30-07-2015, 08:08 AM
(29-07-2015, 09:52 PM)JohnBS Wrote: Nick;I have already got it down to a suspect capacitor. The offset is appearing after the first stage of amplification where there are a number of those tubular capacitors to check out in the HF comp part of the circuit. I also have a bench power supply with center tapped outputs that could easily be clipped onto the test points so that is just less effort than taking the frame down from its new home.
Quote:my shack test bench
That's quite a collection: have you ever tried to estimate how much those instruments would cost if you'd bought them new? (or perhaps you did!)
Refugee:
If you have one channel of your 7A26 working, I'd suggest you plug it into the LH slot of the main-frame with the LH side-cover removed. You can then check the AC and DC's of the two channels with, say, a 40mV p-p signal applied to both and the inputs set at 5mV/div. You should get close to 40mV p-p out of the FET follower (R150)*. By progressing down the signal path you should find where things have gone awry. Most problems with my Y amps have been associated with the input attenuators, and in two instances, a bizarre component failure whereby a small white tubular ceramic capacitor (c 2pF) has grown to about 10x the value!
John
* if not, there's an issue right up front!
