Revox Capstan problems / questions

trancedental

trancedental

New member
My Revox PR99 MK III seems to have some big problems, reverse & fast forward work as normal but play doesn't engage at all, the capstan wheel seems to go around quite slowly I'm sure that this is not normal?

Opened the back & it seems that it's a direct drive machine & doesn't have a capstan belt at all?

Any ideas on what to check first would be great! Fuses all seem OK

Cheers
 
Do you have the service maula for that deck? Sounds like you may have a failing fader start board....


AK
 
I've got the pdf service manual for it. This machine is the line i/o stereo version & does have the fader / capstan speed / tape drive connector inputs at the top back.
 
Are you sure the problem is in the capstan and not in the pinch roller? Have you tried to push the pinch roller againts the capstan by hand?.
 
TheReeler said:
Are you sure the problem is in the capstan and not in the pinch roller? Have you tried to push the pinch roller againts the capstan by hand?.
Click to expand...

All the tape path & roller is 100% clean, it changes nothing if I push the rubber roller against the capstan wheel.

As the capstan wheel is spinning slowly which doesn't seem right it seems to be a problem related to that?
 
I'm going to assume that it's a DC servo motor. I'd check the drive transistors to see if any have failed. If that proves to be the problem, replace them all.
 
RRuskin said:
I'm going to assume that it's a DC servo motor. I'd check the drive transistors to see if any have failed. If that proves to be the problem, replace them all.
Click to expand...
If I remember correctly, it has the same capstan motor than the B77, and that is an AC motor.
 
It's an AC motor, uses an oscillator controlled by a hall-effect tach counter on the outer rotor. I would suspect the controller board, which IIRC is in the middle of the machine. On the B77 this board has a big pot which controls the capstan speed, and I think the design is pretty much the same across the range. Might be worth checking the output of the hall effect sensor, though. I believe the expected frequency is in the manual.


EDIT:
Page 51 of the PDF file, section 6.3. The tach signal should be 800hz.

EDIT EDIT:
See also page 78... that gives you the expected signals throughout the entire drive circuit if you have to go down to that level. Presumably someone with a better understanding of electronics can also spot the power transistors.
 
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Seems that the capstan board has some problem, I've checked the voltages & they seem slightly out of spec.
Only got a multimeter so can't do much more, probably will get hold of a replacement board to try, might have to hand this over to a tech to look at.

Cheers for the help.
 
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Check the large can electroytic caps on the power supply board - they are known to fail on revox machines. The speed control circuit is driven from +21V off that PSU board.
 
OK I'm back on the case with this Revox machine armed with my recently aquired very dangerous DIY knowledge.

Capstan PCB tried a new bipolar disc transistor. As the old one looked a bit rough. Tape drive control PCB looks OK checked with a jewellers loupe.

So now I've got to look at the power supply PCB & look closely at these large caps!!!! Hope I can get all this back together again afterwards!
 
Well I checked 3 PCBs (capstan, tape drive & logic control) & replaced a few parts that looked suspect with no luck. However I have discovered at least one problem a wirewound resistor on the Power PCB which has a totally wrong reading. I need to unsolder a leg so I can 100% confirm the reading though.

Problem with the Power PCB is that it has loads of connections including many to the transformer & mains switch blocks & comes all out together by the looks of it, taking it apart & putting it together looks like a total nightmare!!!!!!!!
 
AFAIK, you HAVE to pull at least one leg of the resistor to measure. It won't give you the right reading with both legs stuffed in the circuit.

Sheesh...good luck with this...I wish I could help more. Keep at it.

Did you measure the voltages off the PSU? If you have access to a scope it may help to use that on the PSU to see how stable the rails are too...
 
To check the resistor or replace it & / or any adjacent parts I have to take the full power section out, usually there's molex type connectors & a few spade cables so it's not too bad but there's also some soldered wires on the underside of the PCB board & too many connections for me to be 100% sure of getting it all back working again. At least I've put it all back together in the meantime & not caused any damage!

I might comeback to it in a few weeks if I can't get the extra money for a tech to look at it, in the meantime I've got a 1176 and a Stereo tube opto pair of compressors to build!!!!!

I actually prefer building something from scratch, as long as you take a bit of care & do plenty of preperation things should turn out OK. Instead of troubleshooting problems, I think I'll leave that to the real experts!
 
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