Revox A77 restoration

SeleniumFalcon

I needed a way to inject an audio signal to my circuit under test via the playback head. The DC feedback is fed via a 1,500uF capacitor back through the head to form the necessary low frequency part of the EQ. I decided to build a "flux loop" which is basically an inductor placed in front of the playback head and fed a signal from a frequency generator. Inductive coupling induces the test signal into the playback head's internal coil and then on to the playback audio circuit. A 681 ohm resistor is placed in series with the inductor to give the audio generator a more desirable load.

pitdogg2

WOW!

SeleniumFalcon

Chris Squire.

SeleniumFalcon

The last goal of this project is to install a separate regulated DC power supply for just the audio sections of the deck. The stock 21V power supply has a simple pass transistor regulation stage but feeds both the speed control circuit board and all of the record/playback circuit boards. I'm baseing this additional supply on a Velleman P1823 adjustable DC supply that is fully regulated. I've increased the smoothing capacitor to 10,000uF and replaced the standard diodes with fast recovery types. Also the 10.0uF electrolytic capacitor has been replaced with a WIMA film type.

SeleniumFalcon

I've added a heatsink to the regulator and replaced the stock 120 ohm resistor with a Dale RN60 series. I also found some small brackets that allowed me to attach the regulator to the underside plate and be able to detach it when needed fairly easily.
Now to finish this up with the upper and lower front panels and the wooden cabinet.

SeleniumFalcon

After seeing this photo I decided I didn't like the wrap job on the group of wires to the right and removed the tape and used wire ties instead. When the wires are sort of recessed it's hard to control the tape as it gets wrapped around.

mhardy6647

SeleniumFalcon wrote: »

Chris Squire.

Note the Quad preamp, too

SeleniumFalcon

Outside of the six screws that attach the two reel platters to the brake hub there aren't any screws visible on the front of the A77. In order to accomplish this the upper and lower faceplate sections are attached with spring loaded grippers. Since these tend to fail over time I bought a few spares from eBay and painted them a flat black (yes, I know a terrible waste of hard to find black paint).

SeleniumFalcon

After I cleaned and polished the lower faceplate I found out that the two VU meters I installed were the wrong vintage. In the MK I A77 there isn't a dividing piece between the two meters, they fit flush against each other. In the MK III version lower faceplate I had the dividing vertical piece separating the two meters. This means there are two different VU meter housings, one with beveled sides and one without. Guess which ones I had, correct the wrong meters for the faceplate I had. So, I had to replace the meters.

SeleniumFalcon

After getting that straightened out and the lower faceplate installed I began cleaning up the upper faceplate. Lots of cotton balls, Flitz polish and two coats of car polish I got the upper and lower faceplates installed. I'm on the lookout for an upper faceplate with a few less scratches and gouges, but this is what I have. Now to finish the wooden cabinet.

SeleniumFalcon

Here's a photo with the cover bar lifted up.

SeleniumFalcon

Since this deck didn't have a cabinet I had to buy one from an eBay seller in Belgium and it arrived in perfect shape (it just took a very long time). I removed the feet from the bottom and since they were a bit scratched I decided to paint them. The rubber inserts were dull so I treated them to a automotive tire blackening treatment.

SeleniumFalcon

The wooden cabinet was in good shape, a few scuffs. The first step was to Flitz polish all of the plastic areas, clean with a plastic cleaner and use a car wax to give some protection. I treated the wooden part of the cabinet with a General Finish brand furniture cleaner a few times and then used Watco Danish Oil.

SeleniumFalcon

pitdogg2

Amazing!

Jetmaker737

Lookin good!

SeleniumFalcon

Thank you!

SeleniumFalcon

After getting some metric sheet metal screws, from McMaster Car, to attach the cabinet to the chassis I can see how things look. In general terms everything looks good, the upper face plate is a bit blotchy depending upon what angle I look at it. The real problem is the capstan motor is making a low level rumbling sound amplified by being inside the enclosure. I can't live with that, I'll have to figure out what's the cause, I replaced the bearing. The shaft may be bent.

SeleniumFalcon

txcoastal1

ewww what a let down. Can you find those?

So close. Love this thread

Jetmaker737

Cliffhanger!

