Magnetophon 97 preamp build?

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Fnord

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Jun 7, 2018
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4
So, I picked up an old Telefunken Magnetophon 97 reel to reel a few years ago, and I figured its time to do something with it.  The reels don't turn anymore, but the tubes all test good, so I thought I might try to salvage the preamps into something rack mountable. I found a pdf schematic (attached).  Is this insane?  Should I just salvage the tubes and put them in something else?  I've done some preamp kit building before, but nothing with tubes.  Any good resources out there for tube noobs?
 

Attachments

  • Magnetophon 97.pdf
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The "dotted circle with a t attached" is the symbol for a wire with a grounded shield .  K is the potentiometer for the recording level "Aussteuerungregler" and M is a tone "Tonblende" potentiometer. Both are far away from their tubes, so it makes sense to shield the wires to avoid radio reception.

PS: what a beautiful service manual!
 
Fnord, I don't know how much you understand about electronics? The schematic is typical for a consumer device of that era: the amplifiers use good, but not studio quality tubes, there are no input or output transformers and I believe the level coming out of it won't be studio level. The typical microphones for these devices had simple transformers included, the one I know had it built into the connector. In order to turn this thing into some sort of mic pre you would need a proper input transformer and some sort of output stage. I would not attempt to take the electronics out and rack them, it seems like way too much work with an uncertain outcome. You could try to get hold of an input transformer, make a reasonable connection inside the unit or try it outside for a test and mod the output. You should also replace all the electrolytic caps in the circuit for a start. If you get this done, you'll be able to hear the amp and decide whether you want to get more invested.

And, just in case you are not aware, you can kill yourself working on tube stuff if you don't watch out or don't know what to watch out for!

Hope this helps,

Michael

PS.: I grew up about 20  or 30 kilometers from where the 'Fernsehmeister' who owned the service manual had his shop. This world is so small  :)
And it is a bit sad that the manual is only in German, it is really an example of a great manual: explaining the unit, the adjustments and possible problems and solutions in detail. Kindof a lost art nowadays...
 
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