[SignalOne] CX7A Drive (was arcing)

[signalone@mailman.qth.net]

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On Wed, 11 Dec 2002 13:42:49 -0500, W2AGN <w2agn@w2agn.net> wrote:

> Well, I finally got the nerve up to take out the driver board, and 
> replace the driver transistor.  Even though I showed "some" drive at the 
> grid, it wasn't enough to budge the plate current above the 100ma 
> resting.
> 
> SO, I replaced the transistor (Q4) with a new 2n5641, put it back 
> together...and still no drive. I show a little RF at the output of the 
> first driver transistor, Q3 (which I didn't replace), and much more on 
> the collector of the new "final" driver transistor, but still not enough. 
> New transistor gets warm, while Q3 stays pretty cool.
> 
> Tried adjusting R19 no difference, although does affect RF level when I 
> look at it with RF probe.
> Bias, Screen Voltages are good.
> 
> I even, just for the heck of it, tried connecting a little QRP transmtter 
> (1/2 watt) to the grid of the final tube,...and darned if it didn't show 
> drive then! So I am sure it is the driver board. Next, I guess I better 
> replace the other 2N5641 (Q3). (I have already replaced C30). 
> 
> Any other suggestions?

I know when I got mine, the first FET on the board had been replaced with an audio FET - no wonder it didn't work! Kludged in a dual-gate MOSFET (both gates tied together) and plenty of drive.

If in doubt trace backwards!

John K0JD
Milwaukee