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CR-620 stuck in protection

falsefruit

falsefruit

New Member
Received a CR-620 in non-working condition, found the output transistors shorted and replaced all four. The unit now powers on but has no audible relay click.
  • I read ~20 V or so on both R775 and R776 with the new transistors installed.
  • Still getting DC voltage on output transistor emitters - the base voltages also read much higher.
  • TR701 measures close to listed values, but getting -30 V on collector.
  • TR703, similarly getting about -30 V on collector and +15V at the base (rather than listed 0.08 V). This is also happening with TR704.
  • TR802, 803, and 804 in the power supply measures spot-on.
Would appreciate a nudge in the right direction here. Thank you for reading!
 
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Measure the DC voltage on the output transistor collectors ( metal case )...
Should be +39v on the 2SD's (NPN). And -39v on the 2SB's. (PNP)..
 
OK Now measure the base voltage on the output transistors.. Should be around + and - .6vdc ...
 
Reading about -40 V on the 2SDs and +40 on the 2SBs for the base voltages - this is just like with the collectors. Getting about 27-28V with all the emitters.
 
Something in each amplifier is partially holding the outputs to rail... Check all the circuit board fastener screws around the amplifier circuit board. Some of the screws are responsible for making a good circuit board connection to chassis ground.
 
Did some testing - relay will click with both the 2SD371s (TR723, 724) in place, but when any of the 2SB531s (TR725, 726) are added, no click.
 
Did some more digging and realized that the output transistors were in the incorrect positions - I don't know if this was an oversight on my part or from the previous owner, but R775 and 776 now reading close to 0 V.
 
I definitely said the above too early. I realized this when idle current adjustment across both sets of TPs read 0 mV/couldn't be adjusted.

A re-check of the transistor voltages shows 40 V on the emitters of TR719 and TR720 (instead of 0.6 V listed in manual). Voltages around TR707, 709 are off as well. Annotated voltages in green below. I measured resistances in-circuit with power off across R749-752 - about 15-20 Ohms. Could some of these TRs be suspect here?
annotated.png
 
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FWIW, my experience with several 620s is the electrolytics aged very poorly or something was done in production that caused them to fail early. Values tended to be low but esr was normal, so measuring with an esr meter is useless. Just replace them all.
 
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