Revox A77 No EOT .....well ....

[NorfolkDaveUK]

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

[ms660]

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.

[ben]

My money's on a 'lazy' LDR.

[NorfolkDaveUK]

Originally Posted by ms660

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.

thanks Lawrence . Much appreciate your explanation !! .

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 .

[ben]

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.

[Restoration73]

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

[Humber888]

Originally Posted by NorfolkDaveUK

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 .

Why replace transistors gratuitously when it's so easy to check the LDR? All you need to do is pull off the two wires that lead from it at the PCB end and measure it's resistance with a DMM under light and dark conditions. With the LDR covered you should expect several MOhms and with a bright torch shone on it, about 2kOhms. If it won't go below, say, 5kOhms when illuminated, it is a bit 'tired', but can probably still be used with a bit of re-biassing of the control circuit as mentioned elsewhere. Replacement LDRs of the right format are available from Nagravox and elsewhere, but I haven't looked at prices lately.

[NorfolkDaveUK]

Thanks all , changing the transistors repaired it . I wouldn't buy anything from nagravox if you paid me to do it but thanks .

Ta !

[Radio Wrangler]

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

[NorfolkDaveUK]

Originally Posted by Radio Wrangler

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

I checked the LDR David of course and it was reading 60k lit , 0.6m something not lit . I have ordered a bunch of ldr`s from RS .

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 .

[Ted Kendall]

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.

[Michael Maurice]

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.

[Ted Kendall]

Originally Posted by Michael Maurice

The biggest problem is shipping which because of distance is expensive.

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.

[NorfolkDaveUK]

Originally Posted by Ted Kendall

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.

My problem is the re-branded cheap Chinese caps in the kits , the man is a charlatan , and there I`ll leave it .

[Station X]

As this tape recorder is now on eBay I'll close this thread.