Revox PR99 MK2 stops after releasing play/rev/ff keys

david winter

david winter

Member
Hi,

I've been using this machine for several years without trouble.
I tried to finish playing a tape yesterday after listening to half of it the day before so the tape is correctly threaded.
It will play without trouble as far as I keep the Play key pressed.
The same problem occurs with the rev/ff keys.

Any ideas what this may be ? Looks like some flip-flop gone wrong, or the stop command locked enabled (maybe the same problem).

Any hints welcome


Thanks,

David
 
A shorted stop switch does sound like a worthy candidate to take a peak at assuming no sticky shed issues are in play, compounding the issue.

Cheers! :)
 
david winter said:
The problem was one shorted tantalum on the ader-start board. I replaced both and the problem disappeared.
Click to expand...

"tantalum"

Was that a shorted capacitor? I had to Google that word, tantalum, to find out what that was. Wikipedia says its a substitute element for platinum and is primarily used in capacitors for computers and cell phones. Interesting to see Studer using this in a 1980's analog reel to reel!

Glad to read you were able to find the short and get your deck operational again.

Cheers! :)
 
It was a 3.3µF under 35V and measured 0.5 so it must have turned very leaky if not shorted at the working voltage.
Indeed tantalum capacitors were used quite early. I have seen them used in late 1960s equipment like Band & Olufsen and even the Revox A77.
Some people say they must be replaced, some wouldn't care as far as the machine works.
If they are of superior quality, they cost significantly more than electrolytics.
I think I had to pay over $100 for a full set of A77 capacitors but I could be wrong.
 
The Tantalum capacitor is INFAMOUS in electronics serviceman circles!
They were almost always bright blue hence the appellation "the dreaded blue Tant!" I am not generally in favour of ad hoc re-capping without good reason but I would always rip out tants from old kit and replace them with modern Aluminium electrolytics.
There are almost no circumstances in common (i.e. not Milspec) circuits where Al caps will not work.

Don't know about the machine in question but the relays in the A77s used to give trouble. You can carefully remove them from their plastic cover and clean the contacts with a diamond dust impregnated stick or a bit of very fine "wet&dry" lubricated with WD40.

Dave.
 
Yeah, from what I have read on the subject, it seems ReVox favored German capacitors (rather than Japanese) and these tend to short rather than slowly die or bulge or leak, causing other serious problems, leading many to recommend all ReVox gear to be recapped, no matter if it's showing problems or not.
 
cjacek said:
Yeah, from what I have read on the subject, it seems ReVox favored German capacitors (rather than Japanese) and these tend to short rather than slowly die or bulge or leak, causing other serious problems, leading many to recommend all ReVox gear to be recapped, no matter if it's showing problems or not.
Click to expand...

Ah yes! The Wimas! (another baddie was Sprague. Looked great in their little metal cans but not to be trusted)
Mostly Wimas were brown or black and almost always "crazed" i.e. covered in cracks. Yes rip those out. Big problem in Grundig TRs.

Wima capacitors are still made and the Audiophool brigade pay silly money for them. I hope their longevity has improved!

Dave.
 
david winter said:
The problem was one shorted tantalum on the ader-start board. I replaced both and the problem disappeared.
Click to expand...

Could you elaborate a bit on how you diagnosed the problem? I'm having the same symptom with my B77 Mk II. What, exactly, is the "ader-start" board?

Thanks for your help!
Howard Ritter
 
I found the explanation on another forum and the guy traced the whole thing during 25 hours.
The board is the one holding the two switches near the LED counted. It's a small board.
Note that I tested the capacitors before replacing them to make sure the problem was at least at this place. One capacitor was almost shorted, the other tested fine but by matter of security I replaced both (and accessing this board is not as easy as the other plugin boards).
 
Hi Guys,

mine is a similar problem. My Revox PR99 MK3 started to stop suddenly during recording or playback randomly, than yesterday it decided to die completely (no play, rev&fwd keys work).
before loosing the controls, the motors were moving slowly themselves (5 mins) and then NO controls any more available, just the main capstan works when i turn on the machine.
I have swapped all the capacitors on the Transport control board with no luck, could it be any motor capacitors or something related to the power supply?
Any suggestion please? Thanks.
 
Hi,

I can't really help you but here are the causes of motor failure that I encountered so far:
- Bad relay contats (seen in A77; don't know if the PR99 has the same relay commands)
- Bad motor capacitors (the cylindrical ones mounted on the chassis)
- Bad motor "switch" capacitors (usually 0.047/250V) used with a 47R resistors in seris, itself often burnt when the capacitor shorts, which is a common failure

I would first replace the three 0.047 capacitors (use the X1 type for mains operation) and check the 47R resistors just in case
If this doesn't help, check the motor capacitors. You will need the appropriate test equipment to check them (not only in value, but also for current leak etc).

Finally, check the relays in case some contacts became resistive or just open. This happens.

Hope this helps !
 
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