Vintage Synth Repair

Does the reverb chip have an external RAM?  There may be a bad connection between the two, perhaps a broken trace?  I have noticed (through my own deliberate experiments, not repairs) that messing with this can effectively rearrange the memory map, scrambling the read and write addresses into some unstable configuration- so it can oscillate like feedback.  Your oscilloscope probe may be acting as a pull-down resistor on a floating pin, putting it back into a stable configuration.  You could try putting a smaller resistor there (10 or 100K), or follow it to see where it goes...

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> To: vintagesynthrepair@yahoogroups.com
> From: mrjdevries@...
> Date: Sun, 5 Jul 2009 14:36:02 +0000
> Subject: [vintagesynthrepair] Roland D-50 repair advice please..
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> Hey all.
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> I recently got a lovely looking D-50, but a few things were wrong..
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> About 13 keys were not working, which was found to be a few tracks on the keyboard pcb which had become corroded..
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> After fixing that, I found a rather odd problem... one side of the reverb FX would "Feedback" rather unpleasantly after a note was played..
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> If the reverb level was taken down, there would be no real problem.
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> I could live with just using the other side, but I thought I'd have a go at fixing it..
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> A bit of google and I found out the MB87126A IC was the reverb IC..
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> A good tech (Steve Jones) advised checking for any lifted pins on the surface mount IC's.. but I could not seem to find any..
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> While I was probing around the reveb IC with my 'el cheapo' scope I probed on Pin 7 and the reverb feedback problem vanished!!.. The reverb seemed pretty even (Left & right) and no horrible feedback.
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> I measured the resistance of my scope ( 1M ) and the capacitance (about 200pf) and tried to simulate that with a resistor and cap, but the problem is still there.
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> Does anyone have some sort of advice about how else I could simulate my scope being between pin 7 and ground?
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> I realise this is a 'workaround' fix, but if I can fix my D50 easily, that would be great.
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> Regards
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> Mike
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