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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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7th Jul 2022, 4:01 pm | #1 |
Pentode
Join Date: Sep 2017
Location: London, UK.
Posts: 115
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Revox A77 Speed Issue
I'm having some trouble with a Mk3 Revox A77 and any advice on what could be the problem would be very much appreciated. The issue is the 7.5 ips speed works fine and I get 1600hz at the blue and brown wires, however 3 3/4 ips is running a bit slow and the maximum reading I can get after adjusting the potentiometer is 750hz. The reading I get also varies a lot - it never remains steady. I've taken the board out and change all the electrolytic capacitors but that didn't help. I've also checked C208 & R218 which are on spec and replaced D201 and D202. I've also changed P201 just in case it was making intermittent contact. I'm not sure what else to check? What's puzzling me is the issue only affects the low speed setting. High speed is fine.
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7th Jul 2022, 5:02 pm | #2 |
Dekatron
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
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Re: Revox A77 Speed Issue
It might be worth checking that the tacho head has not shifted, although this usually affects wow rather than absolute speed.
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7th Jul 2022, 5:45 pm | #3 |
Pentode
Join Date: Sep 2017
Location: London, UK.
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Re: Revox A77 Speed Issue
You might be right there. Went to check and the tacho is broken off it’s mount. I’ll fit a replacement and see whether that fixes it.
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8th Jul 2022, 10:22 am | #4 |
Pentode
Join Date: Sep 2017
Location: London, UK.
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Re: Revox A77 Speed Issue
The problem did seem to be the tacho head. It had cracked at the base although it looked OK in situ. I tried replacing it with a good one from a mk1 but that didn't work so used some araldite to repair the broken one. It all seems to be working fine now. Thanks for the advice - I would have spent hours trying to find a non-existent fault on the board.
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8th Jul 2022, 12:12 pm | #5 |
Dekatron
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
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Re: Revox A77 Speed Issue
Excellent! The damaged mounting will have increased the spacing between head and rotor. At 7.5, there was still enough signal to drive the head amp into limiting, so lock was retained, but at the lower speed the 6dB drop in output took the signal below limiting, so there wasn't a solid square wave for the servo to get hold of.
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8th Jul 2022, 1:38 pm | #6 |
Moderator
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Location: Fife, Scotland, UK.
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Re: Revox A77 Speed Issue
Mine too has its base araldited. I used the original slow type, by the way. It was what I had at the time.
David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
8th Jul 2022, 2:20 pm | #7 |
Pentode
Join Date: Sep 2017
Location: London, UK.
Posts: 115
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Re: Revox A77 Speed Issue
I think I spoke too soon on this. I put everything back together and it worked for about 20mins. Then the main power transistor started smoking. I replaced that but now it’s running slow and variable at both speeds. About 1300 hz at 7.5ips but up and down all over the place.
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8th Jul 2022, 2:33 pm | #8 |
Dekatron
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
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Re: Revox A77 Speed Issue
Check the diodes in the bridge circuit.
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8th Jul 2022, 2:58 pm | #9 |
Pentode
Join Date: Sep 2017
Location: London, UK.
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Re: Revox A77 Speed Issue
I did change the bridge rectifier when the board was out just in case. That was before Q209 blew. Could that have damaged the recitifier? The 21v supplies are working.
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8th Jul 2022, 4:04 pm | #10 |
Dekatron
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Location: Kington, Herefordshire, UK.
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Re: Revox A77 Speed Issue
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8th Jul 2022, 5:07 pm | #11 |
Pentode
Join Date: Sep 2017
Location: London, UK.
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Re: Revox A77 Speed Issue
I've changed the bridge rectifier but the fault is still there. The capstan motor is hunting around with the speed varying up and down. Eventually it seems to settle on a constant low speed (around 1350 hz at 7,5ips) so there's some regulation happening but not at the correct speed.
I've swapped the motor and tacho for a spare just to eliminate the tacho and also no change. Everything was working for a short while after I changed the main transistor (apart from a broken trace on C203). I'm guessing Q209 has taken something else out when it failed. I had real problems with this board on another machine with components testing OK of the board but failing under load. |
19th Jul 2022, 8:44 pm | #12 |
Pentode
Join Date: Sep 2017
Location: London, UK.
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Re: Revox A77 Speed Issue
I've not been able to fix this fault. However much I adjust the discriminator the maximum speed I get is 1440 hz. It's hunts for a bit then becomes steady at least but too slow. All the problems started when the power transistor blew. The tacho is fine. Swapping the motor with a known good doesn't change anything. The voltage readings on the board are B 1.975V (recommended 2V), C 10V (on spec), D 1.245V (recommended 1V) and E 86V (recommended 80V). At the emitter of Q204 the service manual says 11V and I get 10.64V. Not sure if that means anything. Any advice would be appreciated as I'm not sure what else to check.
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21st Jul 2022, 1:06 am | #13 |
Octode
Join Date: Jul 2006
Location: Perth, Western Australia
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Re: Revox A77 Speed Issue
Have you changed C213, the 0.47uF Rifa capacitor? We'd normally change it on sight , as well as the others on the relay PCB.
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21st Jul 2022, 8:51 am | #14 |
Pentode
Join Date: Sep 2017
Location: London, UK.
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Re: Revox A77 Speed Issue
Yes I have. In fact I've checked every electroytic capacitor, diode and transistor on the speed board and the bridge rectifier with a multimeter and swapped them for new as I'd taken them off the board anyway. However the fault is still there. When I turn the machine on the capstan speeds up then slows right down (more than it would normally do on start up), speeds up again, slows down then settles on a steady but slow speed. I've swapped the discriminator for a known good one but there's no change. Fortunately I have a spare working board I can use on my machine so I may give up on this one.
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21st Jul 2022, 11:51 am | #15 |
Pentode
Join Date: Sep 2017
Location: London, UK.
Posts: 115
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Re: Revox A77 Speed Issue
The square wave looks odd to me so maybe the fault is somewhere in the AC amplifier section.
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21st Jul 2022, 11:27 pm | #16 |
Dekatron
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
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Re: Revox A77 Speed Issue
Perhaps the squaring circuit is faulty or not being driven hard enough?
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22nd Jul 2022, 11:31 am | #17 |
Pentode
Join Date: Sep 2017
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Re: Revox A77 Speed Issue
What do you thycould cause it not to be driven hard enough? I’ve changed the transistors and caps. My other thought was perhaps there’s a lose connection or contamination causing a short somewhere on the board. Or maybe a faulty resistor. I’ve fitted a spare board for now so at least the A77 is working again.
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22nd Jul 2022, 12:30 pm | #18 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: Revox A77 Speed Issue
It might be worth disconnecting the tacho and isolating the connection to the input of the discriminator transformer and feeding in a 50mV (RMS) signal in from a sinewave generator and 'scope its progress through the amplifier, if anything's untoward then take a full set of voltage readings for the amplifier transistors (Q201 through to Q205)
Lawrence. |
22nd Jul 2022, 4:07 pm | #19 | |
Pentode
Join Date: Sep 2017
Location: London, UK.
Posts: 115
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Re: Revox A77 Speed Issue
Quote:
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30th Jul 2022, 2:05 pm | #20 |
Pentode
Join Date: Sep 2017
Location: London, UK.
Posts: 115
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Re: Revox A77 Speed Issue
The problem seems to lie around R210. I’ve removed it from the board and it’s fallen apart. Hopefully a new resistor will fix the speed variations.
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