitar]

collector resistor being too big is only one way to get a gated effect. another possible answer is that your BE junction is reverse biased or just close enough to BEvf of 0.7v. on Q1, the 4.7M and 2.2M create a voltage divider that biases the base of Q1. voltage dividers are pretty simple. take your supply voltage multiply it by this number R2/(R1+R2) where R1 is pulled up to V+ and R2 is pulled down to 0v. for example 2,200,000/(4,700,000+2,200,000) = 0.318840. then multiply that by 7.5v and you get 2.39v at the base. I cheated a little since I left out the 27k. in the math, R1 would really be 4,727,000. what if we had %20 resistors? lowest voltage possible at the base is 2.36v.

so yeah back to looking at the collector resistors or bad transistors. it is really old after all. I would say the power supply has a risk of flyback spikes in the 200v+ range when the power is disconnected. this is more than enough to kill even the biggest of the transistors used. that MPSA06 is rated at a max VCE of 80v, see the datasheet here http://www.fairchildsemi.com/ds/MM/MMBTA06.pdf if you are not familiar with flyback spikes that come from switching inductive loads (the power transformer), I highly suggest you look into it. http://en.wikipedia.org/wiki/Flyback_diode