Revox A77 mk.4 plays as soon as power is applied - UK Vintage Radio Repair and Restoration Discussion Forum

quad2fan · 16th Jul 2012, 10:51 am

Hi folks!

Hoping someone can help with my Revox A77 mk.4.

Bought one of these very cheaply recently. Had the usual problems with the Rifa and Frako caps. Every capacitor now replaced (inc. new motor run caps from Nagravox), and new end of tape sensor lamp fitted.

Problem is as soon as I apply power to the Revox, it immediately goes into play mode. Stop button does not work. I can engage rewind or ff, then press stop but it then - again - resumes playback. The only way to stop it playing is turning the power off.

I'm not an expert but guessing it might have something to do with the relays, or the resistors associated with the original Rifa 0.47uF perhaps?

Any suggestions would be gratefully received.

Cheers,

John.

Michael Maurice · 16th Jul 2012, 12:25 pm

Hi John,

When you replaced the capacitors on the control board, did you leave the board in situ or did you remove it?

If you removed it, then check all the wiring is refitted correctly.

One other thing, if you hold the record button in, does it go into record?

quad2fan · 16th Jul 2012, 1:06 pm

Thanks for your reply Michael.

I took the board out to replace the caps, and to also clean the brass switch contacts underneath the board. Will do as you suggest and re-check all the wiring, plus try pressing the record button.

Cheers,

- John

quad2fan · 16th Jul 2012, 3:31 pm

P.S. haven't had a chance to check it yet, but remembered something else I should add. The deck was working fine (after the recapping) couple days back, though rewind seemed a little weak compared to FF. Set voltage to 21v without any trouble. No problems.

However, the bulb between the VU meters was dead, and there was no end of tape bulb either.

It seems since fitting those the deck has suddenly decided to go directly into play mode as soon as it's turned on... I guess I could have dislodged something when putting the covers back on after inserting the bulbs. Or do I need to readjust the 21v setting now the new bulbs are in place?

- J

quad2fan · 16th Jul 2012, 4:18 pm

Update: figured this had to be something simple as it was working fine before replacing the dead/missing bulbs, and all the wiring checked out after a thorough consultation over the photos I took whilst dismantling the deck. Decided to check the position of the white plastic 'sledges' attached to the control levers. Sure enough the middle 'Play' sledge (which holds the brass switch contacts) was not fully out after being pressed, so the machine was in a constant state of play. Not quite sure why it was sticking but it's working fine now without the bottom front cover in place. Fingers crossed it'll be ok when the cover is put back. Will have to make sure none of the buttons are rubbing against the housing. Was only obvious whilst looking internally at the underside of the relay board. No buttons were obviously stuck in the in position from the outside...

Anyway... Whew!

Now all that's left to do in the future is replace all the plug-in pcb board trimmers with modern sealed versions I got from Nagravox (look like unbranded Piher types to me - hope they're not cheap chinese knock-offs). Once done I need to find someone near Basingstoke who can set the deck/trimmers up for me to factory specs, and bias for RMG-911 or other suitable modern tape. Any recommendations for techs who really know their stuff (and won't break the bank...!) ?

Cheers,

John.

Michael Maurice · 16th Jul 2012, 4:39 pm

I'm in NW London and sometime in the next week or so I have to go to fleet.

But you should be made aware that setting up is not easy andcan be time consuming.

I would leave all pots alone unless you need to touch them.

quad2fan · 17th Jul 2012, 2:05 pm

Yes been looking through the service manual and everything bar setting the 21v psu pot is way beyond my means, and I don't have the measuring equipment at any rate. Still the Nagravox 'kit' of parts came with a nifty Excel spreadsheet where one measures the old removed pots very carefully (i.e. end to end; right to middle; left to middle). Then end to end on the new pots. Then the program works out the values to set the new pot to before soldering it in. I guess not as accurate as setting it up from scratch but it's meant to work well. I've replaced all electrolytics and tantalums, and the motor run caps, and about to do R125 as that measures below 1K2, and the rewind is a little weak on 7" spools as a result. Only pots I replaced are the PSU and speed control ones.

If you happened to be passing through as it were, I'd love to have you set these pots up correctly. I'd solder them all in before you arrived to save time. Failing that some other time would be great. Appreciate the offer of help

If you could P.M. me with further details, likely costs etc I'd be grateful. Many thanks!

John.

