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Pride DX300 on the 80-meter ham band? Say it ain't so, Joe.

nomadradio

Analog Retentive
Apr 3, 2005
5,565
8,202
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Louisville, KY
www.nomadradio.com
So here's a problem we see only on rare occasion. The Pride DX300 was sold as a legitimate 5-band ham linear. The design will in fact tune all five bands and perform as claimed.

But most of them ended up on 11 meters. Go figure. Oddly enough, the FCC tightened the restrictions on RF amplifier sales in 1979 and put an end to this product.

The front-panel Load control does not have quite enough capacitance for the lowest band 80 meters. The band selector will connect this oddball capacitor in parallel with it.

Begs the question. What's wrong with this picture?

FiGB2y.jpg


The aluminum chassis deck is the ground side of the capacitor. The large brass plate is the hot side. The white teflon dielectric is less than 1/16 of an inch thick.

The problem occurs at the sharp corners of the brass plate. The tiny radius of a square corner piles too many volts per cubic millimeter of air. It will literally drill through the teflon.

RgDIo9.jpg


This is why the edges of plates on an air-variable capacitor are rounded. It increases the voltage that will ionize the air and cause it to break down.

Got more than one corner.

EheIGv.jpg


This corner was clearly next. Hasn't quite drilled all the way here, yet.

KmrJjl.jpg


The lesson here is that if you really want to use this amplifier on 80 meters, remove this capacitor plate, round the corners and file them smooth before you put it back.

Or choose a different aux Load capacitor. This one has problems, as delivered.

73
 

loosecannon

Sr. Member
Mar 9, 2006
4,218
3,650
273
seems like a variable trombone capacitor commonly seen in linear amplifiers might be a good replacement.
LC
 
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