[sdiy] Juno 106 repair question (beyond simply replacing 80017a/5534a)
jim at antsclimbtree.com
Fri May 18 22:19:44 CEST 2012
On May 18, 2012, at 12:43 PM, Tom Wiltshire wrote:
This is a great bit of info to work from.
> You mean you've tried swapping the 55134 with a clone and the problem remains? That makes it highly unlikely that the problem is in the 55134.
The 55134 / MC5534 (whatever you wanna call) it is actually an original part -- NOS or salvaged, whatever Doug Terrabone sells. But yes, I get the same result from my original IC as the replacement that I recently bought, and it seems unlikely to me that both failed in the same way (unless the comparator is a common weak point here).
> No, you're right, the Juno 106 uses the PWM CV to switch the pulse wave off when required (after all, why use an extra wire for the job?).
> If the PWM CV is between the valid values (6V to 0.6V) then there *ought* to be pulse wave coming out. If it's more negative than that, then it could be ok, but switched off. The Service Manual doesn't commit itself to *exactly* where the pulse switches off - just that it's at 95% at 0.6V and that it's off at -0.8V. Presumably somewhere between those extremes it goes first to 100% and then dies.
> So I'd test those voltage again. If they're between those normal limits that the service manuals give, then that would suggest the comparators on the 55134 are dead.
Recently I've only tested continuity here but given that all 3 55134 chips are given the same PWM CV and 4 voices work, it seems like the problem wouldn't be the PWM CV itself.
> A possible cause of a reduced ramp wave would be a dead 4052 for that pair of voices...
Ah, yeah, I somehow overlooked that chip entirely. It would be easy enough to swap, I have a stack of them. And if not that, there's the op-amps and caps you mentioned.
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