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-   -   Revox A77 refurbishment advice please. (

Andrew64 10th Jul 2019 8:20 pm

Revox A77 refurbishment advice please.
Hi peoples,

I have recently become the owner of a Mk 4 (3 3/4 & 7 ips, two track), Its done very little hours, It's just about pristine externally with no signs of wear to tape path or heads. The down side is although stored in the original box it has at some time been stored in damp conditions, there is a light 'bloom' on the case in places, all the internal cabling has a white deposit that easily rubs of with your fingers and most of the cadmium plated parts have 'fur' on them to a greater or lesser degree, also one or two rust spots on the motors. The aluminium is unblemished.

I have replaced 3 cracked Rifas on the relay board and one on the Capstan motor speed control board, while I had it out I went over it with the meter and found a shorted capacitor, a white 10uF job, C203, this is in the bias network of the speed transducer amplifier IC201a, this was causing the motor to race as full volts were being applied to it.

I have adjusted the brake solenoid and the gap between the bands and the brake drums, this seems to have cured a tape spilling problem in FF & RWD near the ends of a 7 tape.

I notice at switch on the VU meter needles bang against the end stops twice in quick succession, is this normal?

Also I would appreciate advice on what I should replace as a matter of course in this 43 year old machine?

Restoration73 11th Jul 2019 12:57 am

Re: Revox A77 refurbishment advice please.
The brake bands have a cotton tape and if the adhesive gets gooey it can stick to the drum resulting in the band being ripped away from the rivets.
Meter lamps can fail, and replace the counter belt if too loose.
I would not contact the cadmium screws and replace with BZP if of concern.

Michael Maurice 11th Jul 2019 7:25 am

Re: Revox A77 refurbishment advice please.
The VU meters are working normally

early brake bands did not have any tape on them, that came much later.

Nearly all the capacitors will need replacing and the trimmers as well.

I will comment further when I get some time.

barrymagrec 11th Jul 2019 9:27 am

Re: Revox A77 refurbishment advice please.
I`d be surprised if the corroded parts are cadmium plated - more likely zinc and colour passivated I would have thought - cad became very expensive and generally only used on mil spec stuff, quite apart from the toxicity implications.

Welsh Anorak 11th Jul 2019 5:49 pm

Re: Revox A77 refurbishment advice please.
The brake bands should be shiny and springy - it's the drums themselves that have the cotton tape on them and this can cause glue migration with the effect described above.

Andrew64 11th Jul 2019 11:54 pm

Re: Revox A77 refurbishment advice please.
First, thank you all for your helpful comments.

Now brakes, on mine I have the shiny metal bands and the brake drums have cotton tape that seems to be glued on. Due to high forces >200 gms needed to turn the drums in the unwinding direction I removed the bands and ran what I thought were clean fingers around them to see if they were OK, the surface seems very hard, anyway I reassembled them and found the force required to turn them was now way too low so bands off again and I cleaned them with Isopropyl alcohol being careful not to over do it, the cleaning cloth I used snagged quite a bit on the hard rough surface. Seems the little amount of grease on my fingers had a big effect and cleaning restored the forces to near normal. Now because the tape liners seem hard and rough they may be suffering from the glue migration problem. Re-adjusting the solenoid to bring band clearance to near 0.04 thou has restored forces to normal but the brakes do seem 'touchy' hence I will replacement the cotton tapes.

The comment about parts not being cad plated is right now I think about it, my thinking was clouded by my familiarity with cad plate from working with it in my aviation engineering career. I have a stock of stainless screws and will replace the bad cases.

I have done a fair amount of research using the forum search facility and have read many threads on the A77. A common theme seems to be replace the small electrolytics, tantalums and motor phasing capacitors, also the trimmers.

I also need a belt for the counter, I can only see one belt in my machine, am I missing one?, Nagravox supply two in their kits an 'O' ring type and a toothed one.

The pinch roller seems good but should I replace that?

Your thoughts on what to lubricate would be appreciated as well?

