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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 2nd Jan 2019, 4:58 pm   #1
Mikebee88
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Default Revox B77 not recording

I've just picked up a thread from 2015 on the issue of a Revox B77 Mk.1 tape machine not recording but with every other function fully operational. Moderator Michael Maurice provided some extremely helpful information regarding the logic cct. elements required for the recording process to work. I'm currently experiencing the same fault condition and from that thread have made some progress. I have a copy of the Revox B77 Mk.II Service Instructions manual. I am also in the process of replacing all the electrolytic caps from the audio and control PCBs. I also have a 'spare' B77 and a full set of PCBs that enable me to aid the faulting process by substitution. I'd be pleased to hear from anyone who's had similar issues with these machines.
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Old 2nd Jan 2019, 6:40 pm   #2
Michael Maurice
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Default Re: Revox B77 not recording

First, please do not recap the boards until you have fixed the record fault.

We don’t want you making mistakes adding to your problem.

I presume you’ve got the record switches in the on (up) position. These switches can cause problems themselves and should be cleaned to eliminate them.
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Old 2nd Jan 2019, 9:56 pm   #3
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Default Re: Revox B77 not recording

Thanks for picking up this thread. Yes, the record switches are in the ‘up’ position and the two LEDs glow when rec is pressed. Incidentally, the service manual shows a +24V supply to those record switches - should this not be +21V as I can’t find anywhere on the audio interconnection board a supply of the higher voltage! At the moment, I suspect a problem with the tape drive control PCB although the play and record buttons pressed simultaneously deliver 24V to pin 1 of IC4 but I’ve yet to check out Q10. I’ve also checked out the operation of the two relays and their contacts on the oscillator board using a bench PSU feeding its output via a LED and the board’s connecting pins thus proving the relay coils and the associated components.
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Old 5th Jan 2019, 3:16 am   #4
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Default Re: Revox B77 not recording

Recap the boards if you like it makes no difference if you do it before or after diagnosing the fault. I'm not sure what people's obsession with not doing it is. You aren't going to make it any worse by putting good caps in, just make 100% sure you orientate them correctly. I do it as a matter of course and 5/10 it fixes a lot of issues.

Anyway to the fault .

It's not the logic. If the logic fails it won't go into record mode at all.

I need some more info on this before I can advise you further, you say you have no record but do you have any erase? If you have erase but no record the fault is with the record card, swap that out for your good one and see if that cures the problem, if you have no erase and no record, then the issue is probably with the oscillator, it's not a power issue because if you have a power issue the LED's won't light.

Check Q8 on the Tape control board, that supplies the Y-rec to the record amp, Q10 is for the tape end sensor.
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Old 5th Jan 2019, 10:04 am   #5
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Default Re: Revox B77 not recording

Quote:
Originally Posted by NorfolkDaveUK View Post
Recap the boards if you like it makes no difference if you do it before or after diagnosing the fault , I`m not sure what peoples obsession with not doing it is . You arent going to make it any worse by putting good caps in , just make 100% sure you orientate them correctly . I do it as a matter of course and 5 / 10 it fixes a lot of issues.
And some times out of ten it lands you in the because you have used the wrong value or put it in the wrong way round.

Wholesale recapping may be good occupational therapy but it isn't the most efficient method of finding a fault. The cautious squint and the head-scratch are the two best tools in any repairman's box, and there is little point in mauling the board to no purpose.

