Revox A77

[Diabolical Artificer]

Whilst fitting an IEC socket to a Revox A77 yesterday, I found these three leaky caps - see pic - one 3.5u two 4.3u 160v. I could restuff them with radial 3.3/4.7u 105 deg caps, but is it possible to get direct replacements? Hav'nt touched the goop, probably carcinogenic.

Can't find them a quick look this morning in the service manual PDF I have but presume they're something to do with the tape transport circuit. The other suspect caps for replacement I found are several Rifa Miniprint caps, though they show no signs of the usual Rifa peccadilloes.

Are there any other things that might need looking at while I have the back off?

TFL, Andy.

[vidjoman]

They look like the motor run capacitors. Easy replacements available.

[John Caswell]

Hi Andy,
Yes they are motor run capacitors, readily available on eBay from several suppliers, especially in Australia for some reason or other.

John

[Humber888]

The guy in Australia specialises in Revox repairs and has the three motor run capacitors specially made (rather than generic white goods ones). He also does complete refurbish kits. There has been a thread on this some time ago.

However, there are a number of UK-based folks who provide Revox calibration and repair services (Google something like 'Revox refurb'). Some may be able to fix you up with direct replacements without the postage costs of shipping from Oz).

Mike

[Diabolical Artificer]

Motor run, thought so, thanks.

Looking at the rest of boards all the electrolytics will need replacing as they all have high/borderline ESR. Not too bad as I have most of these made by decent manufacturers.

The other note of possible concern are about twenty crusty looking skeleton type trim pots on the amp etc boards. Would rather not replace, but surely modern 10 turn trimmers would be better.

So it looks like a wholesale re cap. Fortunately these decks are easy to work on as some thought has gone into the design to make it easy to remove panels etc, unlike some Jap decks I've worked on, where you need to remove at least twenty screws, five of which are hidden or hard to access. Nice one Studer.

Andy.

PS,Looking at old threads, it appears I have to get the exact value. Would rather didn't have to order from Oz. Early days in the search though.

[Ted Kendall]

Yes, the values have to be exact.

My experience of the Oz connection has been good - the remanufactured bits are excellent and he doesn't supply rubbish for generic components either. Transit is pretty swift considering, too.

[Radio Wrangler]

If you're going to do a general capacitor change, the trimmer pots are also known for disintegrating.

David

[Humber888]

Andy, replacement trimmers are available in the same (15mm) footprint - both carbon and cermet, but cermet is harder to source in small quantities. I have been chasing down stuff recently since my A77 MK3 blew its 21V rail (shorted Frako electrolytic). I may have more information tomorrow that could be useful to us both.

Mike

[Diabolical Artificer]

Hope it didnt take too much out with it Mike. Thanks, that would be great. If you decide on replacing a lot of the caps etc, maybe we can combine orders?

I found the bloke in Oz, he does a complete cap, trimmer, tantalum etc replacement package. Uses Nippon Chemicon caps too.

Started on the main board that all the sub-boards go on, it's tricky to get at without un-soldering all the wires. Made the mistake of popping the volume/ source switches to see if that freed it up. Out fell four tiny ball bearings and springs, one spring still AWOL. These are going to be a b*gger to get back in.

Yep, noticed the trimmers David, though they look ok, think I'll change them. Most are common values, IE 20k and 50k, but there are a few 250/500 ohm jobbies.

Thanks, Andy.

[Radio Wrangler]

I could join in a group order which ought to dilute the postage... I have an A77 in the back bedroom that hasn't been powered since the 80's. Capacitors, presets and a pinch wheel ought to cover everything... oh and the counter belt.

David

[Humber888]

OK, will liaise with you both offline when I get a little more info on options.

Mike

[Diabolical Artificer]

Have replaced most of the caps on the sub boards, rec amp, oscillator etc and replaced a few of the preset pots. One wiper on one pot in particular fell off to the touch.

Recapped the "motherboard" too, and manged to find and replace all the springs and ball bearings in the switches by dabbing a bit of copperslip on the ends of the springs, so that the bearings stuck and didn't fall off.

