Odd happenings on a PR99

[Linnovice]

Hi all, I just spent the last couple of days giving my PR99 a little service. I've changed the capstan shaft, lubricated the spooling motors (I know it's not in the manual but they really benefit from a little oil. Especially after 25 years!) changed the capacitor on the back of the voltage selector and the three motor caps.

Now onto the alignment. According to the manual the initial input voltage is measured from the monitor output. I found myself an old lead fitted with a DIN plug of the correct polarity and connected. Initially it was fine but then I started getting weird readings with the millivolt meter flicking all over place and really odd trace on the oscope. It looked like a conventional sine wave but it was not a clear trace; it was made up of a very small sine wave. Very odd.
After much probing and searching I checked the actual DIN socket and found that all the pins were shorting out to earth! I have a sacrificial machine so took the socket from that and installed it in place of the shorting one. ****** me if it wasn't the same! Eventually I disconnected the signal leads from the socket and completed the alignment by ear/meter using the normal input/output xlr sockets.

I've never seen this before. Can anyone throw any light on why this would happen? I've not had any problems previously with the machine but this is the first time I've used the monitor socket. The machine is ex-BBC and badged as a Revox MkIII but all the level adjusting knobs/cards have been removed. Would that have anything to do with it?

Very odd anyway.

Mike

[Radio Wrangler]

There are some very odd variants on revoxes around, for example a replay only tape editing machine:

http://en.audiofanzine.com/analog-mu...,m.170294.html

What you have could have been modified in almost any way. I've seen ones with no boards except mechanism control used as simple tape transports for use with external electronics.

So the first task is to find out just what you've got.

David

[Linnovice]

Hi David, yes I think you're right. The input is definitely out of phase according to the oscilloscope but the recording and playback is fine. It's a bit weird. I still can't work out how the din sockets seems to short everything out. Eventually I set it up using crocodile clips on the pins themselves. A bit crude but it worked. Sounds good now. M.

[Radio Wrangler]

There's also the possibility that someone got it wrong.

How did that advice go? Never attribute to malice that which can be adequately explained by stupidity - or something like that.

I can't see a reason for a shorting DIN. It doesn't seem worth the trouble of adding switch contacts to ground inputs when nothing's plugged in.

David

[Linnovice]

Hi David,
Good advice . . .
Thing is, the DIN only shorts when a plug is 'plugged in'. I can't fathom any reason for that other than preventing it being used for setting levels. The only thing I can think of is that when in use with the BBC an engineer may have set it like that for a particular application. When the corporation decided to sell off its machines nobody would have bothered to set the machine back to standard?
Mike