When it comes to Revoxes and Studers, certainly the 470nF Rifas with which the transport electronics are sprinkled should be "replace on sight" components in any transport fault or when doing a full overhaul - the youngest of them have had their design life by now. Frakos across supply rails I also be regard as suspect, as I have found these to have a significant failure rate. But to bore in and replace the lot is seldom necessary as a fault-finding strategy and frequently counter-productive. As I have said before, an ESR meter allows you to find duff electrolytics in-circuit, and is thus a most useful tool.
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Old 5th Jan 2019, 12:29 pm   #6
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Default Re: Revox B77 not recording

no it never lands me in the poo , it just doesn't fix the problem , because i`m meticulous about how i put them in , you seem to assume that people are going to make mistakes and you shouldnt , its not difficult to recap revox machines , they are clearly marked, you have the board diags in the manual . IF you are unable to recap them properly , then yes you shouldnt even recap them at all , but if you have the skill set needed to do the work if make absolutely not difference whether you do it before or after fault finding thats the point . The electrolytic caps in the revox machines are crap , they no where near as good as the panasonics of today and if you want the machine to last another 50 years you have to replace them , I do it with every machine that comes through my workshop whether its a revox or not , 74 machines in the last 8 months to be exact . Its never un-necessary , and always productive if done correctly , if you want to spend hours going through the machines with an ESR thats great , but I recap the whole thing in a couple of hours and then I know they are all in new condition and will be for many years to come and the new own wont have to revisit them . its not about anything other than the longevity of the machine and the only way to guarantee your customer they wont be coming back and paying another fault finding fee is to replace them , it cost very little , and its the best way to ensure longevity.
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Old 5th Jan 2019, 12:35 pm   #7
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Default Re: Revox B77 not recording

It's your Revox, it's your choice.

I'm very, very, careful (The day-job is stuff for aircraft) but I still make the occasional mistake. Not all new components are good, there is a small risk of fitting a bad one. There is a risk of getting a fake component. Nothing is so cheap that it isn't now being faked.

The way to control these risks is to do a section at a time and test after doing each section. With a stereo item you can do the L and R channels separately so if something goes wrong, you have the other channel in known working order to compare against as well as working in a small area. Diagnosis is a lot easier.

This approach does rely on having a working machine in the first place. So whether you want to do a minimalist repair or whether you want to do a full reconditioning job, the optimum place to start is by doing the minimalist repair.

Your reconditioning job could work all the way along one channel first and then you can have a bit of fun comparing the two channels. By ear, with test equipment, however you fancy. You'll learn whether or not it made a real difference in your case. Everyone has their theories, but this is reality. It's interesting to check theories with some reality.

Electrolytic capacitors contain a water-based conductive gel. It forms the effective surface of one plate of the capacitor. Over time the water escapes as vapour past the case's seals. The capacitance doesn't decrease significantly until the end, but the resistive component steadily climbs. ESR meters measure this. They are not only brilliant for spotting dead capacitors, they give you reassurance of how much life is left in the living ones. Running over the electrolytics with an ESR meter can be illuminating.

I bought a Revox B261 tuner a while ago at a G-QRP doo at Rishworth. It was ex-BBC and had run continuously for decades mounted in a rack full of hot stuff. It worked, but the mark-1 eardrum said it wasn't working well. There was hum and it seemed a bit light at the low frequency end. Opening it found a couple of previously replaced capacitors in the power supply area. The new parts were not one of the trusted-reliable makes. Prodding around with a scope showed hum on one power rail and led me to a small capacitor in the regulator circuit, the main reservoir was working reasonably well.

Out came the ESR meter and the audio board was first up. Over 50% of the electrolytics were showing ESRs high enough that even fans of the minimalist approach would replace them. The other boards had plenty of examples of the same values.

This whole tuner had been baked for years, and heat greatly accelerates the loss of water from electrolytics. Seeing the pattern, I set to and replaced the lot, comfortable in the knowledge that it wasn't wasted effort. The new capacitors came from a main industrial supplier which deals directly with manufacturers in large quantities. They were of makes I'm comfortable about trusting (I routinely test electronics over -55 to 85C, altitudes to 80,000 plus explosive decompression and crash tests and vibration levels simulating a damaged helicopter). They weren't expensive 'esoteric' ones designed to impress audiophiles.

Job done. Working tuner. No errors introduced, no fault finding needed beyond tracing the hum. No new components were faulty.

