Revox B710

[monaro0162]

Hi all,

Picked this up at the NVCF last week. Ever since I can remember I've wanted a 3 head cassette deck, but I could never afford one, as 3 head decks back in the day were way out of my price range. So I was pleased when I got this home to find that it actually works .. well it almost works!

On powering it up (presumably for the first time in a long while) the audio output was a bit crackly and hissy, and there was an odd regular 'thumping' noise on the output accompanied with full scale deflection on the meters when dolby was switched on and the monitor set to source. After leaving the unit powered up for several hours, and after applying switch cleaner to switches and removing and refitting most of the pcbs, luckily both problems seem to have gone away.

So then I did a first test record using a TDK AD90. Cassette was never anywhere as good as reel to reel, cassette recordings always sounded a bit dull to my ears. However the sound from this 710 seems really good, there is none of that 'dullness' to the recordings with very little sibilance on the trebles. Dolby works fine too.

However it does have issues:

1 - Some Wow and flutter is present, and gets worse towards the last 1/3rd of the tape. I've noticed that, on powering the machine up, the right hand capstan doesn't start by itself, it needs a little push by finger to get it going. The mechanical resistance felt on both flywheels (including the left had one which starts fine by itself) when pushing by finger with the machine powered off feels about the same, so I don't think that the right had shaft is gummed up.

2 - Rewind and fast forward work, but cut out after about 2 seconds

3- Tape counter isn't working

So this is going to be my next restoration project, to complete my Revox collection. I have 3 A77's that I have restored with excellent results, but they are a lot simpler compared to this B710. Things are taken to another level here with the implementation of a microprocessor and extensive logic control, it's a very impressive machine!

Could anybody help on the wow and flutter issue? I have read that bad pinch rollers can increase w&f, but I wouldn't have thought that they would increase it by as much as I am seeing. I've tried increasing the pinch roller to capstan pressure slightly, but it made no difference. I suspect that the problem is with the right hand capstan motor, or the drive circuitry to it. Why doesn't the motor start by itself? How can the capstan motor speed (and any variations to the speed) be measured?

[Marconi_MPT4]

Hi,
It has been some 36 years since I last removed a cover on one of these; but certainly I would start with looking at the reluctant capstan motor and control circuitry. Fortunately as the other capstan motor is working it will be handy as a reference. First check power supplies to the board are present. Now if an oscilloscope is to hand you may be able to ascertain if the tacho signal is being generated at TP2 and TP3.

As an aside, my B760 tuner has many tantalum capacitors that have all failed one by one over time. But not only those, some of the electrolytic capacitors have developed high leakage and one or two have even shorted out. In fact all failures to date have been due to capacitors! I see the B710 control circuit has a few, it might be useful to check those out in the first instance.

Cheers
Rich

[monaro0162]

Hi Rich, thanks for the info.

I removed the tape drive assembly took off the flywheels pulled the capstans out slightly and got some PDP oil into the bearings, all quite difficult to see, didn't make much difference to the resistance felt on the capstans before, they run very smooth.

I took scope readings on TP2 and TP3 which are the tacho signals after amplification ... both capstans (blue and yellow on the scope) are feeding back a 'stable' square wave at 375hz ... with a little jitter. When I compare the two against each other I see more jitter and phase shift. Gently pressing on each of the flywheels causes the square waves for each capstan to stretch out in time as the capstan slows. On another test point there is a stable 375hz square wave that is unaffected by slowing the flywheels. So it seems to be working although the motors (interesting plat panel design!) don't seem to generate much torque? It's very easy to slow them down, I can't seem to feel the motors respond when extra load is applied.

So assuming that the cct is operating properly (all psu voltages have been checked and are fine) is the wow and flutter I'm hearing due to the jitter between each capstan shaft? This brings me to the twin capstan design of the B710:

I have another twin capstan deck, but the application of twin capstans is for auto reverse play, where as here with the 710 it seems that both capstans drive the tape? How is tape tension across the head maintained, wouldn't both capstans have to be running at identical speeds, without any jitter, to maintain tape tension without wow and flutter? The A77 uses back tension from the supply reel motor to maintain tape tension, with the second capstan involved the 710 does it differently?

Thanks, Paul.

[audiomm]

I had a B710 Mk2 way back when..... like 15+ years ago. I recall having a similar problem with mine when I got it and so I made a few phone calls to a Revox expert in Sydney. Upshot was that he was very familiar with the spooling motor issue and suggested I replace the bronze bearings - two per spooling motor.
He said that the bronze bearings wear oval over time especially if the machine has had a lot of use and that in itself causes drag. I replaced them with miniature roller bearings sourced from a local engineering shop! I remember dismantling one motor and then taking out the bearings to provide the shop with a measurable sample. They had them in stock too.
I re-assembled the motor and it certainly ran freely so I then did the other one. That solved the problem completely. It is a very fiddly job to do but well worth the effort. Mike.

[Ted Kendall]

The trick with dual capstan drive is that the leading capstan runs faster than the trailing by a small margin, which tensions the tape across the head in a manner independent of supply or take-up tension.

A playful little complication with the early 710s was the fixing of the hubs to their motors, which was prone to slip. The factory issued new carriers with modified fixing to solve the problem.

