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Old 25th Mar 2023, 12:07 am   #1
Tirebiter
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Default Revox A700 Power Supply Problem

Hello one and all, I was searching for a forum which has a lot of experience with vintage gear, specifically Revox machines, and this looks to be a very informative site. Hopefully I can pick your brains and figure out how I have disabled my A700.

I have had a Revox A700 for a couple of years and began the process of cleaning it up and doing some of the typical measurements and adjustments to get it into top shape. Nothing overtly wrong but wanted to understand where the baseline is before looking at cap replacement. I know that it has all the original caps and have also been mulling over going through a total cap replacement program. Thus my current quest to get some baseline measurement as it is.

Which brings me to a self induced problem. I have the service manual for the instrument and began working through the Electrical Adjustment Section. It starts out with a check of the +12V/-12V/+5V section (1.067.160) and if off, adjust trim on R14 and R23. Initial readings were about -9.9V, +8.7V and +4.9V. So I gave the trim pots small tweaks which resulted in no apparent movement on voltages. Suddenly, the capstan quit running and all of the control push button lights came on. With that I quickly shut the unit off and unplugged it.

No odd smells or plumes of smoke, just the fact that it has suddenly lost it's mind which in and of itself is to me, not a good sign.

Also of note, the SM schematics for A4 show an early and late version. According to my serial # it should be a late version which would have board 1.067.161 as opposed to XXX.160 but 160 is installed. Date stamps on the frame castings say it is '76 vintage. Don't know if that is relevant to my issue but there it is in case it is.

Any help would be appreciated...

Thanks,
Brock
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Old 25th Mar 2023, 2:24 pm   #2
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Default Re: Revox A700 Power Supply Problem

I would start by replacing all 4 2200uF capacitors in the power supply. Also you MUST change all the RIFA capacitors on the motor control boards and the relay board. Failure to do so will cause you a lot of problems when they fail and cost you dearly.
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Old 25th Mar 2023, 3:34 pm   #3
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Default Re: Revox A700 Power Supply Problem

It sounds like you've lost one of the supply rails. It would be as well to check the fuses on the PSU. I'd be inclined to replace the trimpots as well as the main smoothing caps, since the fault came on once you started tweaking and they are open-frame components of a certain age.

The later supply board used chip regulators, if memory serves, but the function is exactly the same.

Oh, and do change the RIFAs as a matter of urgency. These little jokers look like glacier mints and act like smoke bombs. Fortunately, the failure is usually more spectacular than damaging, but you really don't want to take the chance, nor deal with the appalling smell, which can take days to dissipate. I have several of these machines and once tried reviving one without first changing the RIFAs - "just a quick check". Nine of 'em went at once...
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Old 25th Mar 2023, 4:23 pm   #4
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Default Re: Revox A700 Power Supply Problem

Quote:
Originally Posted by Ted Kendall View Post
It sounds like you've lost one of the supply rails. It would be as well to check the fuses on the PSU. I'd be inclined to replace the trimpots as well as the main smoothing caps, since the fault came on once you started tweaking and they are open-frame components of a certain age.

The later supply board used chip regulators, if memory serves, but the function is exactly the same.

Oh, and do change the RIFAs as a matter of urgency. These little jokers look like glacier mints and act like smoke bombs. Fortunately, the failure is usually more spectacular than damaging, but you really don't want to take the chance, nor deal with the appalling smell, which can take days to dissipate. I have several of these machines and once tried reviving one without first changing the RIFAs - "just a quick check". Nine of 'em went at once...
Thanks Ted, I did check the PS fuses and all are good. So for my edification, is it possible that I over adjusted the trimpots in my zeal to make a change or did the change simply precipitate the inevitable?

Thanks,
Brock
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Old 25th Mar 2023, 4:25 pm   #5
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Default Re: Revox A700 Power Supply Problem

Quote:
Originally Posted by Michael Maurice View Post
I would start by replacing all 4 2200uF capacitors in the power supply. Also you MUST change all the RIFA capacitors on the motor control boards and the relay board. Failure to do so will cause you a lot of problems when they fail and cost you dearly.
Thanks Michael, I hear over and over that these caps need to be replaced and have now forced the issue. A new project to keep me occupied.

