[Milsurplus] Dear Vibrating Smart People....

[J. Forster]

A couple of things come to mind:

Are you using the supply loaded or unloaded? The snubbing network has
limited energy snubbing capacity and is designed to absorb only short
spikes. The rest of the waveform should go to the load.

Is the rectifier tube really good? If it starts to fire on each half cycle
more slowly than it was designed to (any radioactive substance may have
decayed by now) it would put more spike energy into the snubber. Try some
SS diodes.

FWIW,

-John

========




> I'm working on an RCA Light Aircraft set using the AVA-126A
> Vibrator Pack power supply.  Here's a drawing of the input
> of that supply, wired for 12 volt operation per the design:
>
> http://home.netcom.com/~arc5/AVRAVT/AVA126Vib.JPG
>
> All the old caps have been subbed.
> The OZ4 rectifier tests good.
> The 10K resistor in the "snubber" is rated at 1/2 watt.
> With or without load, it is drastically overheating.
> I'm having this problem with two of these supplies.
> A .02 temporarily touched across the resistor shows
> sparks, so it's "snubbing" some hefty spikes.
>
> My current theory:  The vibrator is very old and the
> pressed-in, oxidized contacts are creating more "noisy,"
> higher-voltage spikes than those for which the set
> was designed.  These are causing higher spikes
> in the secondary than the snubber can handle.
> I got one of these vibrators working by soldering the
> contact bases to the contact springs, but the top layer
> of the bi-metal contacts themselves will not take solder.
> The other worked without opening it.
>
> I assume the RCA engineers knew what they were doing,
> so I don't want to "(dis)improve" their design.  But they
> could walk to the Supply Sargent and get a new vibrator.
> I can't.  I don't want to fry them out with 115 VAC
> as I've often heard mentioned, and I'd rather not replace them
> with solid-state parts if I can avoid it.
>
> Possible solution:  Modern MOVs (spike killers) are available
> cheap for voltages from 2 to 500 volts that will handle amps.
> Either low-voltage MOVs tacked-across the primary windings
> or a high-voltage MOV on the secondary, or both to eat the spikes.
> That's what A.R.C. did on the MD-7 modulation transformer
> secondary; that large, conical-ended doodad under the chassis
> is a big "varistor" (caveman MOV) that eats audio spikes
> to save the transformer.
>
> Take a look at the diagram.
> Do you think MOVs might solve the problem?
> If so, what values would you recommend?
>
> TNX OM ES 73 DE Dave AB5S
>
>
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