A little info about the Ensoniq ASR-10

I purchased a non-working ASR-10 recently.   It was completely dead.   The problem turned out to be one pin on the connector that feeds the main power transformer secondaries to the rectifier/filter power supply board.  The pin that was not making a connection was the center tap of the winding used to make +5V, so that voltage was not present.   It seemed that the pin and connector contact had overheated, and somehow the metal was no longer making a connection.  Both of the metal pieces had changed color due to the heat.  I replaced both the pcb pin and the connector end, and that fixed the issue.   It makes sense that the center tap contact would be the one to fail, since it is passing current during both halves of the AC cycle, and the two other legs are only used during half of the cycle. 

I read out the V1.50B firmware from the upper and lower EPROMs.  Here are the binary images in case anyone needs them.

Also, I have been helping a friend fix his TS-10, which seems to use the same PolyTouch keyboard as the ASR-10.   His keybed works properly when it is removed from the synth and connected.   If he places it in the synth, it fails calibration.   I think the issue is that one of the oscillators is set a little too high.   Bringing metal near to the keyboard will raise the oscillator frequencies very slightly, and I guess if it's right on the edge of the acceptable range, it could fail in that case.   Anyway, I made some measurements and captured the schematics of both coil pc boards from a TS-10.  Here is the lower one, and here is the upper one.  I powered up both bare pc bds (no keys attached) on the bench, to make these measurements.  This circuit is pretty similar to the one used in the SQ80, except the drivers have been changed from 138 to 238, so the PNP inverters are no longer needed.  Also, a different custom chip is used, with support for interfacing to a 128-byte EEPROM.   I have no idea what that was used for.  The SQ80 keybed schematic is available on the web.

Note:  At the time I was figuring out the coil board schematics, I had only those two boards here, without the rest of the synth.  It seems that I got upper and lower swapped.   So if you are looking at the two schematics, everywhere it says "lower" it should be "upper" and vice versa.   Oops!


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