[BC] Trapezoidal modulation display (was "Re: [BC] Inovonics model 520")

[Phil Alexander]

On 18 Feb 2006 at 9:53, Lamar Owen wrote:

> John Randolph (W4QA, SK) told me back in 1992 about this display.  The few 
> months I was graced with his advice are golden to me; he died in December of 
> 1992 from a heart attack suffered why working at WSKY in Asheville, NC.  I 
> participated in the estate sale a few months later, and got some equipment 
> and mementos from the sale; some items are priceless, some I've sold.
> 
> He had an old Tektronix rack mount that was gutted, with a series blocking cap 
> and direct plate drive, just for this purpose.  Directly viewing the 
> modulated high voltage at the PA plate, to check for PA linearity; comparing 
> with a trace of the various intermediate audio stages to find AF 
> nonlinearities.  The Tek might have had a pair of high voltage pots to adjust 
> sweep size; don't remember.

Possibly, but usually the amplitudes were fixed in the setup. Audio with
a large tapped Ohmite on the horizontal, and fixed caps for the RF (V) side.

> > When monitoring glowing tubes, one was fortunate to get much more than
> > 100% positive mod., although there was no limit to the + allowed. In
> > fact, the rules called for a minimum capability of 85%.
> 
> John also told me about the 10kW rigs ran at 5k to get 250% positive peaks, 
> back in the day, in Washington, DC.  Perhaps the call sign was WEEM, or WEAM.

That was a "special" mod. (available from the factory for those who knew) on
the Gates BC5P2 and probably on the BC5H when it was introduced. Since the
BC5P/H and BC10P/H cabinets were identical, it was a matter using the 10P/H
iron and PS in the 5. A really easy mod. from the factory standpoint. I 
believe this was the reason for the 125% rule. Too many complaints about
"loud" competitors from the whiners who didn't want to buy one. <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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