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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc. |
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4th Jul 2018, 2:11 am | #1 |
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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 .
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4th Jul 2018, 6:07 am | #2 |
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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
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4th Jul 2018, 7:26 am | #3 |
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Re: Revox A77 Capstan
here you go Thanks for your help .
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4th Jul 2018, 8:58 am | #4 |
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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
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4th Jul 2018, 9:21 am | #5 |
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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 ?
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4th Jul 2018, 11:39 am | #6 |
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Re: Revox A77 Capstan
MJE340 might be worth a try
David
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4th Jul 2018, 1:45 pm | #7 |
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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 .
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5th Jul 2018, 6:47 am | #8 |
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Re: Revox A77 Capstan
its only 20w ? Surely thats a bit low no ?
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5th Jul 2018, 8:01 am | #9 |
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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
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5th Jul 2018, 8:31 am | #10 |
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Re: Revox A77 Capstan
TIP47 maybe ?
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5th Jul 2018, 11:44 am | #11 |
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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 |
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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
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5th Jul 2018, 7:18 pm | #13 |
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Re: Revox A77 Capstan
Thanks David , thats great information thank you .
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13th Jul 2018, 10:51 pm | #14 |
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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 |
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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
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