[sdiy] Jupiter 8 not booting

Adam Inglis (synthDIY) synthdiy at adambaby.com
Tue Feb 2 05:03:59 CET 2021

Could I trouble some patient soul to cast their eyes over this circuit and explain what should happen?
The JP8 CPU reset circuit is in the grey box on the right half of the image.

My jupe has been taking its time booting these last few months, 5 or 10 minutes, and now sometimes is refusing to boot at all. (Occasionally it will boot as usual in a couple of seconds after going through the tune routine, so the fault appears to be intermittent). 
Up on the bench, when failing to boot, I measure the reset pin 26 on the CPU as 1.4 v. When it boots successfully, that voltage takes a second to go from 1.4 up to 4.9 volts.
So I’m suspecting something in the reset circuit.

When working:
Transistor 1 measures
E  = 00 v
C = 0.14 v
B = 0.59 v
Transistor 2 measures
E = 00v
C = 4.9 v
B = 0.14 v

When not working the only change is the collector of TR2 doesn’t come up above 1.4  v

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