Vintage Synth Repair

Hey all.

I recently got a lovely looking D-50, but a few things were wrong..
About 13 keys were not working, which was found to be a few tracks on the keyboard pcb which had become corroded..

After fixing that, I found a rather odd problem... one side of the reverb FX would "Feedback" rather unpleasantly after a note was played..
If the reverb level was taken down, there would be no real problem.

I could live with just using the other side, but I thought I'd have a go at fixing it..

A bit of google and I found out the MB87126A IC was the reverb IC..

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..

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.

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.
Does anyone have some sort of advice about how else I could simulate my scope being between pin 7 and ground?

I realise this is a 'workaround' fix, but if I can fix my D50 easily, that would be great.

Regards
Mike

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...

________________________________
> To: vintagesynthrepair@yahoogroups.com
> From: mrjdevries@...
> Date: Sun, 5 Jul 2009 14:36:02 +0000
> Subject: [vintagesynthrepair] Roland D-50 repair advice please..
>
>
> Hey all.
>
>
>
> I recently got a lovely looking D-50, but a few things were wrong..
>
> About 13 keys were not working, which was found to be a few tracks on the keyboard pcb which had become corroded..
>
>
>
> After fixing that, I found a rather odd problem... one side of the reverb FX would "Feedback" rather unpleasantly after a note was played..
>
> If the reverb level was taken down, there would be no real problem.
>
>
>
> I could live with just using the other side, but I thought I'd have a go at fixing it..
>
>
>
> A bit of google and I found out the MB87126A IC was the reverb IC..
>
>
>
> 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..
>
>
>
> 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.
>
>
>
> 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.
>
> Does anyone have some sort of advice about how else I could simulate my scope being between pin 7 and ground?
>
>
>
> I realise this is a 'workaround' fix, but if I can fix my D50 easily, that would be great.
>
>
>
> Regards
>
> Mike
>
>
>
>
>
>
>
>
>
> 
>
>
>
>
>
>
>
>
>
>
>

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--- In vintagesynthrepair@yahoogroups.com, Scott Nordlund <gsn10@...> wrote:
>
> 
> 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...
> 
>
> >
> >
> 
> >
> > 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..
> >
> >
> >
> > 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.
> >
> >
> >
> > 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.
> >
> > Does anyone have some sort of advice about how else I could simulate my scope being between pin 7 and ground?
> >
> >
> >
> > I realise this is a 'workaround' fix, but if I can fix my D50 easily, that would be great.
> >
> >
> >

As someone who's been soldering in professional enviros for 25+ yrs,
I would get some solder flux , lightly dab some around the leads of the reverb chip and resolder all the leads. They may "look" good but at times the solder is covering an oxidized pin that's
 "almost" connected

Scott in Vermont