So, I don't subscribe to the 'Recap everything!' band. This is often the approach of non-technical people coming from the audio hobby direction. It isn't a terrible approach, but it turns into a disaster if workmanship isn't excellent or if the new parts are iffy. It also does nothing about faulty parts which aren't capacitors.

I don't subscribe to the 'only change the minimum' band, either. It has the strength that good components working happily have passed their infant mortality period and have settled down to good proven reliability. They really are best left undisturbed.

I like to find out.

I don't subscribe to the audiophile religion either. On one hand they seem to live in their own mysterious universe where all differences are dramatic an large (Though I often can hear nothing or little). On the other hand, I find the exaggeration of theoretical low probabilities and pseudo science to be generally hilarious. Historically, they have latched onto a few things which were right and of real value. I don't dismiss them out of hand but I do like to introduce a sense of proportion. I'm disquieted by the amount of money they're being fleeced of. They're fragile, insecure, creatures, spooked by the sight of even a multimeter. The sight of an oscilloscope produces the full garlic and Dracula effect.

I strongly support getting an ESR meter.

David
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Old 5th Jan 2019, 12:48 pm   #8
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Default Re: Revox B77 not recording

of course if you replace with worse caps than the ones that are in there you are shooting yourself in the foot. You have to replace with caps that are of good standing , i also dont conform to the audiophile way of thinking , but ive done enough of these now to know they are slowly failing , an esr will tell you what condition the caps are in today , but unless you check them regularly you have no idea what they will be like tomorrow . The caps are 50 years old , and are in a lot of cases dry and slowly failing , use good caps and replace them its the only way to ensure that they are going to be good for a long time to come . Its nothing to do with being non technical or an audiophile , I`m very technical and again its all about longevity . All of my machines are recapped and they always sound better for it . (i have 2 esr`s by the way)
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Old 5th Jan 2019, 2:00 pm   #9
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Default Re: Revox B77 not recording

Quote:
Originally Posted by NorfolkDaveUK View Post
no it never lands me in the poo
Really?

What about the B77 transport logic fault you had a while back?

All human flesh is heir to error, and it is only good sense not to risk causing more faults when you are trying to the first one. Scientific method - change one thing at a time.

Last edited by Ted Kendall; 5th Jan 2019 at 2:16 pm.
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Old 5th Jan 2019, 3:27 pm   #10
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Default Re: Revox B77 not recording

yeah you got me there but that was a while ago now . Since them I have learned a lot and become a lot more meticulous . I have the benefit i having a huge collection of A77/B77 boards now I guess so I have one beside me now when I re-cap and that makes it a lot easier not to get them in wrong , of course I still do but I`m able to check the error quickly and efficiently now before it goes into the machine .

But my point really is that AS LONG AS YOU PUT THEM IN CORRECTLY its going to make no odds putting them in before or after finding the fault and its possible , not guaranteed , but possible that you can save yourself a lot of time in the long run . Its highly possible too that you wont , but at least you can then rule out the caps , again as long as you make damn sure they are in correctly before you move ahead , which anyone with common sense will do.

I recap my machines because I want the buyer to have the best machine possible and not have to worry about rogue caps going bad on them , which you have to agree is inevitable , I dont use cheap Chinese caps , I only use Panasonic , I make darned sure they are in right and , as yet , I haven't had one back or been asked to repair one I`ve sold. So FOR ME the method works . I guess you just have to do what you think is right and let others do the same . Don`t get me wrong , some of the machine I do are an absolute pain in the backside to re-cap . The Teac 4 channel machines especially so , but I would rather take the time and effort to do it now and give someone something that`s going to stand the test of time than have them coming back with constant problems because the caps are failing . Not only that , I enjoy it , a lot , its great therapy for me for sure . Hey we are all in this for the same thing to keep those machines going , and however you choose to approach it . If it works , keep doing it .
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Old 5th Jan 2019, 4:16 pm   #11
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Default Re: Revox B77 not recording

Interesting discussion. I'm not a Compulsive Component Changer myself.

