Revox A77 blown main fuse

[Steve Bass]

Hi

Newbie here, thanks for letting me join.

I’ve had my circa 1969 Revox A77 for about 32 years. Prior to me using it as a hi fi deck it had been used as data/program storage on a DEC PDP/8 minicomputer before then being stored for ten years.

It’s had its ups and downs, but a worrying fault appeared two days ago. After about 2 hours standing unused (reels stopped) I went into the room where it was located and heard a noticeable hum. The top of the case also seemed a lot warmer than usual. I started the reels then it powered itself off. Initial investigation showed the main 500ma fuse in the back had blown.

I know very little about how to tackle a repair, would it be safe to just replace the fuse initially and see how things go?

I’m based in Blackpool and the machine was looked after for many years by Colin at the Tape Recoder Centre in town until his retirement about five years ago.

If attention is needed would anyone within, say, an hour’s drive of Blackpool be able to undertake such work?



Thanks for any help and advice.

[DMcMahon]

Hello Steve,

It is possible that the mains voltage 500mA fuse blowing was just a nuisance age related failure (not common though), however there are potential issues with the A77 that can blow the fuse, so it would be prudent to investigate first before considering replacing the fuse.

On the Relay/Tape Drive PCB there are 3 capacitors C113, C114 & C115, these are located on the edge of the board opposite to the push button switches side.

They are 0.47uF in value and are used for EMI suppression of the AC voltage that is connected (via the electronics) to the 2 reel/spooling motors. The AC voltage is not at mains potential, but lower voltage from one of the mains transformer secondary windings. This secondary winding is not fused, so a fault here can blow the main 500mA fuse.

These capacitors are often referred to as Rifa which is a brand name. These capacitors can short out, sometimes visibly damaging the capacitor body.

So these 3 capacitors should be visibly checked to see if any obvious damage and measured to make sure none of them have short circuited. For future reliability it is also recommended to replace them even if they are OK.

They should be replaced by AC rated "X2" (safety spec) Metal Polypropylene film capacitors, 275 Volts AC is a good voltage rating.

Each capacitor has a 4.7 ohm resistor in series with it, namely R121, R122 & R123 (one next to each capacitor). If a capacitor shorts then it may also take out its resistor, but less likely, but still should be checked while checking the capacitors.

There are also 3 much larger metal can capacitors that are mounted above the Drive board, namely C151 (3.5uF for the Capstan motor), C152 (4.3uF for the right reel motor) & C153 (4.3uF for the left reel motor). These potentially can also be problematic but I think much less likely. These capacitors when bad can rupture their can and leak some of their electrolyte.

If all this means little to you then it would best for you to get a knowledgeable person to look at it.

David

[DMcMahon]

Should have said that even if one of the C113/C114/C115 capacitors do burn/get damaged that does not always result in the fuse blowing. Also note that if these capacitors do burn they can generate a strong smell.

[Michael Maurice]

Your symptoms, certainly the hum, assuming it was coming through the ampifier and speakers rather than a buzzing from the tape recorder suggest the problem lays in the power supply, however the capacitors that Dave mentioned do need replacing.

If you are not confident to fault find, then Brian Reeves in Cheshire is a Revox specialist.

Please note that with the machine removed from its case, it will not work because you will need the safety bridge mounted on the inside of the cabinet. You can remove it by drilling out the rivets.

[Steve Bass]

Thanks David and Michael.

This isn’t something I’m prepared to tackle and I’ve spoken to Brian Reeves who I’m pleased to say is confident he can fix the issue.

For info -

The humming was coming from the machine itself, not through external amp/speakers.

I’ve had caps replaced before. On one occasion this was following a period of a few years when the machine was relegated to the cold attic at home. On being brought back into use it failed to the extent that smoke was visible!

Once again, thanks for the helpful comments and I hope to report back when all’s well.

Steve

[TonyDuell]

I don't suppose you have any information/memories as to how it was used with the PDP8? No I don't have an A77, but I do have a couple of (later) PDP8 series machines...

[Steve Bass]

Hi Tony

Some memories. I recall it from when I worked in our company’s computer section from circa 1974.

It was somehow interfaced with the PDP via that output socket on the top of the A77. It could then be controlled directly from the ‘Teletype’ keyboard, in turn hooked-up to the PDP. Sorry I can’t recall if entire programs were stored on the tapes or whether it was just data. Certainly great fun watching the 10 inch reels spinning about remotely, ‘looking’ for stuff like what computers did in movies of the day!

It fell into disuse in the early 80s or so due to the obsolescence of the PDP 8 and the availablity of floppy and hard drives. From there it lurked unloved in a cupboard until I discovered it again in 1988. Ultimately it would have been destined for the skip, but I persuaded my boss to give it to me.