[Collins] KWS-1 that screeches!

[Dr. Gerald N. Johnson]

On Tue, 2006-05-09 at 20:02 -0400, Roger Shultz wrote:
> Why these things find me I will never know but the KWS-1 which I am
> presently working on actually "screeches"  during tune up (probably out of
> some sort of unrecognized pain) .
> 
> The sound I hear is coming from the RF PA compartment  near the rear of the
> chassis between the 6AL5 ALC rectifier and antenna connector as near I can
> tell.
> 
> As the PA is driven harder in the operate position the screech increases in
> frequency (high pitch) and intensity. The plate current does not seem to be
> affected as this occurs. It also makes the sound in the tune position but
> not as loudly.
> 
> I thought it might be the RF choke L404 under the spaghetti arcing but that
> is not it.
> 
> This sound is not like "frying" that would be associated with arcing but an
> actual "screech" is the best way to describe it. I have been able to
> visually observe the RF PA compartment with the top cover off in near
> darkness and I can't see any arcing or any other visual clue.
> 
> I really need to resolve this because I think it's a symptom of some sort of
> malfunction causing the rig to tune erratically at times. All observations
> have been done on 80 meters so far.
> 
> Has anyone seen this oddity occur and if so what in the world was this
> strange problem? Any suggestion is appreciated.
> 
> 73, Roger, NJ2R
> 
Listen to it with an AM receiver while tuning. I suspect the PA has an
audio frequency parasitic from poor electrolytics though it could be an
oscillation of several stages. The screech you hear then might the disc
ceramic plate bypass in its piezoelectric mode. Lots of ceramic
materials, especially those that make the most compact capacitor show
that piezoelectric effect and will scream when lots of AC is applied.
The 50s vintage B&W TV had lots of 15,750 to be heard because of
those ..... capacitors. Might be the screen bypasses too.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer