Repairing an UN/UM 57

[aiyiadam]

So.. I am trying to make this Neumann/Gefell U57 work and wanted to post here. The customer service of Gefell didn’t help and unfortunately I am not experienced in mics.. I would appreciate any help.

The mic and the power supply don’t look too bad to me, but there certainly are a few problems and modifications:

• An XLR output is installed. There is an 12AT7 instead of the EC92. The last filter cap is a newer 1000uf. And 1n4007’s are installed for the B+ rectification.
• The heater voltage to low. It is around 2,8V (instead of 5.8V) and can’t be adjusted higher. The 12AT7 should pull twice as much current, but it still seems to be too low.
• The plate voltage is 145V (should be roughly 120V). Don’t know yet if the GR 27-46’s are okay.. Possibly not.. One of them is flickering.

Even though the heater voltage is too low, the mic works a bit.. It will probably need a new capsule and the owner wants the EC92 back, but I need to repair the power supply first.

The filter caps must should be changed. I was thinking of designing and etching a PCB board for the filter caps without touching the resistors and rectifiers on the other side much.. Maybe I can also add:

• A LM317 for the heater voltage regulation?
• New rectifier for the heater voltage?
• A few zeners for the high voltage regulation?
• And two additional fuses..

Along with the schematics, I am also adding a few pics of a similar implementation that look okay to me.

 

[shabtek]

only use half the 12at7, and that half of the heater filament

maybe the selenium rectifier is not working

nice mic

 

[pucho812]

low heater voltage is one of many reasons your mic is not passing signal. the tube vacuum does not get up to temperature to allow electronics to flow in the vacuum. If you were to remove the tube I bet the heater voltage would increase but the moment the tube is in play, it drops.
I would bet that a recap is in order from age.

 

[k brown]

Also, the tranformers were known to not age well; maybe check their specs also.

Unfortunately, the link for the Archut upgrade seems to be dead.

Maybe something useful here (if you haven't seen it already):
https://www.tab-funkenwerk.org/articles-tech-documents-2/um57-history/

 

[Accelerator]

Start your re-search from the P.Trofo of the UN 57 P.S.U. and check out firstly the LT heater voltages of the LT heater voltages filament windings of your P. Trofo….

It should be somewhere 6,3 Volts AC, as the EC92 tube use 6,3 Volts DC / 150mA for heater voltage…

From there on, you move your re-search to the heater rectifiers and the heater filter, if they working or not to deliver the 6,3 Volts DC / 150mA for heater voltage of EC92 triode tube use, or the 6,3 / 300mA, that the ECC81 – 12AT7 (the big double triode brother of the EC92 tube) needs…

As I wrote before, the ECC81 – 12AT7 is the big double triode “brother” of the EC92 triode tube, so there is a lot of play that you have the ability to do, as you can “stuck it as split” and use only the one half of the ECC81 – 12AT7 (which is the big double triode “brother” of the EC92 triode) with 47K Plate Resistor, or you can wired it the ECC81 – 12AT7 “en parallèle” (both the 2 anodes wired together, both the 2 cathodes wired together, both the 2 grids wired together, replace the Plate Resistor with a 100K, replace the 1K5 cathode resistor with a 890R Cathode Resistor and double up the Cathode Capasitor to a 50μF…), as this will helps you to reduce nearly in half the internal resistance (RL) of the ECC81 – 12AT7 tube and make a better matching with the BV-U 551 Output Transformer, so you will have a better performance in this way…