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Could use some help on a +B puzzle in a Kenwood KR-5600

tusntuk2

tusntuk2

Active Member
I'm restoring a Kenwood KR-5600 that was in working condition, at least I know the FM and the Tape inputs were working, and I started with the power supply. The KR-5600 has a history of heat problems with its design, so I boosted the clumped resistors to 3 watts and the caps to UPW 100/50v. There was some slight solder-trace burning, so I folded over leads to strengthen connections. The amp passed the DBT test, the relay clicked on, the FM and Tape inputs (Audio amp) work fine, but there is no Phono or FM mute working.

I checked the PS pcb, its a 2630-10 model, Pins 3 and 4 supplying the +/- B voltage for the Pre-amp at 18v, but here is the puzzle - Pin 3 is +12.7v, and Pin 4 is -22.3v. Now, the main rail tested at +/- 40.2V, so maybe the 22V on Pin4 is OK, but what happened to Pin 3? The schematic shows this -B to go to both the Phono Preamp and the Muting circuit, and neither of them work. I'm hoping to find the problem.

The Phono Preamp has two IC's. The Muting circuit has 2 or 3 FET's. I am not an engineer to understand what the off voltage does to these circuits, but it must play some part.

Can I disconnect Pins 3 and 4 in the Power Supply and then power up the PS board to measure these pins? Without other consequences, I mean! That would identify the Power Supply as the culprit.
Is there a way to test IC's? The Preamp board is difficult to extricate, so I'm hoping I don't have to move it.

Any other ideas? Thanks a lot for chipping in!
Jim
 
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That would be my first step. If pins 3 and 4 are only supplying voltage to the FM muting and phono I would just quickly disconnect those circuits and measure pins 3 and 4 unloaded. It could be that something is dragging the positive rail down. Measure 3 and 4 and report your findings.

Dan
 
Well that might have been the quickest fix I ever did. When I disconnected Pin 3 and 4 I got -17.91v on 4 (spec is -18v), and 0.0 (!) on 3 - and the only part that could have failed was a small electrolytic I had just installed. Turns out it was bad, so, in went a new one. Now there is +17.91v on 3, and -17.91 on 4. Perfect! Even better, both the FM muting circuit and the Phono Preamp started working correctly as well. YAY!!! We fixed it!! I love when that happens.

Dan, thank you for your support! Just what the doctor ordered! Good Form!
Jim
 
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