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…

 

[aiyiadam]

Back again for an update.. Thank you for the replies.. I tend to procrastinate, when I have to deal with vintage mics..

There was probably a problem with the heater rectifier. Anyways, I have decided last week to install a complete PCB without removing more components than necessary and also prepared a 2 mm aluminium plate for installing the PCB.



For the heater voltage filtering I have soldered a 5W 10R resistor (under the board) instead of the big choke. I have also used a voltage regulator and set the voltage at 6.0V. On the high voltage side I used a 5W 140V Zener and couple of 18K resistors. There are 2A rectifiers on the board and the fuses are on the AC side. The B+ is now roughly 124V.

Now I can check the mic..

 

[Accelerator]

@k brown​

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

Here is the Oliver Archut UM57 Mod Schematic

 

[tomas.borgstrom]

Back again for an update.. Thank you for the replies.. I tend to procrastinate, when I have to deal with vintage mics..

There was probably a problem with the heater rectifier. Anyways, I have decided last week to install a complete PCB without removing more components than necessary and also prepared a 2 mm aluminium plate for installing the PCB.

View attachment 138652 View attachment 138653View attachment 138654

For the heater voltage filtering I have soldered a 5W 10R resistor (under the board) instead of the big choke. I have also used a voltage regulator and set the voltage at 6.0V. On the high voltage side I used a 5W 140V Zener and couple of 18K resistors. There are 2A rectifiers on the board and the fuses are on the AC side. The B+ is now roughly 124V.

Now I can check the mic..



PS: I have also attached a layout for the UN57. Maybe it helps somebody..

View attachment 138655View attachment 138656

It's too late now but why didn't you just replace the bad rectifier bridge?

 

[aiyiadam]

It's too late now but why didn't you just replace the bad rectifier bridge?

Well.. I don't trust any 70 year old electrolytic capacitor. Couple of them are already gone. I also don't have any spare GR27 tubes. As I have mentioned, one of them is flickering and the B+was 20% higher than what it is supposed to be. It can't be only the higher wall voltage ..

I had designed the PCB with eyelets so that I could just use what I needed. I ended up using everything on the board to make this thing a bit more reliable. In the end this is a just a power supply. I don't expect a secret ingredient for great tone here.

And it is not too late. I didn't even drill a single hole. In 1 hour I can turn this thing in its original form again (including the vintage nail polish ) .

 

[tomas.borgstrom]

I've serviced many of these PSU's. It's well built with a choke in the heater supply. Ask Jan First have can capacitors of very high quality and most likely also the stabilizer tube.

 

[tomas.borgstrom]

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

I have never encountered any problems with the power transformers or the choke.

If your UM57 or CMV563 have low end and sounds good I don't recommend changing to output transformer to Olivers. I did all of the Oliver modifications in a CMV563. I guess it measured better but it lost it's unique sound so I reversed the mod and put the original transformer back in.

I have two CMV563. Both are serviced with the exact same components. When testing them with the same M7 lollipop the only difference is the output transformer. I like both but one has more low end, sounds smoother and can actually live up to the nickname "poor mans U47".

If your microphone sounds honky even after service I guess installing Olivers transformer is the way to go but in general I'm a bit sceptic to his Gefell mods. It's true that these microphones do not sound as good as Neumann Berlin but they won't do that with those mods either.

 

[aiyiadam]

I've serviced many of these PSU's. It's well built with a choke in the heater supply. Ask Jan First have can capacitors of very high quality and most likely also the stabilizer tube.

Unfortunately the high B+ and the drifted capacitor values don't agree with you here. One cap was already gone and a two of them are too low.. The ESR values are probably also all over the place. My equipment is unfortunately not capable of measuring ESR realiably.

So... Some components had to go and there are no spare ones. And since this device is going to be used in professional music recording, I would like to classify its components as "reliable" and "not reliable". I don't care much about unreliable components that still measure okay. We really don't need to wait one more year until the two remaining unreliable caps or the GR27 die..

PS: I have just soldered the heater supply choke back. I had omitted it in the first place, because I wasn't sure about the voltages. And instead of the 37R choke I had used a 10R resistor.. With the choke and the proper load I now have 9,8V at the voltage regulator input...

 

[tomas.borgstrom]

Unfortunately the high B+ and the drifted capacitor values don't agree with you here. One cap was already gone and a two of them are too low..

I think you got me wrong. I encourage to change the capacitors. You can buy new F&T screw caps that probably will last for another 50 years.

Old professional gear needs service and often recap but they are impressingly reliable.

 

[aiyiadam]

. You can buy new F&T screw caps that probably will last for another 50 years.

Where do I get new F&T screw caps for the UN47 ?

 

[Dark1]

Agreed on the capacitors and the stabilizing tube, no reason not to just replace the capacitors with modern DIN-based ones (https://www.die-wuestens.de/k1.htm), IIRC the 2x stabilizing tubes can be replaced with more modern StR75/60 or 75C1, the GR27-56 is just an ancient predecessor to these.