Now sourcing these parts, Nagravox Australia seems to get mentioned a lot, your thoughts on who to use?

Radio Wrangler 12th Jul 2019 6:01 am

Re: Revox A77 refurbishment advice please.
The two belts are both for the tape counter.

You have the components to replace right.

Lubrication is in the Revox service manual.


Ted Kendall 12th Jul 2019 11:14 am

Re: Revox A77 refurbishment advice please.
Bauch recommended acetone for cleaning brake bands. I'd give them a thorough clean with this before condemning the drum linings.

Michael Maurice 12th Jul 2019 11:46 am

Re: Revox A77 refurbishment advice please.
The capacitors I source from RS or Farnell and use either Panasonic, Nichicon, Wurth or Vishay.

The trimmers are not so easy to obtain but can be sourced.

You will need an alignment tape and test equipment to set it up.

The early counters used only one belt, later counters used a second toothed belt which is on the counter itself.

I’ve not personally encountered problems with the motor run capacitors, though I’m sure others have.

You may have to replace the entry guide roller.

Nagravox are expensive as is the shipping from Australia

I know these machines very well and have service many of them over the years.

SteveCG 12th Jul 2019 2:47 pm

Re: Revox A77 refurbishment advice please.
Quote "Its done very little hours, It's just about pristine externally with no signs of wear to tape path or heads... I have replaced 3 cracked Rifas on the relay board and one on the Capstan motor speed control board."

Those Rifas usually only crack after a moderate amount of use - ie not just age. Are you sure about the low mileage of the recorder??

Ted Kendall 12th Jul 2019 6:15 pm

Re: Revox A77 refurbishment advice please.
The failure mechanism on the Rifas is, I think, triggered by damp, and mere inactivity in long-term domestic storage can be enough to set it off. The seal between casing and leads fails, mositure gets in, partial failure causes cracks in the casing as the paper swells, this lets more moisture in, degrading the dielectric further, until the thing erupts. I wouldn't give more weight to duff Rifas than the condition of the tape path when estimating hours, for this reason.

Andrew64 13th Jul 2019 12:07 am

Re: Revox A77 refurbishment advice please.
1 Attachment(s)
Evening All (now who said that?)

Lets put the question of hours run to bed, I don't want it to become a distraction from the original reason for starting the thread! I'm with you on the Rifas Ted. See attached picture of the play head, I can see little, if any wear hence my reason for saying low hours, nuff said, lets move on!

I have checked the counter and I do only have the one belt. BTW the wind motors are marked 2/76 so this must be a late model as production was from 67 to 77, S/N is G2050XX, why I have the earlier counter I don't know, maybe supply problems? Anyhow just a simple 'O' ring belt required.

Thanks for your confirmation David about the components to replace, and to you Michael for your tips, Nichicon caps came to my mind as well. I'll build a list of what I'll need.

Anybody have recommendations on the trimmers?

Now the brakes again, mine seem to be a hard cream coloured plastic bonded all round onto the drum, if they are cotton tape it's impregnated with something that when you gently run a a scapel over them gives the impression of a hard rough surface, is this normal? I'll clean them with acetone and see if that softens them any, thanks for that tip Ted.

Alignment I have a scope and meters, nothing else suitable so I plan to do one board at a time making sure not to mix up CH1 & CH2 ones. I will carefully measure and record the resistances of the trimmers before removal and set the replacements to the same once there installed. This will be easy for some e.g. the four on the oscillator board but not so easy for others e.g. P501/P502 on the record amp boards, in these cases I'll need to record the resistances from each end of the pot to the wipers, noting which way round the meter is so diode/transistors junctions don't mess things up. I know this will not restore everything to exact pre-replacement values, I'll have to put an ask out for somebody with the right gear that would be happy for me to visit with my machine and do the final tweaking.

Ted Kendall 13th Jul 2019 8:22 am

Re: Revox A77 refurbishment advice please.
The drum surface is pretty hard and rough on the A/B77s - the earlier 36s were more a plain nylon weave. You may find that a thorough clean of the bands is all you need to do.

