Revox B77.

[NorfolkDaveUK]

I have two issues currently with my B77.

Firstly is the tape end sensor. It's not functioning at all. The voltage across the IR LED is 29V which is way too high, but I dont know how to go about searching for the fault.

Secondly the relay on the monitor board isnt closing. I have 21V on pin 8 but other than that I really dont know what to look for.

Thanks for any assistance.

[Chris55000]

Hi!

Have you got a Digital Multimeter with a Diode Test Range?

You will need one with a Diode/Semiconductor symbol (usually marked with a black arrow-and-line symbol on the panel) to test the IR diode.

Many mobile phone camera sensors have some sensitivity in the I.R. spectrum, try using a mobile phone camera first to see if there's any trace of light being emitted - you can't see it with the naked eye but you might pick up a tiny white dot on the screen if the sensor is emitting light.

If you cannot see any trace on a phone camera, switch off the mains power to the machine, switch your D.M.M. to "Diode Test", and connect it's test-probes to the leads to the sensor lead-out wires, one direction should show 1.8V to about 2.1V and the other direction "OL" or "1-" (dependent on the overflow indication of your meter).

An overflow reading in both directions indicates the I.R. diode is open-circuit, whilst a very low reading either way indicates a short-circuited sensor diode, in either case the sensor is un-serviceable and will require replacement.

It is possible, in some cases, to have an I.R. diode suffer from internal degradation causing very low emission, even though a seemingly normal forward drop of 1.8 to 2.2V can be read - substitution with a new sensor is the only sure test for this!

I will study the manual and come back later on your other fault!

Chris Williams

[Chris55000]

Hi!

Have you a good copy of the Service Manual to work from?

http://www.johnmcculloch.net/docs/Re...kI-II_Serv.pdf

Referring to p.122 of this for the 1.177.260.00 Monitor PCB, the monitor relay K1 coil is energised by the two transistors Q4/Q6 connected as a thyristor-pair in series with an enable transistor Q7, turned on directly by a +20-21V "y-ON" enable voltage which comes in at pin 7 of the Monitoring Panel.

Q4/Q6, R25/R27 and C14 provide a time-delay of about 1-1.5 seconds after the "y-ON" monitoring-enable line from the control/switching circuitry goes high to allow the amplifiers to stabilise and reduce the possibility of unwanted noises when swiching the monitoring on.

To fault-find this circuitry, switch the machine to "Recording" and select "Monitoring On", (see your operating manual) then use your D.M.M. to make sure pin 7 of the Monitoring Panel has gone high - if this is missing you'll need to trace it's source back from the control/switching circuitry.

Diodes D7, D8 and Q9 are present simply to rapidly discharge C14 when the monitoring is switched off, so the time-delay operates again each time the monitoring is switched on.

If Pin 7 on the monitoring panel is high at +20/21V but relay K1 is not operating, check transistors Q4/Q6 and Q7 by substitution - they are only standard TO-39 small-signal devices - I recommend you get BC107/BC177 originals rather than cheap plastic-case BC547s/BC557s etc., for reliability!

The 560k resistors R25/R27 can go high, but good quality metal-film replacements can be obtained online for a few pence.

It's a very simple circuit to fault-find that shouldn't prove any great headache!

Hope this helps - let us know how you get on!

Chris Williams

PS!

Continental printed-circuit panels of this age tend to have very fine spidery-print that lifts very easily - please be careful when removing or replacing components on them!

[NorfolkDaveUK]

Thanks Chris for such a comprehensive reply. The TES i managed to diagnose as kaput and managed to source another of for the cost of shipping for a kindly fellow Revox lover in Denmark, so that issue is resolved just waiting for it to arrive.

I have all the manuals / schematics from hifi engine, thanks for the link though. I have a decent fluke meter, so will get on to testing everything you mentioned in the morning, again thanks very much for your help.

[NorfolkDaveUK]

Hi Chris ,
Thats now fixed. Q7 was kaput. Put in a new one and its now closing as it should thank you.
Now I have more issues to contend with though, that's how this goes right, haha.

OK so when i put headphones in I only have audio on one side, and only one of the VU meters is working. That's the first problem .

