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CR-820 relay buzzing

Results of todays testing:
Diode test C731: Started at 1.70 and ramped up to O/L. Reversed leads and steady at 0.580.
Resistance check of VR701: 2.4kohms and steady
TR707 pins to ground: B +34.1vdc, C +36.2vdc, E +34.1vdc.
 
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Nuklhed said:
TR707 pins to ground: B +34.1vdc, C +36.2vdc, E +34.1vdc.
Click to expand...
So Vbe is about 0V, suggests "no" current flow so no volt drop.
Looks like TR711 and TR713 need to be ON for current flow, voltages for each, b/c/e to GND, also test R747 and R751 (not open?)
 
R747 and 751 were removed, tested good and reinstalled.

The voltages on all pins of TR711 and TR713 are very unstable.
TR711 all pins reading approximately -21vdc, jumping around a lot.
TR713 E and B approximately -21vdc, jumping around a lot. Collector was so unstable it was hard to assign a value…..
 
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Nuklhed said:
The voltages on all pins of TR711 and TR713 are very unstable.
TR711 all pins reading approximately -21vdc, jumping around a lot.
TR713 E and B approximately -21vdc, jumping around a lot. Collector was so unstable it was hard to assign a value…..
Click to expand...
Make sure your test probes/grabbers are making a good electrical connection. Component leads are likely all suffering from minor oxidation by now.
 
Nuklhed said:
TR711 all pins reading approximately -21vdc, jumping around a lot.
Click to expand...
Unstable TR711b would point to TR705, TR701, TR703.
Measure TR705b, if it's stable then replace TR705. If it's unstable then replace TR701 & TR703 with a hfe matched pair (KSA992?) TR705b is the "input", if the input is unstable then bs in bs out, look elsewhere. If input is stable then TR705 is going intermittent.
 
Thanks so much guys. Pete, appreciate the link, I’ll definitely check it out.
I gave up on trying to hook my grabbers under transistors and have been using needle leads on the back of the board (unless those chassis/ground bars are in the way)
Will check TR705 and I have a bunch of 992’s on hand. Will make sure to find some hfe matches.
 
Wow guys. In the course of working on this bugger I saw something I have never seen. Between the pads of a transistor there is a “trench”. Looks like maybe burned into the board. Hard to see unless you remove the transistor and clean it up good and use my high mag glasses. If I knew how to post pics….
Anyway turns out when soldering the components in, the trench between the pads fills with solder and causes a short. I have never seen this before. After seeing sparks on the underside of the unit I found these “trenches “ in 2 locations!
So have you guys dealt with these? If so maybe some non-conductive epoxy??
Can’t thank you guys enough!
 
Need pix's. Save file on your pc then hit attach file on bottom left of post. Drag and drop probably works.
What pins of what transistors/resistors/... would be short??
After removing the solder bridge is the amp now working normal?
 
This is TR806…I was thinking of some non-conductive epoxy on a toothpick? So I have not yet repaired.
 

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Nuklhed said:
This is TR806…I was thinking of some non-conductive epoxy on a toothpick? So I have not yet repaired.
Click to expand...
Need to totally remove all traces of carbon from that divot! I would use a dremel cutting disk. Make a thin, shallow, slice between the traces and cut just deep enough to clear the burnt part of the phenolic from the board. Carbon is conductive!

A clean cut will not require any filling.
 
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Repairs complete. Soldered the components back in. Checked for continuity. Turned on the unit with zero volume, speakers off….and blew the 4 amp main fuse…☹️
 
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