Fatman stopped making sounds.
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Author:  hemmerlinj [ Sun Oct 21, 2012 3:45 pm ]
Post subject:  Fatman stopped making sounds.

So I was attempting to use the Pitch, Velocity, and Gate outs on my fatman to control some modular stuff I just got. Everything seems to be working fine in that manner. I also wanted to use the audio out of the Fatman to run through that filter in the modular setup, so I added an 1/8th inch jack to the fatman. Hooked things up and I was getting audio out of the fatman and into the filter, but there was a definite difference in volume btwn the fatman, and the sound gen that I have in the modular stuff. So I unplugged it for a few minutes, was thinking of how I could boost the signal into the filter.

I also decided to enlarge the holes where the rca plugs are for the pitch, vel, and gate ports are. Which I did, and proceeded to put everything back together. Afterwards I was getting no sound out of my Fatman. Actually there is a thump with the press of a key, with no change in pitch, same sound everytime I press the key. And very very faintly there is the sound of the fatman, but it's almost inaudible, but I can tell it's there. Obviously I tried messing with all of the knobs to make sure something wasn't set weird.

I've searched for any shorts and can't visually find anything. Was thinking maybe a piece of metal from widening the the holes, or adding the 1/8th inch jack shorted something, but I can't find anything. I blew everything off with compressed air as well, just incase it was something small, but nothing.

Did I fry something here. Hope not. I do have a multi meter to check voltages, but I'm not really sure how to do that, like on one of the IC's or something. I don't want to short something else out.

I hope this made sense. Any help is appreciated.

Author:  PAiA-Scott [ Fri Oct 26, 2012 2:46 pm ]
Post subject:  Re: Fatman stopped making sounds.

Maybe a board to panel wiring connection broke during this operation. Check the ones from AA and AB to the VCA ADSR (amount) control, R102, and, the ones to the controls for A D S and R, N V W X Y and Z.

If the board was powered and solder joints for ICs 17 or 18 touched something metal, this short-circuit could have damaged the LM 13700 IC.

Try probing for audio per the assembly/using manual trouble-shooting section to determine where you're loosing the VCO mix audio-signal.

From the trouble-shooting pdf over at the Technical Bulletins/Assembly Notes, FatMan area:


For a unit that gets a Gate Trigger as indicated by the Gate LED, but has no audio output, a common trouble is an accidental short circuit causing IC17 in the VCF section to fail.. DC voltages for the pins of the IC are (count up in a ccw direction around the part from the notch): Pins 1-16, -10.84, 0.86, 0.22, 0.22, 1.4, -12.06, 1.4, 0.19, 0.13, 1.32, 7.95, 1.32, 0.23, 0.23, 0.87, -10.84.

IC18 VCA readings:

Measure to see that the V- circuit, (-)12 volt dc power supply, is getting to pin 6 and the V+, 8v dc power supply, is getting to pin 11.

Pin 1 is at about -10to11 v dc. The current flow at this pin to Q12 is about -120uA with only the VCA ADSR control, -200uA with both the ADSR and the Velocity.

Look for the V+ on one end of the 12k brown-red-orange-gold resistor R108 and about 1v on the pin 2 end. The 470ohm yellow-violet-brown-gold resistors at Rs 110 and R111 should set the voltages on pins 3 and 4 to about 150mV with about 50mV of audio on pin 3..

Pins 5 and 7 share a connection with the 33k orange-orange-orange-gold. The audio signal should develop to about 0.2-0.3V as the VCA goes 'on'. This audio should wind up at the 4700ohm yellow-violet-red-gold R112 on about a -1v dc offset before running on through the C25 capacitor to the audio Output level control, R113 and output connector J6.


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