Revox A77 Capstan - UK Vintage Radio Repair and Restoration Discussion Forum

NorfolkDaveUK · 4th Jul 2018, 2:11 am

Hi all . I have a A77 mk3 with a big problem with the capstan motor. its running really really fast (i.e. full tilt if I let it which i dont) , would the q209 transistor cause this if it were dead ? Is the this transistor a 2sc782 ? What else should I check . The issue started after I rebuilt the deck and stupidly put wires , E3 and ED2 backwards and blew resistor R226 . I replaced that and now the motor is just flying .

Thanks

Radio Wrangler · 4th Jul 2018, 6:07 am

Like any DC coupled amplifier, it might not just be the power device, the drivers may also have been taken out. My A77 manual is in the attic so I don't have the circuit in front of me and there were a couple of variations in the motor drive board. I'd better not comment in any more detail without the right diagram in front of me.

David

NorfolkDaveUK · 4th Jul 2018, 7:26 am

here you go

Thanks for your help .

Radio Wrangler · 4th Jul 2018, 8:58 am

That could have taken out all the semiconductors on that board, and some of the passive components.

You'll just have to systematically check or replace your way through the thing.

After years of bad experiences I do lots of cold checks with an ohmmeter when I've put something back together before daring to try it with any power. Slow, but not as slow as getting something wrong. I developed a few hundred watt switching supply with a weird 24kHz sinusoidal output without blowing anything up until the time came to do fuse-pulling tests.

David

NorfolkDaveUK · 4th Jul 2018, 9:21 am

Yeah its not a mistake I make often I had been at it all day and should have taken a break really . Lesson learned , do you happen to know what I can replace the 40138 with should I need to ?

Radio Wrangler · 4th Jul 2018, 11:39 am

MJE340 might be worth a try

David

NorfolkDaveUK · 4th Jul 2018, 1:45 pm

Lovely thanks mate . Went over the whole board this morning and its this I need , its short from collector to emitter .

NorfolkDaveUK · 5th Jul 2018, 6:47 am

its only 20w ? Surely thats a bit low no ?

Radio Wrangler · 5th Jul 2018, 8:01 am

Something more powerful would be better, but finding something still available and without abysmally low Hfe is the problem. Any suggestions?

At least worst dissipation in the transistor is with the motor at partial power, and the full motor power is more watts than the transistor's peak dissipation. Parts in this place seem to be seen off mostly through voltage transients rather than over dissipation. Illustrated in fails being shorted not open.

David

NorfolkDaveUK · 5th Jul 2018, 8:31 am

TIP47 maybe ?

NorfolkDaveUK · 5th Jul 2018, 11:44 am

This may be a dumb question ...lol

But could you put 2 MJE340 in series to increase the wattage ? The original part was 35w . Like i say its probably a stupid question but i`m not a newbie for nothing . lol

Radio Wrangler · 5th Jul 2018, 12:03 pm

two MJE340 in parallel with the emitter resistor split to give current sharing between them would be feasible.

Look at the resistor in the emitter of the existing transistor and replace it with a pair, each of twice the resistance of the original. The resistors join at the bottom (as seen on the schematic) and then the top end of one resistor goes to the emitter of one MJE340, the top end of the other resistor goes to the emitter of the other MJE340. The bases of both MJE340s are connected together and connect where the base of the original transistor did. THe collectors of both MJE340s are connected together and connect where the collector of the original transistor did.

The MJE340 has plenty of voltage rating, but better current and power would be good, so parallel connection is what's needed. You can't just connect semiconductors in parallel, one tends to hog a bit more current and get a bit hotter. The hotter one changes characteristics due to the heat and hogs even more current.

So the emitter resistor is a nice opportunity to force good current sharing.

David

NorfolkDaveUK · 5th Jul 2018, 7:18 pm

Thanks David , thats great information thank you .

NorfolkDaveUK · 13th Jul 2018, 10:51 pm

Ended up putting a TIP47 in there , works a treat .

Thanks for all your help .

Radio Wrangler · 13th Jul 2018, 11:07 pm

That's good, and it does it in one transistor. I'd been wary of TIP47 because the minimum Hfe is a bit on the low side, but if the one you've got is a goer, then that's fine, and luckily the old one died short without anything nasty coming backwards out the base connection to share the misery around.

