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Revox B77 Restoration

Finally found a data sheet for the TBA 231A
http://www.bg-electronics.de/datenblaetter/Schaltkreise/TBA231.pdf
Looks like the Fairchild UA739 is the same thing as well
http://www.chipfind.net/datasheet/html/fairchild/ua739.html
And the data sheet for the LM4562
http://www.ti.com/lit/ds/symlink/lm4562.pdf
I have worked out the pin out for both and will fit a socket with the crossover wiring
Works really busy at the moment so don't have time for this right now
The machine sound good as it is right now but will give it more time for all the changes to settle down and then recalibrate


Alan
 
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Been doing some more work on my B77 recently to resolve a few issue’s

I was getting a regular wow from it that I put down to the capstan pinch roller wheel as it was a bit tired but when I got a new one the problem was still there after cleaning the whole tape drive. I thought it might be some oscillation from the capstan drive motor board so I checked out the electrolytics I recapped this board with and they were just general purpose ones from a A77 service kit I bought a while back but they had very low ESR, so I decided to change them back to some new AVX tantalums that I had and this has cured the problem.

Capstan drive board by Alan Towell, on Flickr
 
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I was also getting some intermittent output fro m the left channel and I traced that to the poor quality RCA sockets which were loose and tarnished.

The RCA plugs on the A77 and B77 are crap and too close together to get the plugs in properly so I bought a set of replacement ones from Revox online in Germany. Fitting was straight forward but you have to do some work with a round file to accommodate the extra spacing in the new circuit board

I also did away with the 5 pin din radio socket wiring as it was also knackeered.

I changed the 2 output pots for a couple of Sfernice P11S ones I had form a passive pre amp.

This cured the intermittent output so sounding much cleaner now with even channel balance.

Before

Old in out board by Alan Towell, on Flickr

In out board 2 by Alan Towell, on Flickr


After

In out board by Alan Towell, on Flickr

New RCA Board by Alan Towell, on Flickr

IMG_2456 by Alan Towell, on Flickr
 
My record board didn’t have the muting circuit board so again I picked a one up from ebay and fitted it

Whilst I had everything apart I also changed out all the carbon film resistors directly in the signal path for Takman Rey metal films on the input, record and reproduce boards

Muting circuit board by Alan Towell, on Flickr

Alan
 
So after all this I calibrated the deck for RTM LPR90 tape and the frequency response form it is within +- 1 db and it sounds really good at 7.5ips

Here is a short video of the calibration check after adjustment, input was from an NTI Minirator MR Pro signal generator and the output was monitored with a Leader 186A dual channel AC Voltmeter all set for -10dB


Alan
 
I would expect a B77 to beat the pro PR99 due to not having the balancing transformers at input and output plus any other extra gubbins in the signal path.

Excellent work there Alan!:)
 
Jez it will be interesting to compare the PR99 Mk1 which shares all the same circuit boards appart from the Balanced In/Out boards and transformers as you said
I have a PR99 mk1 but it also needs a full overall so it will be a while before I get round to that unfortunately, I also have a MK 2 in the same state
There were significant changes in the circuit to the PR99 mk 2 & Mk 3 and they added the electronic counter and more opamps mainly RC 4559

BTW I worked out the pinout for replacing the TBA231 opamp with an 8pin LM4562 but havent implemented it yet
I also notices someone has made an adapter with an NE5532 built in so that would be a neat solution
http://rover.ebay.com/rover/1/711-5...0001&campid=5338728743&icep_item=171925845032
Alan
 
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337alant said:
Jez it will be interesting to compare the PR99 Mk1 which shares all the same circuit boards appart from the Balanced In/Out boards and transformers as you said
I have a PR99 mk1 but it also needs a full overall so it will be a while before I get round to that unfortunately, I also have a MK 2 in the same state
There were significant changes in the circuit to the PR99 mk 2 & Mk 3 and they added the electronic counter and more opamps mainly RC 4559

BTW I worked out the pinout for replacing the TBA231 opamp with an 8pin LM4562 but havent implemented it yet
I also notices someone has made an adapter with an NE5532 built in so that would be a neat solution
http://rover.ebay.com/rover/1/711-5...0001&campid=5338728743&icep_item=171925845032
Alan
Click to expand...

I always thought the digital counter and the way it slows as it gets to a pre-programmed point on the tape and then inches back and forth until the counter reads zero was well trick!

I got my MkIII with a horrible intermittent fault which the past owner was "good enough" to keep schtum about...
it would seemingly quite randomly just suddenly stop altogether, as if mains power had been quickly switched off then on...Usually in the middle of a live recording of course! I thought it seemed like an intermittent bad connection probably at the mains inlet socket but further probing found it was not this... It took some finding but turned out to be a small bridge rectifier of about an Amp or two... I wish I could remember exactly which one and where but it was years ago... I wonder though if it was a blessing in disguise as being an unusual and difficult to trace fault it could maybe have meant that the machine had unusually low use...

It's since developed another fault which is that the auto stop has packed up, but as this is of little consequence in normal use, more an annoyance really, and as I so rarely use it, I've never done anything about it... I've actually only used it for maybe 15 mins a year for the last 22 years or so I've had it if averaged out!:oops:

I've certainly had thoughts about replacing all the RC4559's with say LM4562's... and indeed even NE5532's are good and a big improvement on the originals, but the lack of any real use for it beyond it being a nice technical objet d'art means it will probably never happen... I have all sorts of condenser mics, mic stands, couple of mixing desks, loads of cables etc etc as well that have all seen probably only two hours use since I bought them:rolleyes:
 
This site contains affiliate links for which pink fish media may be compensated.
Thats not a lot of use over that time so it should be in good shape mechanically, Im still looking for a Mk3 PR99 and I dont mind if it has faults or doesent work at all PM me if you are thinking of selling it
The cuting out fault was probably the 0.8A Bridge rectifier on the Capstan control board
The not stopping is probably just the end of tape sensor or narrow focus LED, the end of tape sensitivity trim pot could also be flakey all located on the tape drive control board

Alan
 
337alant said:
Thats not a lot of use over that time so it should be in good shape mechanically, Im still looking for a Mk3 PR99 and I dont mind if it has faults or doesent work at all PM me if you are thinking of selling it
The cuting out fault was probably the 0.8A Bridge rectifier on the Capstan control board
The not stopping is probably just the end of tape sensor or narrow focus LED, the end of tape sensitivity trim pot could also be flakey all located on the tape drive control board

Alan
Click to expand...

Thanks Alan. It was an odd fault which was just as if the mains supply had failed for a split second, IIRC the lights flickered off as all the solenoids were heard to release, leaving the machine sitting as if it had just been powered up. It could go as long as 2 hours or happen every 15 mins at random...

I've probably mentioned it before but I do have an A77 that would be suitable for someone looking for one they "dont mind if it has faults or doesent work at all".... Probably a challenge though!
 
I'm still looking for a fully fettled and working B77, one that does 3.75 and 7.5 ips and must be quarter track...pm me if you have such a beast.
 
Just got round to painting the back panel on my B77.
After removing the Nextel coating and a good rub down and de-grese with panel wipe, I gave it 2 coats of Halfords Gray Primer.
The best match I could find for the colour of the front panel was peugeot graphite grey metallic which needs a couple of clear lacquer coat as well.
These aren't the best spray cans but did a half decent job in the end

IMG_2477 by Alan Towell, on Flickr

IMG_2507 by Alan Towell, on Flickr

IMG_2509 by Alan Towell, on Flickr
IMG_2512 by Alan Towell, on Flickr
Alan
 
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