make a good A77 from 3 not so good ones?

[ady]

hello..

i'm the proud but time, space and money poor owner of 3 A77's!

One early silver and white normal speed 1/2 track one which records and plays back OK but seems to have alot of head wear.

One Mk IV or V HS 1/2 track models, one of which has no input working on one channel, head wear and call me stupid but I can't seem to get the case off this at all, maybe it's warped...but cosmetically very good

The final one is a MkIV or V HS 1/2 track again..I haven't tested it yet but seems to have excellent heads but the controls seem to have lost some of their silvering??


Not surprisingly none of these appear to have been serviced or calibrated recently or maybe in their life! Luckily I got them all very cheaply..

My plan was to slowly DIY service / fix them and learn about calibration (I have access to a signal generator and oscilloscope but no test tapes) and maybe make a good HS model to keep and sell the other two..

This plan is looking unlikely now, although I'd love to have a working, well calibrated Revox.

Anyone on this forum offer servicing or help in servicing in the Bristol / Gloucestershire area. It could be that two machines get sacrificed for the life of one and the spares could be offered in part payment??

I'm open to suggestions...


many thanks

adrian

[ruffrecords]

I have a couple of A77s, one high speed and one low speed both with good heads that I am 'restoring' for want of a better word.

I would advise starting with the HS machine with good heads. The input amp should be straightforward to repair and you can always swap the knobs from one of the other machines. AFAIK there is no difference in the input amp cards between HS and LS models so you could try swapping this too.

These are very robust machines even when 40 or more years old and with good heads have the best LF response I have ever seen in a semi pro machine. Heads do occasionally become available on eBay but are very expensive. An alternative is to consider having the worn heads relapped.

The most common problem with these old machines seems to be dried out electrolytic caps. My HS machine ran fast at both speeds and this turned out to be a duff electolytic in the capstan servo board.

Calibration is fairly straightforward. There is nothing to adjust on the replay side except the replay gain - EQ is fixed. Record calibration is largely a question of setting the bias for the tape you intend to use which you can do with a sig gen and scope.

You should first arm yourself with a copy of the service manual - it is available on the web for free - if you cannot find it I'll email it to you if you wish. Good luck - they are wonderful machines and well worth spending some time and effort on. There are a number of published mods that make them even better.

The only thing missing from my two is one of them has no plastic head cover - the one that plugs into the two tape guides. If you want to sell one I'll buy it.

Cheers

Ian

[Tim]

Hi May be able to help if you have a copy of the manual/circuit.
I am in Bradford on Avon(Near Bath)
Cheers
Tim

[ady]

thanks for the advice, and thanks tim for the offer.. i'll be in contact in due course..

had a closer look at the latest A77 HS I picked up and it seems the best of the three.. but one of the rectangular capacitors on the motor or capstan board has split and the capstan seems to be running too fast..

the other problem is that the erase head was loose, on closer inspection the long screw holding it in has sheared off, leaving itself inside the threaded hole:

http://uglypc.ggh.org.uk/revox_erase_screw_sheared.jpg

hopefully i can find a tap small enough..

apart from these two obvious problems it looks like I might get this running nicely with not too much work, other than the calibration..

thanks

[DOFFERY]

Change all the caps. on the motor speed control pcb as they will go down in time.

Doffery

[Michael Maurice]

Originally Posted by ady

the other problem is that the erase head was loose, on closer inspection the long screw holding it in has sheared off, leaving itself inside the threaded hole:

If you cant remove the remains of the old bolt, you can usually find someone breaking a Revopx A77 for spares, just purchase a head block and transfer the heads to the new block if necessary.

[ruffrecords]

Originally Posted by DOFFERY

Change all the caps. on the motor speed control pcb as they will go down in time.

I recently had a problem on my HS A77 Capstan Servo Board becuase it was running too fast on both speeds. There are two versions of this board one, like mine, which uses an LC discriminator to set the speed and the later versions that use an NE555 timer chip to do the same thing. Both use 1% tolerance capacitors in the speed setting circuits. My initial suspicion was that after 40 years these 1% caps had drifted, hence my speed problem. This turned out not to be the case. The 1% caps were still well within 1% and as close as the brand new 1% caps I had bought to replace them.

The problem turned out to be an electrolytic in the filter circuit following the discriminator. So I would advise changing all the electrolytics but leave the 1% caps alone.

By the way, this is a PITA of a board to get access to. You need to disconnect the capstan and one of the reel motors and the start solenoid. Then you need to unscrew the mains transformer on to which the Capstan board is mounted. Even then you cannot remove the assembly completely as there are other wires connected to it. I supported mine on a block of wood while I worked on it. Then you can remove the three screws that hold the Capstan board to the transformer so you can actually remove components. There is no way you can power up the machine in this state so you have to find/fix the faulty component, rebuild and pray you fixed it.

If you are going to work on this part of the A77 you really do need a service manual at your side.

Ian