This Synthex came to me for two reasons... primarily for the replacement of some very bad potentiometers, which was pretty straightforward after I found some adequate replacement ones from Digikey, and secondarily due to some issues with the Square Wave output from the LFO.
This latter issue required a bit of diagnosis, an endeavor which was impeded by the strange (Italian) design of the Synthex. I came to find out that the Synthex is particularly hard to work on because 1) the schematic lacks any labels or circuit descriptions, and 2) the circuit boards are essentially piled on top of one another, so you cannot insert a test probe onto any circuit boards except for those on the very top.
As far as dealing with the schematic, I was able to find the LFO output on the 5850 circuit board, after searching for familiar patterns on the schematic. I found that the various LFO output shapes (sawtooth, sq wave, etc) are routed through a few 4066 IC's on that board, on their way to the oscillators or other point they can effect.
As far as dealing with the inaccessible circuit boards, I was able to do some testing by soldering tiny wires onto various points of the board, running the wires out through the tiny spaces between the boards, and then connecting my scope probe to those wires. This is an arduous process, but it does work.
Ultimately I found that the square wave LFO was getting into, but not out of, IC 2E, which is a 4066 switching IC. .I found that the reason was that IC 2A, a 40174, had a dead output, and was not activating the switch, and not connecting pins 8 and 9 of 2e as it should. Replacing IC 2A solved the problem.
I now understand why Synthexs' have a reputation of being difficult to fix!
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| The circuit boards, except for the top two, are impossible to access with a probe... so I had to pick some test points and solder small wires to them, and then reassemble and test. |
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| The 5850 Circuit board, where the problem was... IC 2E has been removed here. |
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| These Digikey Potentiometers were pretty close replacements... Linear 5k... however the shaft was a bit long and also the diameter of the busing slightly small.. but the shaft was easily cut, and the pots fit securely when tightened up. |
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| The square wave was detected going into IC 2E right here... but not coming out the other side! The problem was a dead output on IC 2A. |
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| The problem ultimately was not the with IC2E, but with the flip-flop IC 2A which was controlling it. |
