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

Neumann Power Supply Corrosion

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Thank You so much for all of the help.   I guess if I had known what that was, (a hand-wound resistor) it might have been easier. :)
This does show the simplicity and how well the cells worked!   So, last question: can the hand-would resistor be removed?

Hand-wound resistor can be removed when installing the LM317 circuit.
Original schematic of NN48hu sent

Well...I thought this was finished.  Checked all voltages, all good unloaded.  I connected my M269....Filament only gets up to 2.8V....checked all wiring, all good.  Tried different cable between mic and supply, same result.  Tried different M269...same result.  Reconnected old cells in place of new circuit...same result.  So, any advice on where I should be looking?  Obviously the mic load is drawing down somewhere...is there an obvious place to look?  Thanks, and sorry to reopen this already too long thread.


This should be easy.
Measure and note idle heater voltage at several test points: at (+) output of rectifier, after first dropping resistor R7, and so on.
Then do the same, but with the load (mic) connected.
Then note at what point/component an unusually large voltage drop occurs.
Then test that component, to confirm whether it performs as specified, and if not, substitute it.

And if there is enough voltage at the input of LM317, let us know why the trim pot that adjusts the regulator's output does not give you sufficient range.

uwe ret:
Sorry for jumping on this late. Check for conductive contamination of the printed circuit board. It may look clean, but the alkaline electrolyte from the obviously leaked NiCd Stabilyts has probably leaked into the phenolic board. If that is the case, repeatedly neutralizing with vinegar, then rinsing with water or alcohol is recommended. The equivalent AC 701 filament load is easily simulated with a 39Ω/5W resistor (exactly 40Ω/4W derived from the 4V at 100mA heater requirement). The LM 317 solid state Stabilyt replacement circuit has been successfully deployed in dozens of old microphone power supplies. Please check the DV potential at GR2 (B30/C250). With old age and possible contamination these selenium bridge rectifiers are known to develop rather high internal resistance, which will drop the available DC-voltage under load. When replacing it with a suitable silicon bridge, make sure to add an inrush-current limiting resistor of 47Ω.


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