[BC] Gates 20B

[Phil Alexander]

On 24 May 2005 at 2:35, Mike Holderfield wrote:

> Oh Yes. Although as a back-up to an FM-25K that replaced it
> as the main. I nursed it along until 1985 when I put it back
> in service as the main for about a week. I was moving the 25K
> to a new tower site at the time. Fingers crossed the whole time!
> It played all week though.
> Later that year I de-commissioned it. The driver cabinet had gone
> bonkers and I just didn't want to fool with it any longer.
> The driver cab & driver ipa was a real b*tch as anyone who has dealt
> with one knows. Oscillation city! 

You had to get into the IPA and hand tune those coils for the 
6146 P-P stage, and I mean squeeze, twist and stretch, plus
add a swamping resistor to the grid circuit increasing the drive
requirement from 1 to 3 W up to about 7 or 8. That made them
stable. Or, in pinch, you could drive the PA directly from the
exciter, but that made it eat 6360's and had to make full power
on some frequencies. This was true of all the FM-1B's. Mostly,
they fixed it in the FM-1C, but the 20B used the 1B as driver.

> For that matter, the entire TX
> was very touchy. When you got it to run...leave it alone!

The real problem was the B+ lead for the P-P 4CX10000D's
The box made the nicest 25 or 35 kW oscillator near the 
80 meter ham band. No driver needed. When it did that, it
would take the breaker out quickly. The fix was a large
pi-wound RFC inserted in the B+ line where it entered the
PA cavity with door knob bypasses. Calmed it down greatly.
That was so late coming (1/68) and there were so few in the 
field that I don't think the fix got into a service bulletin.

> My predecessor, Oscar Lanmon, told a funny story about 
> the 20B. He was having problems he couldn't get a handle
> on and called in a Harris field engineer.
> The Harris tech told him the transmitter was just not designed
> to operate on his frequency (99.7 MHz)!
> Oscar swore he was serious about this statement.

Must have been after those goons from Cleveland ran the 
good guys off to BE. If any one did that while George
Yazell was running the show he would have found George's
boot so firmly planted that he would have cleared the
river and landed in Missouri without getting wet. :)

> I believe the problem turned out to be some door knob
> caps in the (2)4CX10,000 cavity.
> Looking back at the old maintenance logs, lots of problems
> with that TX. I didn't have too many as I only ran it
> once a week on the dummy. At 15 KW though.
> Full power would often invite a trip of the big
> main breaker in the bottom of the right cabinet.

It was trying to tell you it was going into oscillation,
most likely. Meters jumped and KA-bam, down it went.

> BTW..There was also a BC-5B when I arrived there. 
> Nice big walk-in TX. 

Oh, that was a beautiful old box from the '50's made while
Parker owned the company. A nice art deco design, bigger 
than many later 50 kW boxes. If your wife threw you out 
you always knew where you could set up housekeeping. <g>

Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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