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

[Lamar Owen]

On Saturday 04 February 2006 18:45, Phil Alexander wrote:
> On 1 Feb 2006 at 19:33, Alan Alsobrook wrote:
> > You should use sample of the audio being applied to the input terminals
> > of the transmitter.

> That depends on the linearity of the modulator. "Back in the day" when
> such things were less dependable, we would tap a blocking cap on the
> top side of the modulation reactor and feed a big voltage divider with
> it. Thus, you had a pure analog of the B+ going to the PA driving the
> H plates of the CRT and we used a pick up coil in the tank compartment
> or a tap off the mod. monitor sample for driving the V plates. See,
> those were the days when most scopes had jumpers on the back so you
> could drive the deflection plates of a CRT directly. Just remove the
> jumper from the V amp and sweep output and put enough voltage to get
> a screen filling pattern.

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.

> 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.

-- 
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC  28772
(828)862-5554
www.pari.edu