That head looks close to zero hours, too - and the width of the guttering suggests it hasn't been re-lapped, either.

Restoration73 13th Jul 2019 10:06 am

Re: Revox A77 refurbishment advice please.
Sorry , I should have said "brake drums" but I only encountered it once. I think it might
be an Akai with the other way round.
Whilst your heads look in good shape, earlier I had one with head wear. I sent it off to
and received the re profiled head that looked and functioned as new. Recommended.

monaro0162 13th Jul 2019 11:17 am

Re: Revox A77 refurbishment advice please.
3 Attachment(s)
I have three A77's that I restored a couple of years ago, a MK2 a MK3 and a MK4, great machines. As Michael says, the 15mm trimmer pots can be hard to find. Piher make suitable replacements, although they don't seem to be available via RS or Farnell. Piher PT-15 is what you need, they are available via Mouser UK or Reichelt in Germany. If you order via the Mouser UK website, they will actually be shipped from Texas in the States, will take a few days to arrive. Make sure you order the correct versions, as there are 100's of permutations!

Michael Maurice 13th Jul 2019 11:29 am

Re: Revox A77 refurbishment advice please.
Cricklewood electronics also sell them

Diabolical Artificer 14th Jul 2019 9:03 am

Re: Revox A77 refurbishment advice please.
Cricklewood as mentioned for the Pihar trimmers. Before you replace each one take resistance measurements both ways and set the new trimmer to these readings, then you will be in roughly the right ballpark.

Nagravox in Oz are good, takes a while and not cheap. there is also a place in Germany does Revox spares, sorry can't find link.

there is one really hard to get at screw/bolt on the RHS if I remember right that secures the motherboard, you need a really long screwdriver. You don't need to take the motherboard out to recap, just loosten in order to get at the underside to solder.

From memory there a big cap on the motherboard with a stud fixing, I got a new cap that fits inside the old cap shell and secured with big heatshrink.

Watch out for the switch board, be very careful the SW's don't pop open, there are tiny ball bearings that jump out and hide in darkened recesses : )

That's all I can think of for now, my old thread will be here somewhere, should be some info on there and there's a very good later thread on setting up too.


PS Take lots of good pics of where wires go before you start.

ben 14th Jul 2019 1:13 pm

Re: Revox A77 refurbishment advice please.
On the last one I did (Mk I so no pitch control), I had to replace a dozen trimmers on the daughterboards (originals were crude, open types that disintegrated, use new Piher), half a dozen Rifas. The front panel pots and selector switches are pretty crude and needed a good clean of the contact surfaces. No problem with motor run caps, pinch roller was in good shape. I also replaced the mains inlet socket for the more common IEC type. Once these things are attended to, these should soldier on for another few decades!

Andrew64 14th Jul 2019 5:40 pm

Re: Revox A77 refurbishment advice please.
Thanks to all for your contributions!

I've have the acetone on order Ted and will give the linings a good clean and see what happens. Your observations of the head agree with mine, I obtained the A77 from another forum member and though I have only met him a couple of times now I have complete faith in his history of the A77.

Thanks Tony for info about Summertone, I also have a Teac A3440 that's next on my list, ex-BBC, now that does need re-lapping! It will complement the 2 track A77 nicely, shame it's 7 & 15 ips but you can't have everything.

Piher pots seem to be favourite, thanks monaro0162, Michael, Andy & Ben, I'll use them. I have also noted your tips Andy & Michael, I think I'll leave the stud mount cap but will check the ripple though, I'll search out your threads for info as well.

There's no pitch control on mine Ben, my pinch roller seems OK so I'll leave that. I've done the FF, RWD and play tests with no tape and the sensor covered and the motor motions/torques are in spec. so will leave the motor run caps as well.

All for now, I'll get my parts lists done and ordered and let you all know how it goes but I may have a query or two on the way!

Michael Maurice 14th Jul 2019 8:43 pm

Re: Revox A77 refurbishment advice please.
I’ve also had the capacitors on the motherboard fail, I think they’re 100uF/35V axial types

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