Revox A77

MKIII 2 track

This machine came to me via a trade and cost me around €50. The previous owner told me he and his grandfather had been working on it for a while and couldn't get the noise out of the left channel. His grandfather had worked in electronics repair for years he said and he hoped I could find the problem. It ran, but the noise on the left was very bad.

What I did not know was that most of the repairs had been done, how should I say that nicely, done well. Also he gave me a plastic bag with new capacitors they bought. That was good, since they were audio grade electrolytic capacitors. I later found out the bag only contained some of the right values.

On the outside it looked fine with minor scratches on the wooden case. It missed the aluminium (aluminum for US folks) handle, but the plastic parts were there.

I got to work and boy, I had my work cut out of me.

I also found out that it did not have the loudspeaker amplifiers installed. They were optional back in the day and these were never installed. I was going to design/replicate the board and make them, but most people with connect the machine to a separate amplifier anyway. It would be a waste of my time and money to do so. I did make the Kicad project schematic, so whoever is interested, let me know.

Oh boy... immediately after checking the boards and the general state of the machine with the covers off. I found one of the motor caps was leaking. Not badly, but finding good replacements was added to the todo list.
I found them on eBay by a German seller heroms Professional Supplies.

While waiting on the arrival of the motor caps, I started to replace the capacitors on the easily accessible boards on the bottom of the machine.

The noisy playback was mentioned in the Revox service manual on page 39 (page 41 in the PDF).

So replacing Q803 and Q804 on playback and drive amplifier boards (1.077.720) did the trick mostly. After replacing the 1500uF caps with 1600uF new caps... the noise was almost gone.

I made sure I replaced the tantalum C802, C810 and C814 as well.

Some capacitors are quite difficult to access, for instance I had to remove quite a bit of other parts before I could access the power supply board. C101 (1000uF) was ok, but replaced anyway. The large bridge rectifier turned out to be a B40 that was cracked. I started to wonder why this machine had been able to play at all. I had a new B40 laying around so replaced that as well. The large capacitor in the back showed a low ESR and still the right value. I decided not to replace that.

All the tantalum capacitors I came across I also replaced. Even since one almost blew up in my face I replace them as well. These are ticking time bombs.

The Speed Control Board normally a 1.077.724/729 in a MK4 was a 1.077.540-01 was an indication it was a MKIII. This seems to be the only difference so far I could find electronically, but will stand corrected by expert historians. The 1.077.724/729 have a NE555 timer chip as an oscillator, the board in the MKIII has a circuitry to perform the same task.

The case was another story. Although I looked ok, it had scratches and the feet were pretty bad. At some point in its life, someone made some repairs to the plastic vent and voltage selector door. I decided to remove all plastic parts, to be glued back in later, and sand and re-varnish the case. The result is pretty cool. It looks like it just came from the store.
Note: the picture shows the varnish still wet. I used a mat finish varnish.

Work is still in progress... <watch this space>

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