Revox A77

[Diabolical Artificer]

Have had a crack at putting video's on Utube, which has been,... interesting ! Not the most photogenic
person, and don't like camera's. Anyway, here's part one of my Revox A77 recap/rebuild, apologies for the dodgy camera work and forgetting what things are called. Its not easy, this video blogging.

https://www.youtube.com/watch?v=xf-Pac5OvZQ

Recapped most of the PCB's, should be able to test the audio later. I would think it's ok to power it up with a no load on the motor PSU's PCB, IE, no motors connected.

Andy.

[Humber888]

Andy, I had a look at your YouTube video. The thing that hit me is that, as part of replacing the original mains socket with a 3-pin IEC one, you have now earthed the Revox. It was never connected via the power plug to a mains earth originally and (I assume) double insulated instead. Someone will correct me if I am wrong here. You could be at risk of introducing earth loops when used with a complete HiFi system.

[Diabolical Artificer]

Good point. I earthed because it has a metal back being a rack mount A77 and sits in the living room at present, and I have a daughter. I need to re do my hifi "cabinet" that's made to fit 19" rack gear, then if I do get any noise from earth loops I can always disconnect it.

Thanks for pointing that out, Andy.

[Radio Wrangler]

If the ground pin is not connected, then the system is just the same as if you had managed to find the right two-pole cable (mine's lost in the attic, somewhere)

I've recently got a Revox B261 tuner and the normal version has the 2-pole connector, but being ex-BBC, this one has the 3-pole fitted, and an earth wire on it.

David

[Restoration73]

Using a 2 pole mains lead, it should pass a portable appliance test as Class II Metal.
There was quite a useful article on the A77 in "Electronics and Beyond" the Maplin
magazine Sept 1999.

[Humber888]

Would you be able to scan that article if rules allow?

Mike

[Restoration73]

Can do, but too large to post here. Could email.

[Humber888]

OK, will PM you my email.

Mike

[Diabolical Artificer]

Could you possibly do one for me too please?

PM, sent, Andy.

[Restoration73]

Now available as pdf;

[Diabolical Artificer]

Managed to get the Revox to run as in the motor's are turning, however I'm sure it's running too fast. Tapes recorded on two other machines at 3.75 and 7.5 ips play on the A77 too fast.

So, looking at the service manual it appears that the tape speed is regulated by a pickup coil (tachometer) that picks up pulses from the capstan motor, which has ribs on it's other surface. The manual says at 7.5 ips this should be at 1200hz and at 3.75 ips 800hz. I've clocked the output of the tacho at 2.85 khz. The manual goes on to say to adjust the speed, adjust T201 for 7.5 and P201 for 3.75 ips. what it doesn't say, is where the heck they are. I presume T201 is the tacho, which doesn't appear to be adjustable. Can't find P201 anywhere on any board. It says to clock the frequency at E1 and 2, which are the wires from the tacho, which I did. At both speeds the tape seems to play at the same speed.

The manual I have is a bit vague in lots of respects, it's the one from Hifi Engine, has any one a paper copy or a more complete copy?

Thinking about this, AFAIK John who had the deck before me said it played ok apart from some issues, which I took to caused by the leaky motor run caps. so I can't understand how it's come to change so much between then and now. I thought I might have connected the wrong motor run cap to the capston, I havnt.

Any input most welcome, Andy.

[TonyDuell]

According to the A77 service manual I have (which I found on-line somewhere, can't remember where), T201 is the inductor/transformer on the speed control board (adjust the core), P201 is the (vertically-mounted) preset potentiometer next to it. The manual also says that the 7.5ips speed must be set first.

Before adjusting anything, I wonder if you have a fault on that board.

[Restoration73]

There are two different versions of speed control pcb, the early one has a large inductor
and the P201 mentioned. The later and more common board has a 555 chip and R217
preset. I have a spare copy of circuits if you need. The frequencies are 800 and 1600Hz
for 50Hz mains.

[DOFFERY]

Alas if you do have the inductor/preset version of the speed control item then capacitors ( a couple are 1% from memory}on the board will have to be changed . Also T /P 201 are a pig to adjust without removing the board, best of luck.

Colin.

[SteveCG]

Just a thought: I wonder if the wiper arm on the speed-adjusting preset pot has fallen off - in classic Revox corrosion-due-to-age style?

[Diabolical Artificer]

Mine has the 555 etc IC's. I've found R217, turning of which does nothing. The pot in question I've replaced, however, I set it to where the old one was before fitting to the board.

The manual I have mentions T/P201 in the adjustments section - 6.22. But it doesn't have a parts list, but it does have a German bit "bolted" on later in the PDF where the schematic is given.

I'll recheck all my work to see if any wires are mis - positioned, I took lot's of pics before doing any work. From the sound of it though, I'm going to have to read up on how the speed control board works to get a better idea of of what to look for, then have a probe about. The board isn't easy to access being restrained from lot's of wires soldered on the bottom.

Thanks, Andy.

[Humber888]

Andy, I believe you sent off to Nagravox for a refurbishment kit. If so, you should have access to the file 'A77 Cap Trim Kit ver 16.pdf' which is available to download - about 400MB. This has quite a lot of useful information on the different speed control boards and faultfinding, starting around page 60. I have the older transistor (not IC) control board on my MK3 and so cannot help fault finding directly.

Mike

[Diabolical Artificer]

Thanks Mike, I was unaware of that.

Managed to sort the problem by the good old method of poking it with a stick, well sort of. I jiggled all the leads and spade connectors, re seated the IC's and on the next power up, I noticed the capston motor was a bit quieter. A looked at the counter and the frequency had dropped to around 2khz and even better, turning the pot changed the frequency. Set it to 1600hz and all was well. Post 31 above is wrong, I wrote 1200hz, which was the erase F for another deck I was working on the other day. These frequency counters are very useful.

There's still a few issue's, EG, it's very noisy, with a loud hiss; on playback it's not too noticable. Are all A77's like this? I've yet to set up the bias and adjust everything, so it might improve.

Andy.

[Humber888]

There's nothing like the brute force method at times! When you say it's noisy, I assume you mean electrical noise and not something like a motor squealing away. You might expect a slight background whine from the capstan motor depending on the speed setting, but that's about it. My A77 doesn't have much backgound hiss even when cranked up, but it is a Dolby version. There's a little hum at full gain, but I'm not too bothered about that until all the electrolytics are replaced. What is peeving me at the moment is that I seem to have stuffed the tape counter. It won't count backwards i.e. the least significant digit goes from zero to 9 and then sticks with a clicking sound. Not happy.

Mike

[Radio Wrangler]

Some years ago I put a new tape counter in my A77 because the first carry gear had worn out and was causing that problem. Acoustic noise, the only sound from it is a 1.6kHz whine from the capstan rotor being excited by the magnet in the speed sensor. It has a characteristic ring from the big steel cup.

Electrically, both channels are definitely quieter than the background noise from bulk erased tape.

If it's electrical noise, does it appear similar on both channels. Does pausing the tape motion by lifting the pinch roller do anything? Switching to direct monitoring the input?

David