Roland SH-3A with ADSR issues

Hild · Joined: Jan 04, 2020 Posts: 1 Location: Brooklyn

Hi all,

First post y'all, and I'm comin' in hot.

I'm working on a Roland SH-3A. It mostly works out pretty good, but there are some issues in the ADSR circuit. The attack and Release controls work fine, but the Sustain control does nothing. There may be an issue with the decay, but if the sustain feature isn't working, who knows?

Anyway, I'll attach a schematic. You can see the sustain pot does little more than act as the bottom of a voltage divider to the base of Tr126. The pot checks out OK, and I've subbed in another unit just to be sure. You can also see that further downstream, there is the circuitry for the attack control, which works just fine, so it does seem like the signal is getting through. I've also swapped the tr126/127 pair just to be sure, and still no dice.

I'm banging my head against the wall. This must be something obvious I'm missing here. The circuit is rather simple. Any ideas?

(the section of the schematic has some high lites on it which can be ignored)

IMG_8492.JPG

Description:

SH-3A ADSR schematic

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1.85 MB

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Regebro · Joined: Sep 12, 2004 Posts: 61

I don't know much about ADSR circuits, but it seems to me that Tr-126 will drain the ADSR voltage until the voltage between the sustain pot and the R201 resistor gets so low the transistor stops conducting.

And now it never stops, if I understand you, so there is no sustain. That could happen if the sustain pot is shorted (which it isn't, you say) or the R201 resistor is blown out and doesn't conduct, or if the Tr-126 transistor has shorted internally (or there is some other short, or the R201 pot has bad solder).

So I would first check the voltage on Tr-126 "input" (uh, uhm <googles>) oh, right, it's called a "base" actually. It should go up and down with the envelope.
I think. It's just a wild guess, really.