Revox A77 MK1 Power Supply - UK Vintage Radio Repair and Restoration Discussion Forum

hallwod · 9th Nov 2019, 8:37 pm

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

ms660 · 9th Nov 2019, 9:54 pm

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

Lawrence.

hallwod · 9th Nov 2019, 9:59 pm

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

ms660 · 10th Nov 2019, 12:45 pm

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.

hallwod · 10th Nov 2019, 1:23 pm

Lawerence,

Thanks and appreciated

David

hallwod · 24th Nov 2019, 7:15 pm

Ok, Time for an update

I solved the fuse blow issue (cap on PSU inserted with wrong polarity)

My machine had a vari speed mod which i removed and put back to original state only to find that the PO had also modded the Reels on/off switch. Looks as if it was bypassed as the large left motor cap and capstan cap had been re-wired as spade connector FJ1 and F4 had been removed with F4 and F5 (from the motor) being joined at the motor capacitor and FJ1 missing completely!

With Power, Capstan and Transport Boards now properly(!) recapped, I tested all AC and DC voltages with the transformer out of the machine and all spades disconnected - all within Service Manual tolerences.

Put everything back together and re-wired the motor caps along with FJ1 and F4 back to original configuration.

Now the problems start!

Capstan motor spins at both tape speeds (increased revs at higher speed)
Tape transport buttons all work in that PLAY engages pinch roller and activates brake solenoid
FF and FR activates brake solenoid but.....
NO movement from either reel motor (bearing changed and they spin freely by hand) in PLAY or any other mode!
After engaing PLAY the Capstan motor slows down to a halt (at both speeds)
The transformer becomes abnormally hot and at one point there was a bit of smoke (possibly from a wire sleeve beginning to melt)

At this point I switched off and started to scratch my head whilst looking at the schematic for clues.

Has anyone and suggestions as to the best place to start troubleshooting? Connectors from the motors mis-connected? All the wires are discoloured to the point of being unidentifiable with the exception of the two striped wires from each and even those you can't identify the base colour.

It's a late Mark1 machine.

Thanks

David
________

9th Nov 2019, 8:37 pm	#1
hallwod Triode Join Date: May 2019 Location: Flitwick, Bedfordshire, UK. Posts: 21	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

9th Nov 2019, 9:54 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	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.

9th Nov 2019, 9:59 pm	#3
hallwod Triode Join Date: May 2019 Location: Flitwick, Bedfordshire, UK. Posts: 21	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

10th Nov 2019, 12:45 pm	#4
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	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.

10th Nov 2019, 1:23 pm	#5
hallwod Triode Join Date: May 2019 Location: Flitwick, Bedfordshire, UK. Posts: 21	Re: Revox A77 MK1 Power Supply Lawerence, Thanks and appreciated David

24th Nov 2019, 7:15 pm	#6
hallwod Triode Join Date: May 2019 Location: Flitwick, Bedfordshire, UK. Posts: 21	Re: Revox A77 MK1 Power Supply Ok, Time for an update I solved the fuse blow issue (cap on PSU inserted with wrong polarity) My machine had a vari speed mod which i removed and put back to original state only to find that the PO had also modded the Reels on/off switch. Looks as if it was bypassed as the large left motor cap and capstan cap had been re-wired as spade connector FJ1 and F4 had been removed with F4 and F5 (from the motor) being joined at the motor capacitor and FJ1 missing completely! With Power, Capstan and Transport Boards now properly(!) recapped, I tested all AC and DC voltages with the transformer out of the machine and all spades disconnected - all within Service Manual tolerences. Put everything back together and re-wired the motor caps along with FJ1 and F4 back to original configuration. Now the problems start! Capstan motor spins at both tape speeds (increased revs at higher speed) Tape transport buttons all work in that PLAY engages pinch roller and activates brake solenoid FF and FR activates brake solenoid but..... NO movement from either reel motor (bearing changed and they spin freely by hand) in PLAY or any other mode! After engaing PLAY the Capstan motor slows down to a halt (at both speeds) The transformer becomes abnormally hot and at one point there was a bit of smoke (possibly from a wire sleeve beginning to melt) At this point I switched off and started to scratch my head whilst looking at the schematic for clues. Has anyone and suggestions as to the best place to start troubleshooting? Connectors from the motors mis-connected? All the wires are discoloured to the point of being unidentifiable with the exception of the two striped wires from each and even those you can't identify the base colour. It's a late Mark1 machine. Thanks David ________