[MRCG] HO

[Dennis DuVall]

Jim,

Sounds like weak output from your oscillator, PA won't show much of a  
dip unless it's driven hard which is what class C is all about.  Also,  
presume the RF you are getting is on the right frequency?  Guess I'm  
going to have to get my 375 down off it's perch and make some  
measurements to give us a baseline here.  Also, you might try your 40M  
TU again with the one tube that oscillates and see what you get.

Dennis, W7QHO

******

On Jan 6, 2014, at 10:10 AM, Falls, Jim at DOC wrote:

> My BC-375-E blew something and I've stuck with the TCS since late  
> October.
> Dennis W7QHO and I have been trying to figure this beast out via  
> email and no luck so far: The rig output went erratic/"flickery" on  
> the scope one net and then red-lined the Total Plate Current Meter.
> We figured MO wasn't oscillating and thought a blown/leaky bypass  
> cap was keeping the MO from doing it's thing. In this rig, without  
> the MO drive provided to the PA, the PA doesn't generate the bias  
> required by the Modulators/Speech Amp and they redline the Total  
> Plate Current meter as a result. Tested all associated bypass caps  
> and resistances in the MO/PA side and it all checked out nominal.  
> Traded TU's to no effect. Swapped tubes around to no effect.  
> Isolated the MO stage (like doing a neutralization by the book) and  
> found it truly wasn't oscillating: 40mA indicated and flatline on  
> the scope.
> Got out an electric drafting eraser  and burnished (again) all tube  
> pins and contacts. Traded 211s again, and found that 2 of the 4 will  
> oscillate per the scope using a probe to the MO grid. Re-connected B 
> +, but  the plate current is still very high (~400mA) and won't dip  
> at all. I developed the theory that that the MO is oscillating, bias  
> is generated by the PA, but is leaking off somewhere along the line  
> to the SA and MOD stages. Checked/bypassed the Modulator and Speech  
> Amp grid bypass caps (1uf bathtubs) to no avail: CW mode shows 80mA,  
> VOICE shows 400mA. Neither will dip at apparent resonance on the  
> scope as measured at the output lugs.
> An interesting point is that if I sing loudly into the mic, the  
> plate current drops to 250-300mA, and the scope shows a decrease in  
> output. Going to hang a wattmeter off of it and see what's actually  
> coming out the tailpipe.................
> This is getting weird...................Any ideas out there?
> 73 ALL DE JIM K6FWT
>
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