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Fatman trouble

Posted: Tue Jan 04, 2011 10:42 am
by alexhiam
I received the fatman kit with the desktop enclosure for christmas (great present!). I got it wired up and turned it on, and the power light came on, so I plugged it into a midi source, but no luck. The midi light flashes correctly at times, but very faintly, and at times fades completely or stays on continually. Every once in a while the gate light comes on, faintly as well, but doesn't seem to correspond with any change in the sound. Every few seconds, although not at even increments and with or without the midi source, a noise fades in that sounds like something's not grounded right. During this noise the VCOs can be heard very faintly, and their pitch sort of steps up and down slowly. I was able to test the pitch knob while listening to it, and it seems to tune right. I checked over every wire, and they are all connected to the right places. I double checked every solder point and all polarities. It turned out I had put VCO2's 555 in backwards (how embarrassing!), but correcting that made no noticeable difference at all. I did discover, while poking around with a voltmeter, that the the voltage coming out of R117 (30 ohm 2W series) is 4.5V (13.1V coming into it), so that the 7805 is only putting out just over 2V. I am hoping that fixing this will fix everything, but I can't figure out what's causing the voltage drop!

Thanks in advance to anyone who can provide any insight,

Re: Fatman trouble

Posted: Thu Jan 06, 2011 8:26 pm
by alexhiam
I've been looking closer at the synth and thought I'd give an update.

First I should correct myself - I said that the gate led doesn't correspond at all with the random self triggering, but it does.

The midi led no longer responds to any incoming messages, it only lights when it triggers itself. I tested the amp and filter with a signal generator - the amp seems to work ok, but the filter lets no sound through. I then watched the outputs of the envelope generators, and when the synth self triggers the ADSR seems to be working perfectly, but the filter's ASR doesn't respond. I pulled out the 7805 and tested it off the board and it works as it should. The other two regulators are putting out what they are supposed to.
Besides the possible digital issues, the main problem still seems to be the low voltage coming out of R117. I even tested the resistors to see if they were incorrectly color coded, but they of course are not. I checked every other component in the power circuit and it all seems fine. The only thing I've found that varies from the schematic is that it shows a power source supplying 500mA, while the adapter supplied with my kit is 12VAC at 1000mA. I don't see how this would cause the problem, but I might as well give all the details.