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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 9th Nov 2019, 8:37 pm   #1
hallwod
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Join Date: May 2019
Location: Flitwick, Bedfordshire, UK.
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Default Revox A77 MK1 Power Supply

I completed a recap of the Tape Transport board, Speed Control (.725) and PSU (.540). Also recapped the Switch Board (.435)

This machine had the vari speed control mod which I have removed and (tried) to return to the Speed Control board to its original state. The original mod invloved moving resistors R212 and R213 to a jack socket added to the front panel. So all I did was to put these two resistors back on .725

I then put the transformer etc back in the machine and carefully re-wired all the spades.

Switched on an noticed the EOT lamp illuminated for a few seconds, then went dim followed by the unmistakable sound of a fuse blowing!

Even so, the tape transport buttons still activated Play and Stop and the pinch roller arm and brake solenoids worked but seemed very sluggish. No power to the reel or capstan motors.

I tested fuses F102 and F103 and found that F102 (630Ma) had blown. Unwired all the spades, removed the transformer and attached boards and found that:

1. C213 was not soldered properly as was R213
2. E7 and E8 wires had detached themselves from the under side of the board (probably as a result of to much brute force in threading the wiring looms beween the boards!)

I carefully re-soldered everything and checked I'd put the re-caps in the right way round. With everything disconnected from the PSU and Speed Control boards and them and the transformer out of the machine (with a new F102 fuse) I powered up and - F102 blew again straight away.

So my plea for help is where do I start looking for what apears to be a short linked to F102. I should add that F103 is OK but I don't seem to be able to get any sensible voltages at the test points linked to that circuit.

Hoping that someone with more experieice can point me in the right direction.

Thanks

David
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Old 9th Nov 2019, 9:54 pm   #2
ms660
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Default Re: Revox A77 MK1 Power Supply

Have you checked the bridge rectifier (D101) If it's ok does the fuse blow with ED1 & AD1 disconnected?

Lawrence.
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Old 9th Nov 2019, 9:59 pm   #3
hallwod
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Default Re: Revox A77 MK1 Power Supply

Lawrence,

Thanks for the response. Everything is disconnected anyway and the fuse still blows. Bit of a novice with this - how do you check the Bridge Rectifier? I have DMM and scope.

David
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Old 10th Nov 2019, 12:45 pm   #4
ms660
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Default Re: Revox A77 MK1 Power Supply

A couple of videos (other videos are available) showing the testing of a bridge rectifier and a single diode component using a DMM's diode test function:

https://www.youtube.com/watch?v=dqkT6hF0O3E

https://www.youtube.com/watch?v=gzh8ICLMXzQ

Lawrence.
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Old 10th Nov 2019, 1:23 pm   #5
hallwod
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Default Re: Revox A77 MK1 Power Supply

Lawerence,

Thanks and appreciated

David
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