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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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8th Apr 2019, 2:26 pm | #1 |
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Revox A77 No EOT .....well ....
Can someone help me really understand how the EOT circuit works on these please .
I have one here that`s playing up . With no tape in it wont engage play (thats correct) , put a tape in and it will play (again correct) , but it doesn't stop the machine at the end of the tape so its a bit confusing , I think if I really under stood the circuit fully I can figure it out without randomly testing parts . Thanks guys . David |
8th Apr 2019, 2:40 pm | #2 |
Dekatron
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Re: Revox A77 No EOT .....well ....
In effect, the sensor shut off is in series with the stop switch, with a tape loaded there's no light reaching the LDR, that in effect turns Q112 hard on due to Q111 being cut off due to the high resistance of the LDR in the no light condition, Q112 is in effect in series with the stop switch, breaking the stop switch contact stops the machine, breaking the same series path due to Q112 turning off also stops the machine, Q112 turns off when light reaches the LDR when the end of the tape has passed through, the LDR then supplies enough bias to Q111 to turn it on, turning Q111 on pulls its collector voltage down thus turning Q112 off.
LDR...no light = high resistance. LDR...light = low resistance. Lawrence. Last edited by ms660; 8th Apr 2019 at 2:51 pm. Reason: addition |
8th Apr 2019, 11:05 pm | #3 |
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Re: Revox A77 No EOT .....well ....
My money's on a 'lazy' LDR.
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8th Apr 2019, 11:15 pm | #4 | |
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Re: Revox A77 No EOT .....well ....
Quote:
Ben you could be right the transistors are ok , so thats really all thats left . although its a bit odd that it doesnt work at ALL no matter how long i leave it , so i may just replace the transistors just to be sure before messing with the LDR . Thanks guys . |
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9th Apr 2019, 8:50 am | #5 |
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Re: Revox A77 No EOT .....well ....
See also this thread.
Incidentally, what would be a suitable LDR to use in this application? I'm likely to need one myself in a different machine I am working on with a similar problem. Been years since I even used n LDR! Size is of course quite important given where it has to fit in the machine.
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9th Apr 2019, 12:42 pm | #6 |
Nonode
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Re: Revox A77 No EOT .....well ....
I remember this came up before;
https://www.vintage-radio.net/forum/...d.php?t=124351 Plenty of LDRs around- https://uk.farnell.com/c/optoelectro...hoCIpAQAvD_BwE |
9th Apr 2019, 2:12 pm | #7 | |
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Re: Revox A77 No EOT .....well ....
Quote:
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10th Apr 2019, 1:04 pm | #8 |
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Re: Revox A77 No EOT .....well ....
Thanks all , changing the transistors repaired it . I wouldn't buy anything from nagravox if you paid me to do it but thanks .
Ta ! |
10th Apr 2019, 1:16 pm | #9 |
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Re: Revox A77 No EOT .....well ....
If the LDR is degrading, swapping transistors might shift the threshold enough to make it work again, but it will likely only be temporary. It is well worth sticking an ohmmeter on the LDR to check if it's going to be a boomerang repair. (returns soon and hits you on the head)
Whatever have Nagravox done to you? Their claims that no-one else's capacitors will work properly seem a bit over egged, but then that's advertising for you. David
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10th Apr 2019, 2:01 pm | #10 | |
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Re: Revox A77 No EOT .....well ....
Quote:
I dont really want to get into the whole Nagravox debate David , but those "caps " they sell (and most of their other stuff) , let`s just say they aren`t quite what they report to be and there`s a reason all his stuff has nagravox branding on it . I`ll leave it there because like I say I dont want to get into a big debate over it , if you wanna use them all good by me but I wouldn`t . Last edited by NorfolkDaveUK; 10th Apr 2019 at 2:08 pm. |
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10th Apr 2019, 3:17 pm | #11 |
Dekatron
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Re: Revox A77 No EOT .....well ....
I can't complain about the quality of the Nagravox bits I have used - some of the marketing-speak and pricing policies do stick in my craw, though.
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10th Apr 2019, 6:11 pm | #12 |
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Re: Revox A77 No EOT .....well ....
Because spares for Revox tape recorders are no longer available from Studer, Nagravox has embarked on remanufacturing some of the parts, ie VU meters, sensors and other mechanical parts.
Motor run capacitors have odd values and again Nagravox will have had these manufactured to the same spec. All other parts will probably be standard components. The biggest problem is shipping which because of distance is expensive. |
10th Apr 2019, 7:54 pm | #13 |
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Re: Revox A77 No EOT .....well ....
Granted, but I object to the recent and arbitrary "minimum order charge" applying to those who don't buy full kits - enough, in the light of all the other expenses involved, to be a deal-breaker.
Last edited by Ted Kendall; 10th Apr 2019 at 8:00 pm. |
10th Apr 2019, 10:27 pm | #14 | |
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Re: Revox A77 No EOT .....well ....
Quote:
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15th Apr 2019, 9:26 pm | #15 |
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Re: Revox A77 No EOT .....well ....
As this tape recorder is now on eBay I'll close this thread.
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