Secondly, the machine is acting quite bizarrely, I will try and describe it as best that I can. If I connect a line in source (in my case from my TV, which is my pc) to the line in and select AUX on the RIGHT channel I get a good signal, and sound is normal and is adjustable via the correct pot. However, if I select AUX on the left channel I get nothing on the left side, I have to select "RADIO" on the left channel and THEN I get audio coming through and it can be adjusted via the correct level pot. Which of course is incorrect. Someone has been in here before me and there is a lot of lifted traces on the boards, but i have been through them all and tidied them up and they are all repaired ok. So its not that.

Play back is great. Stereo, clear, eq is nice.

[NorfolkDaveUK]

The weirdness has been fixed, I'm waiting on a sig gen and a scope to progress. I'll report back when I get those tomorrow.

[Chris55000]

Hi!

The same monitor Panel I described for fault-finding the Relay is also used for the VU Meter/Headphone Output, the LH Line Signal comes in at Pin 16 and the RH signal at Pin 17.

If you have a good-quality oscilloscope you can trace the LH and RH direct, (I believe you're in the process of getting one) but if they're held up for any reason, simply turn the LH and RH Monitor Volume Controls to max, put on headphones and touch Pins 16 and 17 with a screwdriver held in your fingers - a faint click/buzz should be audible in the 'phones and a twitch/deflection on each VU meter.

Q8 is a simple common-emitter variable gain stage whose gain is adjusted by theform a "Meter Cal" preset R33 providing variable negative current-feedback, whilst IC3/D3/D4 form an active audio full-wave meter rectifier.

The Headphone output stage is a simple four-times op-amp gain stage, transistors Q1/Q2 are a simple emitter-follower complemantary buffer to provide the necessary current output to drive the headphone drive units.

I've described the LH channel only (RH identical!), fault finding this is simply standard audio fault-finding principles of voltage-checks, etc.

I'd be very wary of the machine's internal wiring if there's signs of someone having been in it before you acquired it - if one of the I/P signals is missing from the monitor PCB I'd look especially at wiring and/or lifted print!

I must admit whilst I'm not a fan of "primary-skool" lettering Studer/Revox use in their bookwork, me preferring Uno-Stencilled British styles, you can't fault the clarity of the diagrams!

Chris Williams

PS!

If you find any PCBs too badly butchered to consider repairs, replacements aren't too difficult to come by - as well as eBay, etc., other Members may be able to help!

[NorfolkDaveUK]

Thanks again Chris . So far I`ve managed to save all the boards . they are a little but butchered but the traces at all there right up to the component most of the time its just the rings around the legs that are missing . I have had to use a couple of resistor legs and put the traces back in but they are all good now .

The wiring for the input had been switched (i take photos before I start and was putting the wiring back as I found it ) but the input for the mic and aux on the right side had been reversed so that fixed that issue .

Now I just need to sort the monitor board out .

What i`m going to do though is adjust the recording / input with a signal generator and scope to where they should be and then address the monitor board because the recording of the deck is very very low so someone has been twiddling the VR`s i think . I`ve replaced all the VR`s with piher ones now so they need adjusting anyway , i did resistance match them when i put them in but ofcourse thats not going to help if they are all wrong in the first place . I`ll report back when I`ve tried to get it into spec and see where I go from there . Thanks again for your help !

[NorfolkDaveUK]

You mention the Schematic here , I`m really struggling to get to grips with it to be honest . A lot of whats supposed to be there isnt , and some of what shouldnt be there is . Its an absolute mine field to navigate through .

I`ve attatched 3 pics . I have the mark 1 , 2 track , with a 310 power board . So if you look at the schematic , then look at the diagram , then look at what i actually have , nothing matches up .

[NorfolkDaveUK]

Ok . Monitor board is back up and running . It was actually the input board than had the problem . Q4 and Q7 were both borked . Someone had replaced them with bc238`s . I`ve put the 107`s back in there . I now have audio both sides in stereo . Now for calibrating the old girl . Thanks again Chris .

[NorfolkDaveUK]

-Just a quick update for you . https://youtu.be/iUQKzTBthAk This is prior to calibtating it . I had more issues , mainly that the right side bias coil was broken so i was only getting recorded audio one side . Took 3 days to find that .

Feeling proud . Thanks for your help Chris .