16th Jul 2012, 10:51 am	#1
quad2fan Retired Dormant Member Join Date: Sep 2008 Location: Basingstoke, Hampshire, UK. Posts: 52	Revox A77 mk.4 plays as soon as power is applied Hi folks! Hoping someone can help with my Revox A77 mk.4. Bought one of these very cheaply recently. Had the usual problems with the Rifa and Frako caps. Every capacitor now replaced (inc. new motor run caps from Nagravox), and new end of tape sensor lamp fitted. Problem is as soon as I apply power to the Revox, it immediately goes into play mode. Stop button does not work. I can engage rewind or ff, then press stop but it then - again - resumes playback. The only way to stop it playing is turning the power off. I'm not an expert but guessing it might have something to do with the relays, or the resistors associated with the original Rifa 0.47uF perhaps? Any suggestions would be gratefully received. Cheers, John.

16th Jul 2012, 12:25 pm	#2
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox A77 mk.4 plays as soon as power is applied Hi John, When you replaced the capacitors on the control board, did you leave the board in situ or did you remove it? If you removed it, then check all the wiring is refitted correctly. One other thing, if you hold the record button in, does it go into record? __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

16th Jul 2012, 1:06 pm	#3
quad2fan Retired Dormant Member Join Date: Sep 2008 Location: Basingstoke, Hampshire, UK. Posts: 52	Re: Revox A77 mk.4 plays as soon as power is applied Thanks for your reply Michael. I took the board out to replace the caps, and to also clean the brass switch contacts underneath the board. Will do as you suggest and re-check all the wiring, plus try pressing the record button. Cheers, - John

16th Jul 2012, 3:31 pm	#4
quad2fan Retired Dormant Member Join Date: Sep 2008 Location: Basingstoke, Hampshire, UK. Posts: 52	Re: Revox A77 mk.4 plays as soon as power is applied P.S. haven't had a chance to check it yet, but remembered something else I should add. The deck was working fine (after the recapping) couple days back, though rewind seemed a little weak compared to FF. Set voltage to 21v without any trouble. No problems. However, the bulb between the VU meters was dead, and there was no end of tape bulb either. It seems since fitting those the deck has suddenly decided to go directly into play mode as soon as it's turned on... I guess I could have dislodged something when putting the covers back on after inserting the bulbs. Or do I need to readjust the 21v setting now the new bulbs are in place? - J

16th Jul 2012, 4:18 pm	#5
quad2fan Retired Dormant Member Join Date: Sep 2008 Location: Basingstoke, Hampshire, UK. Posts: 52	Re: Revox A77 mk.4 plays as soon as power is applied Update: figured this had to be something simple as it was working fine before replacing the dead/missing bulbs, and all the wiring checked out after a thorough consultation over the photos I took whilst dismantling the deck. Decided to check the position of the white plastic 'sledges' attached to the control levers. Sure enough the middle 'Play' sledge (which holds the brass switch contacts) was not fully out after being pressed, so the machine was in a constant state of play. Not quite sure why it was sticking but it's working fine now without the bottom front cover in place. Fingers crossed it'll be ok when the cover is put back. Will have to make sure none of the buttons are rubbing against the housing. Was only obvious whilst looking internally at the underside of the relay board. No buttons were obviously stuck in the in position from the outside... Anyway... Whew! Now all that's left to do in the future is replace all the plug-in pcb board trimmers with modern sealed versions I got from Nagravox (look like unbranded Piher types to me - hope they're not cheap chinese knock-offs). Once done I need to find someone near Basingstoke who can set the deck/trimmers up for me to factory specs, and bias for RMG-911 or other suitable modern tape. Any recommendations for techs who really know their stuff (and won't break the bank...!) ? Cheers, John.

16th Jul 2012, 4:39 pm	#6
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox A77 mk.4 plays as soon as power is applied I'm in NW London and sometime in the next week or so I have to go to fleet. But you should be made aware that setting up is not easy andcan be time consuming. I would leave all pots alone unless you need to touch them. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

17th Jul 2012, 2:05 pm	#7
quad2fan Retired Dormant Member Join Date: Sep 2008 Location: Basingstoke, Hampshire, UK. Posts: 52	Re: Revox A77 mk.4 plays as soon as power is applied Yes been looking through the service manual and everything bar setting the 21v psu pot is way beyond my means, and I don't have the measuring equipment at any rate. Still the Nagravox 'kit' of parts came with a nifty Excel spreadsheet where one measures the old removed pots very carefully (i.e. end to end; right to middle; left to middle). Then end to end on the new pots. Then the program works out the values to set the new pot to before soldering it in. I guess not as accurate as setting it up from scratch but it's meant to work well. I've replaced all electrolytics and tantalums, and the motor run caps, and about to do R125 as that measures below 1K2, and the rewind is a little weak on 7" spools as a result. Only pots I replaced are the PSU and speed control ones. If you happened to be passing through as it were, I'd love to have you set these pots up correctly. I'd solder them all in before you arrived to save time. Failing that some other time would be great. Appreciate the offer of help If you could P.M. me with further details, likely costs etc I'd be grateful. Many thanks! John.