[Boulevardier]

"...the bronze bearings wear oval over time especially if the machine has had a lot of use and that in itself causes drag."

Yes, those sintered bearings were a problem, especially when operating the deck horizontally. I had the problem with a Brennell Mk5, and, since Brennell had by then shut up shop, like audiomm, I had to get a work colleague to turn me an aluminium housing so that I could replace the top bearing with a ball-race. Seemed to work well. I concluded that the deck had not been adequately designed for horizontal working - but then I just wasn't impressed by anything much in the 10.5 inch spool, 15 i.p.s. version of the Mk5. Terrible tape handling, and almost hourly maintenance needed on those pressure pads and static tape guides.

[Ted Kendall]

Originally Posted by Boulevardier

I just wasn't impressed by anything much in the 10.5 inch spool, 15 i.p.s. version of the Mk5.

You and me both.

[mole42uk]

With the Revox B710, there is the possibility that the sintered bearings become stiff. I used a precision 6mm reamer to bring them back to line to cure a capstan motor that didn't self-start.
Wow and flutter is affected by the position of the flywheel relative to the shaft, before you take the spring clip off the back to release the flywheel, mark where the clip is with a pencil.
The other thing that affects running of the tape is the pinch wheels. I have one here that plays about halfway through a cassette before it auto-stops. The pinch wheels are quite hard and cause all sorts of wow problems. You could fiddle with the pinch wheel pressure springs but it probably won't help.
There is a B710 update document on the web, if you have trouble finding it I can send you a PDF.

[TIMTAPE]

Cassette design makes a true constant tape tension difficult to implement so as the relative tape pack diameters change through the course of a side, it's harder and harder for the capstans to pull the tape through. Slowing or inconsistent tape speed towards the end of a tape is very common.

Before diving in with modifications to capstan shaft bearings etc I'd be checking all relevent bearing/ pivot surfaces for dried/contaminated oil or grease, clean and relube, including the pinch roller rubbers which over time tend to go hard or contaminated in one way or another. Dont forget the pinch roller bearings which can also gum up due to dirt and old lubricant. Slight drag on a number of bearing surfaces can add up to a large total drag which will have more effect towards the end of a tape reel.

So many tape machine mechanical problems can be fixed with a minimum of materials, just a lot of careful, systematic checking, cleaning and relubing.

[monaro0162]

Thanks to all for the advice on this, I think that I have identified the problem.

I have another cassette deck that doesn't suffer from wow and flutter and remains stable right to the end of the tape. For ease of comparison, to avoid having to change the time base on the scope too much, I recorded a 375hz square wave on to a C90 cassette using the good deck (the same frequency as the capstan tacho's on the B710, and conveniently within the audio spectrum). I am able to play back the recording and monitor it with a scope. Whilst the waveform is not perfectly square, and there is some jitter on it, the scope reports a stable frequency of 375hz right to the end of the tape.

If I rewind the tape and then play it in the Revox, initially the frequency reported is about 380hz. It looks like more jitter is present too. So it's playing a bit faster compared to the other deck, fair enough. As the tape progresses to the end, the frequency drops as low as 369hz with more jitter, so the tape must be running through the Revox slower.

As the tacho signal from the capstans does not drop and stays at 375hz toward the end of the cassette, when the wow and flutter gets really bad, this can only mean that the capstan/pinch wheel is not gripping the tape properly, and is not able to pull it through at the right speed, it's loosing the battle against the tape back tension (from the 'supply' capstan or 'supply' reel?)

Looks like it does need new pinch rollers, I'll order some, hope that fixes it!

eta: since I got some lube into the capstan bearings, the one that sometimes didn't start by itself, seems to be starting up ok now, so I think that the bearings are ok.

[monaro0162]

I fitted the two new pinch rollers ordered from Germany and adjusted the pressure that they apply to the capstans using the springs. It made a noticeable improvement although the machine still didn't sound quite right towards the end of the cassette.

So I revisited the capstans and (despite lubing them a few weeks ago) found that the supply capstan now was intermittently not starting, and didn't spin as much freely compared to the other caostan. So I bit the bullet and fully removed the motor from the chassis this time, to get good access to the sintered bearings. Inspecting the capstan revealed a very slight darkening of the front bearing surface, which I very carefully cleaned with some wire wool soaked in oil. I wasn't able to get lube fully into this bearing last time, this time I did, and the capstan felt a lot smoother afterwards.

After getting it all back together it now plays ..... perfectly! The scope traces of both capstans now show less jitter, and the phase shift has reduced too

Next on the list is to get the counter working. I've read that this may be caused by an opto diode failure somewhere.

[monaro0162]

Try as I did, I could not fix the FF & REW problem on this machine. The opto sensors were dead even after they were replaced and with the signals restored it still would not work properly. Then at some point afterwards it decided it didn't want to hold in play either.

I spent a lot of time probing around trying to find out what was wrong, but without a decent manual it isn't easy. I really needed a logic analyser (and a decent manual) to continue neither of which I was willing to invest in, so I sold the 710 for spares or repairs. They are hellishly complex machines, good luck to the new owner. My quest for a 3 head cassette deck will continue. I think that I am going to look for a Japanese model. The B710 seems too complicated for it's own good.