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

[David Stinson]

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."

Here's my "test bed" with a pair of suppressors tacked in.
Primary Voltage was held to 12.5 VDC in all cases.
Now don't laugh or snear at the "new" caps tacked-in here 
and there;  the idea is to keep the original parts in place, 
disconnect them and tuck the replacements in where 
they will fit:

http://home.netcom.com/~arc5/AVRAVT/AVA126Asup.JPG

and here's where the suppressors went:

http://home.netcom.com/~arc5/AVRAVT/AVA126VibS.jpg

The last one I worked-on had all the old caps removed 
and the new ones didn't look any better ;-).
The gray wire-wound is the "new" snubber resistor with
a temperature probe inserted.  The large, orange disc ceramic
under it is the new snubber cap (.02 @ 1.4 KV) 
Without the suppressors, the temperature of the resistor 
stabilized at 155 degs F.

I had some 15-Volt, 1500 Watt transient suppressors 
and some 500 Watt, 22 Volt ones.  
I first tried the 15 Volt set with load resistors behind them
to "sink" the nastyness, starting at 47 Ohms.  No joy-
15 volts is too low.  They sink too much of the waveform
and overheat unless one uses 33-47 Ohms, and that reduces
their effect to insignificance.  Lowering to 7-10 Ohms
to make them effective and the suppressors will fry.  
A 15-Volter without the resistor makes the waveform 
look sweet... for about 3 seconds.  *POOF!*  ;-)

I switched to the 22-Volt set.  These are taking far less of 
the waveform and run very slightly warm without "sink" resistors.
They clip the worst of the spikes at about 22 volts.
Here is contact #1 with the suppressor:

http://home.netcom.com/~arc5/AVRAVT/C1S.jpg

The negative spike is clipped at 0 Volts, somehow.

Here is contact #2 with the suppressor:

http://home.netcom.com/~arc5/AVRAVT/C2S.jpg

I've run the supply for an hour without significant heating
in the 22 volt suppressors.  I get a slightly better waveform
on the second leading spike on Contact #2 with a .047 uFd
cap across that suppressor.  Dunno why just that one spike.
Other values have no effect.  Curious.

With the 30-Volt-plus spikes clipped, the temperature
in the high-Voltage end snubber only dropped a small amount,
leveling at 145 degs F.  
So there's still a lot of  "nasty" over on that side.

I'm planning to get some 20 volt, 1500 watt or higher 
units and give that a go.

OK all you folks smarter than me-  Comments?

73 DE Dave AB5S