[sdiy] OB8 question

[Magnus Danielson]

On 03/02/11 21:12, klosmon wrote:
> Hello-
> I've dealt with a dozen or so of these, and while most can be repaired,
> scaled & tuned to behave correctly, I've seen a few (like the one I'm
> working on now) which have one voice which will fail to pass tune --
> even though re-enabling the voice in page 2 shows the voice to be correct.
>
> I've tried the voice board in another synth with the same problem, so
> it's not the power supply or CPU board; I've swapped out ALL the ICs on
> the board (along with all the capacitors), and the problem still
> remains. I've measured the resistors & traced the tuning signal with a
> scope -- everything appears normal, but that one voice still won't pass
> tune.
>
> Any ideas as to why this might be happening?
>
> It wouldn't bother me so much, but I've seen it in several OB8s now, and
> would like to have some idea what's going on.

Well, for the voice to be trimmed in it will be selected and counted 
using the 8253 counter chip. Check to see if the signal comes through 
properly during the test-sequence. Follow the signal-path in the 
schematic. If that voice won't make it through properly then it will 
fail testing and be selected out.

This is just answering from the back of my head, but I can dig up the 
schematics and help you even more if needed. I could probably trace it 
down in the code as well, as I've done a fair amount of reverse 
engineering on it.

Cheers,
Magnus