UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players

Notices

Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

Closed Thread
 
Thread Tools
Old 20th Nov 2018, 11:38 pm   #1
NorfolkDaveUK
Banned
 
Join Date: May 2018
Location: Norwich, Norfolk, UK.
Posts: 605
Default A77 HS Capstan Issues

Ok I got this one in and it was working just fine and sounded great . I went to start the calibration procedure and for some reason the capstan stopped .

So heres what I`ve done so far :

Check the A/C coming in 135 vac
Changed all the caps (i do that anyway when i change the rifa. was done days ago all caps are good and soldering is fine )

checked the power supply , noticed the trimmer had no wiper so changed it , no change .

changed the capstan board for another , same problem .

If I take the tacho sensor off and move it away from the motor , then the capstan will spin to full revs , as soon as i move the sensor back to the motor , the motor will stop and wont start again until I turn off and turn on again even if i move the sensor back away from the motor . This happens with 2 different sensors and 2 different boards . I got to admit defeat and ask for some assistance on this please . Voltage remains fine on the power supply . 2 different speed control board tried , one with IC`s , one without . Also happens at both speeds.



thanks

Last edited by NorfolkDaveUK; 20th Nov 2018 at 11:45 pm.
NorfolkDaveUK is offline  
Old 21st Nov 2018, 1:53 am   #2
NorfolkDaveUK
Banned
 
Join Date: May 2018
Location: Norwich, Norfolk, UK.
Posts: 605
Default Re: A77 HS Capstan Issues

Ok i have movement back on the original board , still no speed sensing on either of the two sensors I`m trying so its not the sensors , movement was down to a dead short transistor . Looks like I have a burned out resistor too , I`ll change that and see if sensing comes back. I have no clue what happened here , my best guess is the transistor just decided it was going to fail . I have no other explanation . Nothing had been touched since I recapped it . I spent the day after recapping it enjoying it , put it to bed , got it back on the bench this morning , and no capstan , so there is no other explanation .Certainly wasn`t the re-cap because it worked just fine for at least 12 hours after . One of those things I guess . I`ll report back after changing the resistor . I dont have the schematic for this Capstan board so can`t give you the part numbers , only have the schematic for the board with the IC`s if anyone has the schematic for this out that would be great .
NorfolkDaveUK is offline  
Old 21st Nov 2018, 9:54 am   #3
NorfolkDaveUK
Banned
 
Join Date: May 2018
Location: Norwich, Norfolk, UK.
Posts: 605
Default Re: A77 HS Capstan Issues

OK I changed the resistor (for one of a similar board, no change . The resistor (although cooked) tested 50k , which i assume is correct , not having the schematic I can`t tell (its burned so I can`t read it) . I connected the sensor up to my frequency counter and its sensing 3.3khz so I guess that`s the problem with the board should be sensing its too fast and slow it down . I`ll try a third sensor but it must be some thing on the board not adjusting the speed in the circuit somewhere . Hell hath no fury like a revox scorned .

Last edited by NorfolkDaveUK; 21st Nov 2018 at 10:18 am.
NorfolkDaveUK is offline  
Old 21st Nov 2018, 11:26 am   #4
NorfolkDaveUK
Banned
 
Join Date: May 2018
Location: Norwich, Norfolk, UK.
Posts: 605
Default Re: A77 HS Capstan Issues

Rectifier has tested good on the speed board . All transistors good . a real head scratcher

Last edited by NorfolkDaveUK; 21st Nov 2018 at 11:40 am.
NorfolkDaveUK is offline  
Old 21st Nov 2018, 11:44 am   #5
KeithsTV
Nonode
 
KeithsTV's Avatar
 
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,571
Default Re: A77 HS Capstan Issues

I had a similar problem with my A77. It would start up correctly at 7½ but not at 3¾ but if started at 7½, then switched to 3¾ it would run correctly. It has the earlier speed control board with the tuned circuit. I found the speed sensor amplifier was oscillating at around 1kHz causing the speed control to shut down the motor at the lower speed but was OK at the higher speed. I ended up disconnecting the original amplifier and building a new amplifier/comparator which solved the problem.

Keith
KeithsTV is offline  
Old 21st Nov 2018, 12:26 pm   #6
NorfolkDaveUK
Banned
 
Join Date: May 2018
Location: Norwich, Norfolk, UK.
Posts: 605
Default Re: A77 HS Capstan Issues

Thanks Keith , I just can`t figure this board out at all bud , everything checks out apart from the big round thing (i assume thats what you are talking about ) so it could be the case . It doesnt "whine" like it used to , you know the whine it makes when its checking the speed . So I`ll replace it with another one and see what happens .

Its working fine with another board in it now , so the problem is definitely localised to that board .
NorfolkDaveUK is offline  
Old 21st Nov 2018, 1:56 pm   #7
KeithsTV
Nonode
 
KeithsTV's Avatar
 
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,571
Default Re: A77 HS Capstan Issues

Sounds like yours is the earlier board with the tuned circuit. The service manual I found had the later board but I did manage to find a circuit of the earlier version. If you don't have a copy PM me and I'll send you a copy.

I couldn't find the reason for my board oscillating even after I'd replaced most of the electrolytics so decided to try the alternative amplifier which, after a bit of experimentation worked.

Keith
KeithsTV is offline  
Old 22nd Nov 2018, 1:21 am   #8
NorfolkDaveUK
Banned
 
Join Date: May 2018
Location: Norwich, Norfolk, UK.
Posts: 605
Default Re: A77 HS Capstan Issues

I finally got to the bottom of the "no sensing speed" issue , can you believe ALL THREE of the sensors I have had a break right at the connector to the board , 2 on the blue and one on the brown . People yanking on the wire instead of the plugs I guess . wow . Can`t believe all 3 have the same problem thats just insane . So if you have a speed issue on the A77 check those plugs . I now have tried the 3 boards with 3 different sensors , all boards and sensors working fine after taking the plugs off the sensors and soldering direct to the post. What caused the initial fault , i dont know as I say , probably just a transistor went rogue and took out some other parts with it . but , nevertheless , we are back in business with a really nice original 15ips A77 ....thank BEEP for that ! Oh and I have now put the later IC controlled speed board in this one .
NorfolkDaveUK is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 5:37 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.