Vintage Synth Repair

From: "'Lorne Hammond' lhammond@... [vintagesynthrepair]" <vintagesynthrepair@yahoogroups.com>
To: vintagesynthrepair@yahoogroups.com
Sent: Tuesday, September 16, 2014 6:25:03 PM
Subject: RE: [vintagesynthrepair] ARP Quadra distortion on strings help (ARP Omni circuitry)  














so its a divide down from those notes? ie c in both octaves and the note next to it?   


could be at source.  Or are those notes  summed together?


  lastly the quadra is a weird keyboard mechanism with an up and a down connect might be the IC for the key:


 


Read this:  http://forum.moogmusic.com/viewtopic.php?f=1&t=13154


 


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 Thanks for the forum info, I'll be checking through that.






 I always like to start at the beginning and follow the signal to where it first has the problem,  then look closer there . Did you say you had the Quadra schematic? If so, the "UPPER VOICING BOARD" page 1 of 2 shows a MK50240N at the top supplying all the 12 notes, highest frequency only. Then down lower, you can see twelve CD4520BE counter chips. These each give out 4 lower octaves of the notes it receives. Really common organ technology for that time. Checking there with an oscilloscope is the best start to troubleshot this. If you have no scope, all the inputs and  outputs from the dividers should be square waves. You could listen to them and make sure they all have that clean clarinet-like tone. Those constantly running notes then get mixed selected according to what keys are depressed on page 2 of 2 as well as the "LOWER VOICING BOARD" page 1 of 2. That page 2 of 2 shows they 4' final mix and  the 8' final mix. They go off somewhere and get processed by that one amplifier and one filter for ALL the polyp h onic notes.


 


 The ICs may be the cause of the problems, but you must first be absolutely certain all the key contacts are working perfectly. They have a very small space to work with and just a slight bend can really  mess things up. That's also a good opportunity to clean the contacts if you haven't already. Just like an organ, it will never work 100% without clean contacts. I always do that whenever I first get a vintage electronic keyboard  instrument, even before I plug it in to see if it works.