[sdiy] Juno 106 repair question (beyond simply replacing 80017a/5534a)

Tom Wiltshire tom at electricdruid.net
Fri May 18 21:43:50 CEST 2012

Hi Jim,

> I'm trying to troubleshoot a pair of voices on my Juno 106 with no Pulse output. The saw on voices 1 and 2 work fine, the square just makes a click. Because the same issue appears on both, the VCFs/VCAs work fine on the voices, I take this to be a problem with the 5534a which produces both voices 1 and 2. (After all the outputs of 5534a pins 14 and 16 are the sums of PWM and saw, the latter of which comes through fine.) 

Your logic so far seems flawless!

> Both 80017a's had been replaced with clones and seem to work fine now. (I really bungled the replacement of one but think I repaired the traces etc sufficiently and again, it works fine.) I tried replacing the 5534a with a replacement from a reputable source (Synth Parts) but the result is the same.

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.

> What next? I've attached the relevant portions of the schematic at Muff's (see link below). I can verify that pin 12 of all 5534a's (PWM CV) is getting signal. I see no specific PWM control (on/off etc) otherwise.

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. If the problem remains with a new 55134 (which you say it does) then it would have to be the ramp that going into the comparator that's the problem. Although you say the ramp is ok, is it same level as the other voices? If it were quieter, the pulse wave would reduce to a brief pulse (a click) long before it were supposed to. You've been through some of this process already, but if you'll bear with me, I'm thinking aloud.

A possible cause of a reduced ramp wave would be a dead 4052 for that pair of voices. If it were providing much more resistance that it's supposed to, the voices would have a reduced ramp volume, which might give you the results you see. Some weirdness of the capacitors *might* just about cause similar symptoms, but it's pretty unlikely that they'd both go down in the same way at the same time.
It's also possible that the problem lies further back. If the 4052 is not receiving the sufficient voltage, the ramp would be low. That would suggest a dead op-amp or dead caps in the S&H that feeds that pair of voices. So test the DCO CVs that are feeding those voices. If they're low, the problem is not in the waveform generator chip.

I'd start with the DCO CVs and work forwards from there.

Good luck!


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