[Milsurplus] RCA AVR / AVT Set - Vibrator Spike Suppression

[Bruce Gentry]

On 12/12/12 10:45 AM, David Stinson wrote:
> Been working on the "spikey" AVA-126A vibrator power supply for the 
> RCA AVT / AVR series of light aircraft radios.  You likely remember 
> the issue
> with the small snubber resistor on the high-voltage side
> overheating, which I addressed by replacing it with
> a 10-watt wire wound.  The set seems to function normally.
> As a reminder, here's the transformer input functional diagram:
>
> http://home.netcom.com/~arc5/AVRAVT/AVA126Vib.JPG
>
> I've begun looking at the spikes on the primary side of the vibrator 
> transformer and have some scope photos
> of the waveforms at the vibrator contacts to the primary.
> One of my probes has "joined the choir invisible" so I can't give you 
> a dual-trace look, but the waveforms
> sync OK.  The scale is 10 volts per division and the waveform base 
> line is 0 volts. Here is contact #1:
>
> http://home.netcom.com/~arc5/AVRAVT/C1NS.jpg
>
> Pretty nasty on "make" and a 30 Volt spike on "break."
> Note the negative spike at the start as well.
>
> Here is contact #2:
>
> http://home.netcom.com/~arc5/AVRAVT/C2NS.jpg
>
> This one is also nasty on the front end and has a 35 Volt
> spike on "break."
I think the contacts in the vibrator are scratchy, deeply pitted, or 
bouncing. There should be little or no spike of any kind when a contact 
makes, that should occur when the magnetic field collapses on contact 
break. You might also try another 0Z4, if it is "backfiring" or unsteady 
in ionizing it could cause spikes as well. I have never seen a 0Z4 that 
didn't have  a glass tube inside the metal shell, so if you carefully 
remove the shell, it can be observed. When the vibrator first starts, 
there will be intense flashes between the plates and filament until it 
heats up, then it should settle down to  a soft, steady glow (like an 
866 or 872)  between each plate and the filament. Ragged, flickering 
glow or especially arcing and flashes from plate to plate, are signs of 
bad tubes. Another thought comes to mind. Does the power supply have a 
filter choke in the B+ from the rectifier?  If it does, and the input 
filter condensor has failed, you would have a choke input filter, and 
who knows what strange things could happen.  Ionic heated rectifiers 
depend on a stable minimum load current for stable operation. It is 
normal for the vibrator to stutter and sound different for a second or 
two when it first starts and the 0Z4 filament is cold because the plate 
voltage is arcing across everything until the filament heats up. The 
tube "backfires" to start up. If you listened carefully to an old auto 
radio, you would hear a "buh-uh-ZZZZZZZZ" sound from the vibrator as the 
rectifier tube arced and sputtered to life.  Doesn't any else but me 
remember these things?  I'm not THAT old am I ?     :)

        Bruce Gentry,    KA2IVY