Friday, 27 July 2018

Repairing my VCO

A-110-1 standard VCO

Maybe you followed this blog for a while and wondered why I didn't write more about the Doepfer A-110-1 Standard VCO that is shown on some photographs.
Or maybe you don't care.

This module is the first module I bought new  It's also the first I fried : I plugged the connector in reverse during one of my trials, before I have a case.

The module began to act odd.  Plugging its output to Rings input halted the later.  It became very hot.
I stopped everything and began to investigate.

 -12V pin of the module connected to an unconnected pin of the bus board, presumably close to ground voltage.  Gnd pin was at 12V and 12V pin at 0V.   I headed to the A-110 service manual from Doepfer to find out which parts could have been stressed.

I spotted two components that would certainly suffer to be powered in reverse (inversion of the 12V and gnd pins) : the CA3046 transistor array (IC3) and the 78L10 10V regulator (IC5).

The two components I changed.

I reckoned the two TL074 op-amps wouldn't have been stressed as they were powered between 0V and a floating voltage that would most certainly be close to 0V.  I removed them and tested them on a breadboard : they were OK.

I also checked the TL431 Reference Voltage and it gave me the proper 3V it should.  So it was OK as well.

Maybe other transistors were stressed but I have no idea how to verify without desoldering them.  The plan is to change the CA3046 and 78L10 first and test the module.

Calibrating the module.
After some difficulties to get a CA3046 (they are reported as obsolete in DIP14 package by main distributors) and de-solder the 78L10 (I guess my de-soldering braid was oxidized), I finally replaced the two parts. 
Et voila.  A quick check at the oscilloscope showed all outputs seemed fine.  Maybe only these two components were affected after all.

Finally, I re-calibrated the module, following the service manual, skipping step 1 though (too complex).

I can finally use my VCO.  End of the story.

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