[Milsurplus] Dear Vibrating Smart People....

[Bruce Gentry]

On 12/5/12 11:34 AM, David Stinson wrote:
> 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.
>
>
> 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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The buffer condenser is not primarily a noise suppressor. It tunes the 
transformer so it "rings" at a frequency giving a wave shape ready for 
the next alternation of DC from the vibrator. This is usually not the 
exact frequency of the vibrator.  The transformer does not receive a 
true square wave because there is a time when there is no power flow as 
the reed and contacts swing from one contact to the other.   If you 
haven't already, change the resistor. They can do weird things when 
exposed to high voltage spikes. The buffer condenser must be able to 
stand serious spikes,  I very rarely see one today that is good, and 
replace them as the first step in rebuilding the power supply.  Is the 
vibrator the correct frequency,  are it's contacts making properly?  One 
set of contacts not making or being "scratchy" will cause unbelievable 
harmonics and endless troubles.  When I was a kid, I used two  auto tail 
light bulbs- the large ones for the brake/turn and marker lights with 
both filaments in parallel- as imitation transformer windings to test 
vibrators. If you can, use an oscilloscope across each of the bulbs to 
check for bouncing and scratchy contacts. Use clean DC for this test.  I 
have never has any problem "smoking" the contacts on an old vibrator 
with light bulbs and 120 volts, just don't lay more than about 10 
seconds.   This method is to blast the oxide off of otherwise sound 
contacts,  usually the bulbs will flicker and flash like crazy for a few 
seconds and settle down. If they don't, more serious repairs are needed 
and you will have to open the vibrator. Most older ones use zinc sheet 
for the case, bend it gradually away from the wafer, going round and 
round with diagonal pliers until you have spread the can lip enough to 
withdraw the internals. If the foam rubber has turned to goo or powder, 
I have had great success cutting out replacements from the foam soles of 
old flip flops with a razor blade or Xacto knife. This suspension is 
very important, especially at the coil end, the internals can't be 
flopping around and must be held firmly  without transmitting excessive 
vibration and noise to the case. The contacts are usually made of 
tungsten, like ignition breaker points. Some newer vibrators use flat 
springy contacts ( I THINK they are stainless steel, just a guess!)  
that wipe a good deal when they touch. NEVER use sandpaper on contacts 
except to get thick crud off, and you MUST use a metal and/or diamond 
burnisher to thoroughly remove all traces of abrasive afterward. The 
abrasive particles from the paper will stick in the contact metal , and 
cause them to burn like crazy. Not only does the abrasive prevent the 
metal contacts from touching fully, it is chemically changed by the 
current and arcing and burns itself into a scale that is extremely 
difficult to remove.  I have spent too many overtime hours- which I 
hated- cleaning the scale off elevator controller contacts after  a 
mechanic who knew fully better used sandpaper or non-metallic burnishing 
sticks. Vibrator power supplies are an unusual art, but can give good 
results.

    Bruce Gentry   KA2IVY