There are three of four Rifa Miniprint 470n caps one of which has started to bulge that will also get replaced. I havn't bothered with replacing axial for axial caps, and have used Rubicon/Panasonic 105 deg C radials of higher voltage ratings for most replacements. Had to parallel up a 100u and a 22u to replace two 125u caps on the input amp PCB. Most caps are standard values apart from the above and two 1600u jobbies which I've ordered from Farnell.

One other thing concerning the pre set pots. I replaced some 20k's but when I measured the existing pots in order to set the replacements around the same setting, I found they measured 17k with the replacements at 16k. I know pots have tolerance too, but in my experience theyre usaully higher in value and not out by much. Could this be an issue?

Andy.

[Michael Maurice]

I wouldn't have expected replacements to be as much as 25% out, are you measuring them out of circuit?

[Diabolical Artificer]

No out of circuit Michael, there marked Piher Spain, one 50k measures 66k and a 22k,.. 20k.

Will get some new ones from Farnell.

Andy.

[Humber888]

Andy, I just got my Nagravox refurbishment kit and its good to see they use long-life 105C electrolytics (Hermei) and Class X2 polypropylenes. I looked at the trimmers (stated to be cermet but can't tell) and they all seem to come out about 10% high or low - no real medians. So your 22k is probably well in for 20% tolerance, but the 50k is iffy.

I don't see Farnell doing the 15mm trimmers you want, but Cricklewood seem to sell the Piher 15mm PT15V, which is a direct drop-in for the original - also carbon not cermet. I may have to go to Cricklewood as well since my machine has the transistor speed control board variant and hence I need two 2.5k trimers in total and the kit only has one. It seems to have gone a bit OTT with 20k trimmers instead.

Mike

[akaigxfan]

hello, i thihk rs do the piher pots too, also digikey do, whether theyll do the ones you want i dont know. just a thought.

[Diabolical Artificer]

Just had an email to say replacement motor caps are going to be delivered, got most of the other caps, but waiting on Cricklewood for the 22k pots and Banzai for the other pots. Would have gone to RS, but got turned off their site a few years ago as it wasn't that easy to use. Farnell now is a doddle to use as it has a really good filter tool.

Anyway, while all the various bits are winging their way to sunny Lincs, I've been recapping various boards with varying degrees of difficulty. In an effort to not unsolder the wires on the bottom of the PCB's, I unscrewed ( which involved a screw right in a corner, which meant taking out a motor) two PCB's disconnected wires, then poked the iron in to get at a big 1000u axial and other smaller readials.

As regards the big 2200u 40v cap that has a bolt and nut on the bottom, I might have been better to leave in place as it had good ESR and wasn't leaky ( no idea of capacitance) but I'd bought a replacement. Using a clamp proved problematic, mainly due to starting the job when I had toothache and wasn't concentrating. I drilled holes in the PCB,fitted the clamp, put the PCB back,with the motor, then realised I'd forgotten to fit the 1000u axial. PCB back off, clamp tightening screw now in way of 1000u cap. Reversed clamp etc, now the lugs, tightening screw stopped the PCB lining up properly.

After a bit of thought, I chopped the top off the old cap and restuffed it. Much happier with the result. I used the fleece wadding, that secured the old cap in the can, wrapped round the new cap and tamped down to secure the new cap inside the old, then used some big heatshrink to finish off the top. Didn't want to use glue, thinking ahead to the next recap and the next repairer.

Re the power PCB with the big 2200u cap, see pic, there are two unpopulated slots, what would/should go in them. One other question, on two of the sub PCB's, there are two 1600u axials. Couldnt find a 1600u so ordered 1800u, which following the 20% "rule" should be ok.

[Diabolical Artificer]

Forgot to add this pic. Any idea what these are called and where I might get a spare? Broke one when moving the chassis. It holds the front panel on.

Andy.

[vidjoman]

Those slots are there to take the built in power amps, when fitted. Always fitted in the 'portable' with built in speakers or can be added in the 'studio' version if you want them.

[Diabolical Artificer]

Ok, don't really need power amps, the mixing desk/amplifier does all that.

Andy.