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[Bassman65]

Hello,

I have a pair of KM254's that I power with the N52a supplies in the attached photo. The N52A power-supply on the left side is not powering the microphone and I was wondering what the first step should be to troubleshoot it?

The bulb is out, and I found the following post on this forum which seems to indicate that this may be the issue?

 https://repforums.prosoundweb.com/index.php?topic=36966.0

Should I go ahead and replace the bulb as my first step, or should I probe some voltages first? The other power supply in this box is working perfectly.

Thank you very much, I really hope this is something simple!

[klaus]

Please read and study the thread you cited, especially post #6 and follow-up posts!

One relevant excerpt:

Uwe pointed out something very important: On most N52a, the lightbulb is in series with the heater voltage supply. That means, if the lightbulb fails, the heater voltage fails. This was not remedied until the N52a with the embossed "t" after the "a" came out (this was a modified N52a with solid-state heater voltage stabilization added), and all units of the final N52 version, N52t.

What complicates this issue is that TAB (maker of the N52) delivered early N52a/t with the old lightbulb wiring (my schematic), before switching (Uwe's schematic).

Identify the N52 version you have, as it may indeed be the blown bulb that caused the heater voltage failure, and thereby the shutdown of the mic.

There is a simple test to see whether that is your dead N52's problem: mount the working bulb in the dead N52*; if it now works, you know the answer why!

* After unplugging the unit from power, remove the front panel (four slot screws, two on each side). Now you can extract the bulb with a pointed instrument from its fitting. Use a continuity tester or inspect the bulb's filament for breakage. In any case, use the good bulb as confirmation of the diagnosis.

[Bassman65]

Hello Klaus,

Thank you for your quick reply and directing me how to remove the faceplate. I attached another photo showing the faceplate off, but now I'm unsure how to delicately remove the bulb.

What tool and it what manner is the bulb removed?  Does it pull straight out without the need to release a twist lock or some other securing mechanism at the base?  By "pointed instrument" do you mean perhaps a thin hard piece of plastic that I will insert inside the black plastic housing along the side of the bulb and try to pry it out?

Thank you, and apologies for the silly questions.

[klaus]

The bulb pulls out straight. I usually have luck with thin needle-nose pliers wrapped in rubber glove material for better friction.

But the spring contacts in the bulb housing can be a bit firm. You can also access the bulb by poking it forward from behind (thorough the processor side) with a thin, long screwdriver or similar.
 

[Bassman65]

Thanks Klaus, I used pliers with a rubber glove as you described and it pulled out very easily. 

The bulb is dead - no continuity between sides. 

It turns out that the working power supply has a replacement bulb in the base.  I was unable to get a hold of the base to pull it out and test it on the non-working side, so I think my next step will be to check with the bulb supplier (illi.eu) you found in the post I referenced from 2016 to see if they have any left, and if they can fulfill a low quantity order.

If there is any other supplier for these bulbs anyone is aware of, I appreciate the link.

I have attached two photos of the non working bulb, and a photo of the replacement bulb in the working supply.

Thank you!

 

[klaus]

Get a few of these bulbs as specified in the linked post.
Let us know the outcome, and good luck!

[uwe ret]

The N52 missing the filament supply is probably the -a version, which features a 4V/25ma bulb in series between the transformer secondary and the rectifier for the filament supply.

Also beware the NiCad Stabilyt cells smoothing and regulating the filament voltage! This supply may have been updated with a solid state regulator circuit in place of the Stabilyt cells.

The other N52 is probably the -t version, which uses a 12 V/20mA bulb wired across (parallel to) the low voltage mains transformer secondary and featuring a transistorized filament voltage regulator. The bulb from the N52t will not work correctly in the N52a!