Brock
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Old 25th Mar 2023, 5:12 pm   #6
Ted Kendall
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Default Re: Revox A700 Power Supply Problem

Quote:
Originally Posted by Tirebiter View Post
Quote:
Originally Posted by Ted Kendall View Post
It sounds like you've lost one of the supply rails. It would be as well to check the fuses on the PSU. I'd be inclined to replace the trimpots as well as the main smoothing caps, since the fault came on once you started tweaking and they are open-frame components of a certain age.

The later supply board used chip regulators, if memory serves, but the function is exactly the same.

Oh, and do change the RIFAs as a matter of urgency. These little jokers look like glacier mints and act like smoke bombs. Fortunately, the failure is usually more spectacular than damaging, but you really don't want to take the chance, nor deal with the appalling smell, which can take days to dissipate. I have several of these machines and once tried reviving one without first changing the RIFAs - "just a quick check". Nine of 'em went at once...
Thanks Ted, I did check the PS fuses and all are good. So for my edification, is it possible that I over adjusted the trimpots in my zeal to make a change or did the change simply precipitate the inevitable?

Thanks,
Brock
No, excessive tweaking of itself shouldn't do harm, but if the pot fails the supply won't work properly, as a feedback path goes only through the slider. I'd regard these pots as life expired and replace.
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Old 28th Mar 2023, 10:46 pm   #7
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Default Re: Revox A700 Power Supply Problem

Busy getting the overhaul started...so far, all of the Frako and Rifa capacitors from the Power Supply, Relay and Capstan Motor Control boards have checked out good using my Fluke 187...and visually, none of them have looked worse for wear.

I have some Panasonic film capacitors ordered up but have had trouble finding axial electrolytic caps. I see plenty of choices for radial and even noticed that the Nagravox kits appear to use radial caps. Other than the obvious orientation question, any issues I should be aware of if I go the radial route?

Also, some time prior to my current problems, I got "bit" while I was doing some electrical checks when I brushed against one of the motor transistor covers on the top rail. I did some checking and found that there was significant DC voltage to ground on all three...is that to be expected?

Thanks,
Brock
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Old 29th Mar 2023, 11:52 am   #8
Simon Gittins
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Default Re: Revox A700 Power Supply Problem

Rifa capacitors can look good and test good right up to the moment they let the smoke out!
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Old 29th Mar 2023, 12:32 pm   #9
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Default Re: Revox A700 Power Supply Problem

Good to know, the replacements are on the way, except for the 0.1uF that was hidden behind the plastic cover for what once housed the mains voltage selector....
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Old 29th Mar 2023, 9:42 pm   #10
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Default Re: Revox A700 Power Supply Problem

Quote:
Originally Posted by Tirebiter View Post

Also, some time prior to my current problems, I got "bit" while I was doing some electrical checks when I brushed against one of the motor transistor covers on the top rail. I did some checking and found that there was significant DC voltage to ground on all three...is that to be expected?

Thanks,
Brock
Yes and you can get quite a belt off them. Guess how I found out!
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Old 29th Mar 2023, 10:34 pm   #11
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Default Re: Revox A700 Power Supply Problem

Quote:
Originally Posted by Michael Maurice View Post
Quote:
Originally Posted by Tirebiter View Post

Also, some time prior to my current problems, I got "bit" while I was doing some electrical checks when I brushed against one of the motor transistor covers on the top rail. I did some checking and found that there was significant DC voltage to ground on all three...is that to be expected?

Thanks,
Brock
Yes and you can get quite a belt off them. Guess how I found out!
Duly noted if I have to take mine out of it's case
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Old 30th Mar 2023, 7:17 am   #12
Ted Kendall
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Default Re: Revox A700 Power Supply Problem

The Studer A80 does the same sort of thing, which is why BBC machines have vented covers and warning stickers. No room for those on the A700, unfortunately.
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Old 15th Apr 2023, 10:47 am   #13
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Default Re: Revox A700 Power Supply Problem

Hi Tirebiter,

all electrolytics are of the age where attention is required. The Rifa Bombs should be changed on first sight. When I got the smoke, the machine was in STOP.

The 5v supply is referenced the +12v with hi tolerance resistors. From past experience in dealing with TTL Logic chips, I adjusted the 12v supply to make the 5v supply read 5.1v and then adjusted the -12v supply to match the reading of the +12v supply.

For the first power up following the component changes, I ran the machine up on a Variac whilst monitoring the 5v supply and set it to 5.1v once it was seen to regulate.

As a tip, when in play if there is any instability in the +/-12v supplies, the tape tensioners wiggle.

Cheers

Alphonse01
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