Can we now get back to assisting the OP please. It's up to him to decide whether he replaces all the caps.
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Old 5th Jan 2019, 5:50 pm   #12
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Default Re: Revox B77 not recording

Quote:
Originally Posted by Mikebee88 View Post
Incidentally, the service manual shows a +24V supply to those record switches - should this not be +21V as I can’t find anywhere on the audio interconnection board a supply of the higher voltage!
I think it might be a misprint, the layout drawings of the boards (the power supply board & the audio interconnection board) and their interconnects show 21 volt (from the stabilized supply...IC1) so far as I can make out.

Lawrence.
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Old 5th Jan 2019, 8:36 pm   #13
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Default Re: Revox B77 not recording

Is the oscillator actually running?
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Old 5th Jan 2019, 10:13 pm   #14
Mikebee88
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Default Re: Revox B77 not recording

Yes, the erase function operates correctly for both channels but fitting a known working Record Amplifier card still results in no recording to tape!
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Old 5th Jan 2019, 11:32 pm   #15
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Default Re: Revox B77 not recording

Ah, the B77 has a 100 Ohm resistor to ground in series with each record head winding. This can be used to monitor the recording current right at the head. If a record amp is sulking, or if a record head is open circuit then you'll get nothing here.

It's a good place to go looking. If you see nothing, when check head continuity and work your way backwards along the record and input amplifiers.

There will also be a lot of bias signal on here.

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Old 6th Jan 2019, 10:19 am   #16
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Default Re: Revox B77 not recording

Hi all,
I assume that the continuity of the record head has been checked? Obviously degauss after checking with a multimeter.
If this is OK then you need to look at the bias oscillator PCB as the record feed from the record amplifier PCB is fed onto the mother board and thence to the bias oscillator PCB and then via contacts K1 and K2 on the oscillator PCB then out via the multi-pin connector to the erase and record heads.
I would lay odds on the relay contacts or the relay itself not functioning.
If that is the case several suppliers on eBay provide new mini piggy back PCBs with modern PO type relays. I have just fitted one to my B77 but actually removed the relay and refitted it in a DIL socket. I just don't trust relays!

John
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Old 6th Jan 2019, 3:20 pm   #17
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Default Re: Revox B77 not recording

Just in case .....This might be worth mentioning: Only ever use a digital meter to check the continuity of a tape head.

An analogue meter uses more current when testing than a digital one. This can be enough to leave the head permanently magnetised, where it will end up gradually erasing your tapes: they will get quieter and quieter each time you listen to them. This effect is barely perceptible, but still pernicious.
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Old 6th Jan 2019, 11:41 pm   #18
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Default Re: Revox B77 not recording

Now let me put in twopence worth!

First I'm not sure how competent the O/P is repairing these tape recorders, and I dont want him introducing faults inadvertently which is why I suggested not to recap the machine UNTIL HE HAD FOUND AND REPAIRED THE ORIGINAL FAULT!

Now from what I can see, the fault affects BOTH channels and I'd be surprised if both relays failed at the same time. I would also also be surprised if both sections of the head had failed.

As the erase function works, we can assume that the oscillator is running.

You might want to check/replace C1 and C2 on the oscillator board as I've known them to fail.

I think you should also check that audio is getting into the record amplifier and is being outputed to the oscillator as a failure there would give the same results as you have.

Do the VU meters respond to signals when the monitor switch is set to INPUT?

Do you have access to an oscilloscope? You really need one to find where this fault is.
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Old 8th Jan 2019, 12:55 pm   #19
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Default Re: Revox B77 not recording

Silly question I know but is the record head clean and is the tape making good contact with it? One pass of a Sticky Shed tape can put so much muck on head faces that recording or playback are virtually impossible. B77 heads are partially concealed and it's not always easy to see how clean they are.

Also is the tape you are using to record threaded with the oxide side facing the heads?
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Old 9th Jan 2019, 11:50 pm   #20
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Default Re: Revox B77 not recording

Three posts moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=153014
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