R/E/P > Klaus Heyne's Mic Lab

Neumann Power Supply Corrosion

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rdraudio:
Hi Klaus,
I've moved on to my other M269, which has a NN48hu power supply.  I am yet to go through and measure voltages with the mic hooked up, but opened the power supply to test voltage and found quite a mess.  I believe this has been discussed here before, and will search.  I believe that this still has the Stabilyt cells installed.  I would like to clean all of this corrosion off, get rid of the cells, and do anything needed to get it up to snuff.  The voltages at the supply with no mic attached currently read :

B+ 126.7
Filament 4.251

Thoughts on how to proceed?

-Mark

klaus:
Remove the Stabilyt cells, clean up the mess, and follow Uwe Sattler's recommended solid state replacement stabilization:
https://repforums.prosoundweb.com/index.php/topic,37395.msg538543.html#msg538543

rdraudio:
OK...so something is not right.  I removed the cells, and did the best I could to clean up the mess and corrosion.  I built Uwe's circuit, and decided before hooking all back up, to make sure that everything was still ok in the power supply. 

From the schematic I can find online (NN48a, mine is a NN48hu ) voltages are not good. I am reading 36V @ R7, as well as where the circuit would connect, and at Pin 4 of the connector ( obviously not hooking up a mic! )  B+ still at 127, unloaded.  The voltage, even without the circuit shouldn't be that high, should it?

Mark

klaus:
Mark, I'll let Uwe chime in with his voltage calculations of an idle power supply that has been converted to his heater circuitry.The LM317 should regulate the heater voltage quite well, so something went wrong in your implementation.

In the meantime, if you look around on this forum, a while ago someone posted the dummy load resistor value that simulates the current draw of an AC701 tube. Connect that load and then measure again.

By the way, the B+ /high voltage you measured seems about right for AC701 power supplies without load.

rdraudio:
Thanks Klaus.  Yes, the B+ didn't change, but as I posted earlier, I was close to the 4V even with no load for the filament...but this is crazy high.  Something has changed.  Would the supply voltage be that different without the stability circuit of any kind? According to the schematic, no.  Anybody have an idea what would cause this?  I hope Uwe responds.  I appreciate all the answers.

Mark

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