Nice fix!

David

4th Jul 2018, 2:11 am	#1
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Revox A77 Capstan Hi all . I have a A77 mk3 with a big problem with the capstan motor. its running really really fast (i.e. full tilt if I let it which i dont) , would the q209 transistor cause this if it were dead ? Is the this transistor a 2sc782 ? What else should I check . The issue started after I rebuilt the deck and stupidly put wires , E3 and ED2 backwards and blew resistor R226 . I replaced that and now the motor is just flying . Thanks

4th Jul 2018, 6:07 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Revox A77 Capstan Like any DC coupled amplifier, it might not just be the power device, the drivers may also have been taken out. My A77 manual is in the attic so I don't have the circuit in front of me and there were a couple of variations in the motor drive board. I'd better not comment in any more detail without the right diagram in front of me. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

4th Jul 2018, 7:26 am	#3
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Capstan here you go Thanks for your help . Attached Thumbnails

4th Jul 2018, 8:58 am	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Revox A77 Capstan That could have taken out all the semiconductors on that board, and some of the passive components. You'll just have to systematically check or replace your way through the thing. After years of bad experiences I do lots of cold checks with an ohmmeter when I've put something back together before daring to try it with any power. Slow, but not as slow as getting something wrong. I developed a few hundred watt switching supply with a weird 24kHz sinusoidal output without blowing anything up until the time came to do fuse-pulling tests. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

4th Jul 2018, 9:21 am	#5
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Capstan Yeah its not a mistake I make often I had been at it all day and should have taken a break really . Lesson learned , do you happen to know what I can replace the 40138 with should I need to ?

4th Jul 2018, 11:39 am	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Revox A77 Capstan MJE340 might be worth a try David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

4th Jul 2018, 1:45 pm	#7
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Capstan Lovely thanks mate . Went over the whole board this morning and its this I need , its short from collector to emitter .

5th Jul 2018, 6:47 am	#8
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Capstan its only 20w ? Surely thats a bit low no ?

5th Jul 2018, 8:01 am	#9
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Revox A77 Capstan Something more powerful would be better, but finding something still available and without abysmally low Hfe is the problem. Any suggestions? At least worst dissipation in the transistor is with the motor at partial power, and the full motor power is more watts than the transistor's peak dissipation. Parts in this place seem to be seen off mostly through voltage transients rather than over dissipation. Illustrated in fails being shorted not open. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

5th Jul 2018, 8:31 am	#10
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Capstan TIP47 maybe ?

5th Jul 2018, 11:44 am	#11
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Capstan This may be a dumb question ...lol But could you put 2 MJE340 in series to increase the wattage ? The original part was 35w . Like i say its probably a stupid question but i`m not a newbie for nothing . lol

5th Jul 2018, 12:03 pm	#12
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Revox A77 Capstan two MJE340 in parallel with the emitter resistor split to give current sharing between them would be feasible. Look at the resistor in the emitter of the existing transistor and replace it with a pair, each of twice the resistance of the original. The resistors join at the bottom (as seen on the schematic) and then the top end of one resistor goes to the emitter of one MJE340, the top end of the other resistor goes to the emitter of the other MJE340. The bases of both MJE340s are connected together and connect where the base of the original transistor did. THe collectors of both MJE340s are connected together and connect where the collector of the original transistor did. The MJE340 has plenty of voltage rating, but better current and power would be good, so parallel connection is what's needed. You can't just connect semiconductors in parallel, one tends to hog a bit more current and get a bit hotter. The hotter one changes characteristics due to the heat and hogs even more current. So the emitter resistor is a nice opportunity to force good current sharing. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

5th Jul 2018, 7:18 pm	#13
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Capstan Thanks David , thats great information thank you .

13th Jul 2018, 10:51 pm	#14
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Capstan Ended up putting a TIP47 in there , works a treat . Thanks for all your help .

13th Jul 2018, 11:07 pm	#15
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,874	Re: Revox A77 Capstan That's good, and it does it in one transistor. I'd been wary of TIP47 because the minimum Hfe is a bit on the low side, but if the one you've got is a goer, then that's fine, and luckily the old one died short without anything nasty coming backwards out the base connection to share the misery around. Nice fix! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done