Revox D36 : motor circuit

[ricothetroll]

Hi,

I'd like to resurrect an old Revox D36 (circa 1960, all tubes).

I thought this was a nice occasion to learn how those reel to reel tape recorders work, especially the mechanical part I'm not familiar with.

The schematic can be found on page 20 of this pdf (the ones on the following pages are for the later versions E36 and F36) :
ftp://ftp.studer.ch/Public/Products/Revox/Revox_D36/Revox_D36_Op.pdf

On page 3 of this document, the mechanical description says : "The wind motors are designed to provide pulse free torque. Back tension is applied during the record and playback functions but not during Wind"

Those motors are the 3 phase ones right near C63 and C64, on the middle-right part of the schematic.

I don't get how this works to provide the back tension during in play mode. I can see that in this mode the switches g1-g2, g3-g4 and h5-h6 are all closed, so the two motors look like they are wired in series. Is the back tension provided by the motors running against the capstan motor ? Isn't it kind of brutal ? In the mechanical description they talk about "brakes". What's the role of those than ?

The tape recorder isn't working for the moment, one of the resistors in the RC network (10R/0,1uF) surrounding the motors burnt. My guess is a shorted cap but I prefer to make sure I understand the circuit correctly, especially as this one works directly on the mains without isolation...

Best regards.

Eric

 

[PRR]

Forum software garbled your FTP link. (It does not handle FTP links properly unless you use the

FTP code.) Trying again:

ftp://ftp.studer.ch/public/products/Revox/Revox_D36/Revox_D36_F36_Serv_E.pdf
(Wonder why it prints so small and grey?)

However the part below seems to be the key to your questions.

 

[PRR]

And the path in PLAY: (below)

The 10R+0.1u networks just reduce sparking on the relays- forget them.

While the motors are 3 windings and a capacitor, we only switch two wires, so forget that other stuff.

In PLAY, both motors are connected in series, pulling in opposite directions. Just like that they would split the supply voltage in half, and give maybe half of the full torque. However the supply motor is shunted with a resistor, so gets less than half voltage, gives less torque than the take-up motor which is getting a bit more than half voltage. Tape tends to move forward, but not firmly enough to disturb the capstan motor.

 

[PRR]

> In the mechanical description they talk about "brakes". What's the role of those than ?

Mechanical brakes tend to give a "violin bow" scraping effect. Back-tension is not smooth. (Some fine decks have done it this way however.)

Torque-motors can give smoother back-tension.

Torque motors do NOT bring the reels to a full stop without more complication. The Crowns used two aircraft lamps, two rotation detectors(*), and a simple little "computer" to pull the reel speed down, without spilling tape (too often), and killing the motors when the spindles came to a stop. This Revox is pre-computer and it was simpler to just use mechanical brakes to get the spindles stopped.

(*) THAT's the mechanical contraption I was thinking of in the other thread. The motion detector disks were "free" on the spindles, but bushing friction turned them a few degrees in the direction the spindle was turning. This was detected with photo-eyes. When the reel-motors pulled the spindles to a stop, they started to turn the other way. The Crown detected this change-of-direction (usually) and killed the motors. Incredibly smooth handling, but you had to keep the eyes and bushings very clean or it would come down to zero speed and take-off full-speed the other way.

 

[ricothetroll]

Thank you so much for that very instructive answer PRR !

BTW I was able to follow the line to the motors and cut it, to see if the rest functions correctly, and it does ! I'll replace the RC network and the 2k2 resistor (the one across the rewind motor) that looks pretty damaged too. Well at worst I'll have a nice preamp

Is there a type of capacitor that's more appropriate for this use, or I could use some regular HT film capacitor (polystyrene for example) to replace it ?

Something else looks strange to me : the 2k2 resistor is placed directly across the mains when in rewind mode. This means it consumes a power of 230²/2200 = 24W. The resistor I have on this position looks like a lot smaller than 24W (5W tops I'd say). Is there something I've missed ?

Best regards.

Eric

 

[PRR]

The capacitors have to stand full line voltage, plus surges, and should not fail short.

Since 1959 a line of "across the line" capacitors has been standardized. I know them as X and Y caps, which I think is an international standard. Here is an example:
http://www.digikey.com/product-detail/en/B32023A3104M/495-3801-ND/1648118
DigiKey, EPCOS (TDK) B32023A3104M

Note in the picture the "Y2" code. There are also X1 and X2 codes, and I do not know which is for what.

> the 2k2 resistor is placed directly across the mains

Ha! It is! How strange!

A "24W" resistor will dissipate 24W day and night for months. How long can you re-wind? A minute? Then maybe play for a half hour? A large resistor can be overloaded for quite some time. However it is still odd, because you might be assigned to re-wind a whole shelf of tapes at once, or the tape-end sensor could fail and leave the reel spinning (and power on the resistor) until somebody noticed.

But it is 50+ years old. If this were a major design flaw, it would have failed long ago. And if the resistor fails open, the tape deck still runs. If the resistor is anywhere near rated resistance, I would leave it alone. It may be some special type that can stand the heat in a small package, or that goes up in resistance (reducing power) when it gets hot.

 

[ricothetroll]

Well i have to replace this resistor too, it burnt along with the RC resistor, there's lile a big hole in it as if it exploded ;-)
I'll see if I can bolt an 25w resistor somewhere, I'd be more reassured :-S