[MRCG] Need BC-375-E advice
Falls, Jim@DOC
Jim.Falls at conservation.ca.gov
Mon Jan 6 13:10:28 EST 2014
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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