Saturday, December 20, 2014

Juno 60:Dead notes, distortion.

This Juno 60 came in with a bunch of weird key scanning issues; some notes didn't work some that did were distorted. It was clear that the issues were more serious than dirty contacts, so after briefly looking at them I started probing with the scope on the data lines. Finally I found that the VCC for IC 52 & 53 was low (about 3 volts when 5 volts is expected). Although I initially thought that it was simply a broken trace, on careful inspection of the schematic I noticed that those chips have a separate VCC, produced by OP amp IC 51. The 15 volt rail is voltage divided down to 5 volts at the input to this op amp, and the resulting 5 volts coming out on pin 7 is used as the VCC for these chips... which serve as a DAC for the KCV (filter control voltage).
It turned out the real issue was with capacitor 128... it was leaking and dragging down the voltage at that voltage divider. This caused the KCV (filter control voltage) to come out at 3 volts instead of 5, and caused distortion coming through the filters.
It is rare to see a ceramic capacitor fail, but clearly with an older instrument like this components that are usually reliable can ultimately go bad.

In red are the OP amp VCC supply for the filter DAC

Here is where it is located on the actual PCB.

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