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[MikeCheck]

Hello All!
Long time reader, first time poster.

I have a NN-48 power supply that is intended to be used with a KM56. I am checking it before using it with the mic. Plate voltage is good, but my heater was very low (~2.2V unloaded). I believe it should be 4V after loading.

I quickly traced the culprit to 3 caps underneath the board (pictured). They are not labeled clearly. Any ideas?

Note: I could not find an accurate schematic for this supply. It does not match nn48a schematic which is quickly accessible when searching.
The caps sit after R7 to ground. Heater voltage is taken from this point. There is no trim pot (R8), or diode between the caps and ground.

 Thank you all for the help!

[MikeCheck]

EDIT:
Just realized that the third component is not a diode on the schematic, it is in fact the third cap. Duh!
and...... it seems to read either 1.5 or 7.5/300.

Any thoughts?

[uwe ret]

Looks like someone performed a previous 'repair' and replaced the original Stabilyt cells.
Stabilyt cells have a typical life expectancy of 2 to 3 years and been discontinued for nearly 50 years. These replacements (1,5/300 = 1.5V/300mA) are certain to be dead by now also.

At the time, Neumann made available for a brief transition period a solid-state replacement board SK33d, before redesigned successor supplies were introduced, among them NN48b. With modern voltage regulator devices more effective solutions are possible.

For the rehabilitation of vintage power supplies for use with the precious AC701(k) tube I have developed another 'Stabilyt Replacement' circuit with gently ramping supply which reduces stress on a cold tube filament. While working on these power supplies it does make sense to examine the high voltage plate/bias voltage stabilization and consider replacing the gas discharge tube (150B2) with a suitable 150V/1W Zener diode.

File size restrictions will result in multiple postings for the attachments.

[uwe ret]

NN48b

[uwe ret]

SK33d

[uwe ret]

SK33d instructions

[uwe ret]

Stabily Replacement

[klaus]

Thanks, Uwe.
I suggest that anyone who owns or repairs N52 or NN48 supplies copies these schematics, for future reference!

[MikeCheck]

Awesome info Uwe! Thank you very much.

I think I'll build and install your circuit. I'll update when done.

I've attached more pictures of the PSU for every ones enjoyment. The pics are a little dark. Sorry. Note that the serial number is "011"

Also, no markings on the power transformer. Anyone know if this can be changed to 110VAC ??

[uwe ret]

All the schematics for this family of power supplies I have do show the mains transformer to be capable to be configured for operation with either 110 to 120Vac/60Hz or 220 to 240Vac/50Hz.
Your photo of the mains transformer clearly shows the corresponding dual primary arrangement with its current implementation for 220-240V.

[Jim Williams]

The 317 regulator design is the better choice of these. I would probably pop for an LT1086 instead.

If you use the zener clamp solution, place a small film cap across it like .01 uf. Zener's generate a lot of noise and that will shunt that noise off to ground.

[klaus]

The 317 regulator design is the better choice of these.

Thanks, Jim.
That's my biggest complaint when using IC chips for regulation: switching noise. Good filtering plus your Zener suggestion needs to be implemented to avoid that.

[Kai]

... biggest complaint when using IC chips for regulation: switching noise.

The LM317 is an analog linear regulator, no switching inside. It does not generate lots of noise. What's left is filtered out by the final R2-C2 combination, no further improvements necessary.

[klaus]

Good to know. Do you have a schematic for that chip? I am curious how it regulates.

[soapfoot]

Good to know. Do you have a schematic for that chip? I am curious how it regulates.

page 3:

https://www.onsemi.com/pub/Collateral/LM317-D.PDF