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CR-620 Troubleshooting

Brian J

Brian J

New Member
Hello all, first-time poster but (relatively) long-time lurker here!

I just picked up a CR-620, and I plan to restore it and gift it to a close friend. Unfortunately, the receiver was DOA. Here are the particulars:
  1. It powers on,
  2. The red faceplate LEDs light,
  3. The incandescent lights do not light (several/all are burned out),
  4. All of the fuses are intact,
  5. The speaker relay clicks on several seconds after power on, and clicks off upon power off,
  6. There is no output at the speaker terminals or the headphone jacks (no noise, hum, absolutely nothing)
I tested the E-C-B voltages of all transistors in the power supply, protection, and amplifier circuits while the receiver was powered on (per Avionic's instructions to Jpowell and others in the various threads about CR-620 repair). Those voltages are available at the link below. I also removed the TO-3 package transistors and tested them with my DMM in diode mode. Those results are on a separate sheet also at the link below.

https://docs.google.com/spreadsheets/d/1bcu5s3qDzLy90Lml4l0vjf40njHhhYlz0aoS6pTJ0iI/edit?usp=sharing

It looks like there are multiple shorted transistors, which I will replace, along with all of the electrolytics. My questions are:
  1. Are there any other bad transistors besides the shorted ones?
  2. What other diagnostic work should I do?
Any advice/feedback would be greatly appreciated :)
 
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Thank you for the replies, Avionic! I was hoping you would respond, given your great advice in other threads :)

avionic said:
Which transistors do you believe are shorted?
Click to expand...

You've got me there. I didn't make that now-dubious claim based on the measurements I made, but because I remembered someone said in another 620 post that if you find +/- 40VDC at R775/776 that there are shorted transistors upstream from them. So, last night when I was poking around, I thought I measured +40V at R775/776. But I didn't, it was two other resistors. I guess I just got confused. R775/776 measured at 0.018V and 0.007V, respectively.

avionic said:
Which audio source are you using?
Click to expand...

Tuner. Seller said he "couldn't get any sound out of it" but I didn't vet his claim 100%. I will try all inputs and report back.
 
I have good news: I managed to get sound out of the receiver! I think the volume pot is extremely dirty. I didn't get anything out of it, then I got just the tiniest bit of volume at all sources with the volume all the way up. Working the volume up and down seemed to finally get it working.

The other controls are scratchy/dirty, so I will clean it and the other pots with a suitable contact cleaner. The loudness knob causes crackling on the left channel only for some reason.

There are still quite a few small tasks I'd like to do :) all of the thermal grease is dried up, and the output devices don't have insulators -- should they?
 
The TO-3 transistors case is the collector. Collector always has + or - rail voltage on it. The heatsink is chassis ground..Shorting raw +/- B to ground = vaporized fuse ( hopefully thats the extent of the damage. )
 
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