SeleniumFalcon

I believe I've found a solution. There's a fellow who will do a very thorough evaluation of an A77 motor, rebalancing the rotor (if needed) installs a custom made bearing, restores the capstan surface uses a proprietary lubricant and adjusts the tachometer. And has found a way to reduce the whirling sound the slots on the rotor cause when spinning. I'm going to send him a second motor that goes with the next unit I'm working on, then exchange it for this one.

pitdogg2

My man you have bottomless patience!!
I love watching this come together pitfalls and all.
Hats off to you Ken 😍

SeleniumFalcon

I suspect the left VU meter is stuck, there's usually a pegging of both meters when it's turned on. The right one reacts but not the left. Somebody has a modification circuit that eliminates that turn on surge. I noticed there's a illuminated red tape counter available from another source. You can go real high end and add a digital time keeping meter.

Rhodes

I dropped to this forum just by chance searching information on the net for my future REVOX A77 mk3 restauration project. Recently I have done full mechanical and electrical restoration and bring life into for decades forgoten dead beatiful REVOX A700 (to the moment in calibration and testing stage), but that is another story.

After reading and enjoying this thread in one breath, I simple can not find the words to say how much I appreciate Your hard work. First of all, pictures are very clean and informative. Cleaning old audio equipment inside is not a strange thing to me , but painting parts !!

That idea never crossed my mind. That is impressive and high profile job and I simple can not resist that watching your progress on this restoration give me a inspiration how my next REVOX A77 project should be done...

No you did not carried away with these projects and for shure it will help and motivate me to restore another one.
Impressive and awesome. Keep up the good work!

SeleniumFalcon

Thank you for posting, I'm glad the information was helpful. I'm waiting for the replacement capstan motor to be finished. The eBay person (thermionicist ) has kept me appraised of what he's been doing and I made the right choice sending him the motor. He has worked out a special method of removing any of the old oil from the sintered bronze bearing that takes around three weeks to do. Previously I've applied heat to the bearing and slowly added new oil drawing the old oil out. Evidently if the process isn't done correctly the metal can become brittle and the new lubrication won't be as effective as possible. Also he removed some rust from the outer rotor and is re-applying bluing to restore the finish. This along with the custom ball bearing should make for a quieter motor. I'll measure the record/playback wow and flutter and then compare this to the new motor. Should be fun.
Please post some details and photos of your A700, a wonderful deck. That would be great.

Rhodes

Maybe your REVOX A77 restoration post isn't right place to post some steps in my REVOX A700 restoration project, but anyway....you ask

Unfortunately I did not photo-dokument all stages in restoration process, but basically everything was out of the recorder , inspected , cleaned and so on ( you know the drill ) except capstan and two reel motors. Reel motors were very noisy, so for reel motors I dismantle only top components till to the upper bearing and drop inside few oil drops for Revox machines which I bought on e-bay years ago. Yes I know that service manual say „no oil“, but that was 50 years forgotten old deck with a lot of problems.... Problem list for that deck was very long.

So to make long story short, transport at the moment work like a breeze. You can hear only „wind „ from the reels in forward and rewind mode and everything is silent in play.
Photos show some steps, from everything out (except motors), traffo overhaul, cleaning transport control board switches to everything „in“ from the back side.

SeleniumFalcon

Thanks for posting, great photos, very clear. This is the first time I've seen inside the A700 deck, I can see the evolving of the design from the A77. More advanced shielding of the electronics from the fields created by the motors. Of course adding servo control to the transport to better regulate tape tension.
The selector switches almost look like tube sockets. That's quite a fuse panel!
Very nice job!

BenHer

A77 tape speed problem.
Hi all. A most interesting site. Perhaps some of you "experts" can give me a hint on what is wrong with my trusty A77 I bought in 1971. When it is started up and cold the tape transport is very slow. After say 15 minutes runtime it works just fine at play. It must be a heat issue but all capacitors on the motordrive board looks fine. I have not checked the driver transistor yet. any suggestions would be most welcome.

SeleniumFalcon

Hello,
It has been my experience that this kind of problem is related to the lubrication of the capstan and supply and take up motors. In all likelihood the bearings have become gummed up and should be replaced or re-lubricated depending upon the version you have. In order to check this you would need to remove the chassis from the wooden cabinet and remove the front top faceplate to check.