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Uncle Bulgaria · 2nd Nov 2019, 3:31 am

I found it! Tracking back with my newfound oscilloscope skills, it turned out R66 was pulling the input to ground because of a short, probably from the point when the smoke escaped.

It now plays on both channels and I'm going to go through the service manual to check the operation points.

Off topic, and perhaps the subject for another thread: what could cause the hiss in the headphones when there are no audio boards inserted (so nothing behind the volume control is in circuit) and how could I ameliorate the +2.5V thump spike on switch on, and the -2V decay spike on switch off? I've replaced the coupling capacitors C35 and C36, also C31 and C33 from ICs 1 & 2. Perhaps it's something to do with IC1/A and IC1/B that have some role behind the volume control? Are they some pre-amp circuit for auxiliary inputs that are injecting noise further down the chain?

2nd Nov 2019, 3:31 am	#6
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,339	Re: Uher CR 240 solenoid does not disengage. I found it! Tracking back with my newfound oscilloscope skills, it turned out R66 was pulling the input to ground because of a short, probably from the point when the smoke escaped. It now plays on both channels and I'm going to go through the service manual to check the operation points. Off topic, and perhaps the subject for another thread: what could cause the hiss in the headphones when there are no audio boards inserted (so nothing behind the volume control is in circuit) and how could I ameliorate the +2.5V thump spike on switch on, and the -2V decay spike on switch off? I've replaced the coupling capacitors C35 and C36, also C31 and C33 from ICs 1 & 2. Perhaps it's something to do with IC1/A and IC1/B that have some role behind the volume control? Are they some pre-amp circuit for auxiliary inputs that are injecting noise further down the chain?