Revox A77 capstan / motors weak - UK Vintage Radio Repair and Restoration Discussion Forum

NorfolkDaveUK · 7th Sep 2018, 3:31 pm

The capstan on this is pulsing , and the motors are really weak . Will this be a prob with the run caps ? It could be a number of things I guess I just wondered if the Revox guys here knew "oh that always this ". ..I vaguely remember replaceing the power transistor on the capstan board one one not too long ago. I think the motors are down to the bearings , no free spin with the brakes disengaged . So I`ll be swapping those for ones with new bearings .

David.

NorfolkDaveUK · 7th Sep 2018, 8:16 pm

OK some more info and update . I`ve got the transport ticking along nicely now , as expected it was just the bearings , so swaped the motors out for ones with new bearings , still having the issue with the capstan though .

I`ve changed the power transistor and thats not cured it even though the emitter to base was open . I`ve replace the rifa cap and thats not cured it , but i`m not surprised the rifa`s all have the 2015 date stamp on them . I`ve changed the start/run caps and thats not cure it so I`m now looking for some advice and to what to check now .

Thanks folks .

David

Michael Maurice · 7th Sep 2018, 9:04 pm

Does the capstan motor turn freely when the machine is switched off?

If so, if you ground the collector of the output transistor on the capstan control board, the motor should run at full speed, if it does not it means youve got a faulty capstan motor.

NorfolkDaveUK · 7th Sep 2018, 9:33 pm

Its not that I`ve tried 2 now , same thing .

Changed the capstan board out for another one . Jobs a good`n ..

Thanks guys .

Michael Maurice · 8th Sep 2018, 10:32 am

On some of those control boards, there is a tantalum capacitor which when it fails can produce some very odd results.

NorfolkDaveUK · 8th Sep 2018, 10:48 am

Quote:

Originally Posted by Michael Maurice

On some of those control boards, there is a tantalum capacitor which when it fails can produce some very odd results.

Yeah I will diagnose it later on I just had a donor machine so it made sense just to pull it out . I changed the power transistor i thought it was that because it was open from emitter to base . So it need changing , I guess that may have taken out a cap or a resistor when it went open I dunno . Its working great now thats the main thing . I need to change the bearings on the motors , the good drums I have done fit the shafts so they will need changing .

7th Sep 2018, 3:31 pm	#1
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Revox A77 capstan / motors weak The capstan on this is pulsing , and the motors are really weak . Will this be a prob with the run caps ? It could be a number of things I guess I just wondered if the Revox guys here knew "oh that always this ". ..I vaguely remember replaceing the power transistor on the capstan board one one not too long ago. I think the motors are down to the bearings , no free spin with the brakes disengaged . So I`ll be swapping those for ones with new bearings . David. Last edited by NorfolkDaveUK; 7th Sep 2018 at 3:54 pm.

7th Sep 2018, 8:16 pm	#2
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 capstan / motors weak OK some more info and update . I`ve got the transport ticking along nicely now , as expected it was just the bearings , so swaped the motors out for ones with new bearings , still having the issue with the capstan though . I`ve changed the power transistor and thats not cured it even though the emitter to base was open . I`ve replace the rifa cap and thats not cured it , but i`m not surprised the rifa`s all have the 2015 date stamp on them . I`ve changed the start/run caps and thats not cure it so I`m now looking for some advice and to what to check now . Thanks folks . David

7th Sep 2018, 9:04 pm	#3
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox A77 capstan / motors weak Does the capstan motor turn freely when the machine is switched off? If so, if you ground the collector of the output transistor on the capstan control board, the motor should run at full speed, if it does not it means youve got a faulty capstan motor. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

7th Sep 2018, 9:33 pm	#4
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 capstan / motors weak Its not that I`ve tried 2 now , same thing . Changed the capstan board out for another one . Jobs a good`n .. Thanks guys .

8th Sep 2018, 10:32 am	#5
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox A77 capstan / motors weak On some of those control boards, there is a tantalum capacitor which when it fails can produce some very odd results. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/