[Central-Electronics] More 100V Fun and Games

[GRANT YOUNGMAN]

This radio is slowly turning me into a blithering, drooling idiot.  It seems that everything I touch changes everything, or something, or some things.  I’m finally at a point where 160/80/40/15 are producing some output — around 65/80/80/80 watts max respectively in CW.  20 and 10 which were working fine day before yesterday (low output on 10 though) are now essentially dead.   20 just barely flickers the Bird, and 10 is producing around 10 watts out.  10 for 10.  I think when I started all of this that there was “headroom” in the RF output, but these numbers are with everything maxed out.  And there’s no info in the manual that I can find regarding calibrating the power input meter scale.

I’ve sweep aligned all of the coil sets per the manual including the VFO coils— and then had to manually re-tweak them to get the output up.  Replaced the carrier osc tube.  Peaked the 8 Mhz coils.  Cleaned all of the RF coil plugin pins, tightened up the sockets, and there’s no longer any change by wiggling tubes, coil boxes, anything at all.  All of the 1st/2nd mixer injection frequencies are where they are supposed to be, so I suspect sections 3R/4R of the band switch, but if that was broken I would expect not to get the injection freqs measured correctly at the respective mixer grids/cathodes.  I’ve also replaced ALL of the tubes in the RF chain, including the 6550’s.  Everything made small incremental changes, but nothing solved anything really.  I still need to revisit the sweep alignment again since I changed tubes, but just doing a simple “tune for max smoke” at a single frequency hasn’t gotten me any more smoke, and I can’t get any improvement at all on 20/10.  The spurious signal adjustments on the 1st/2nd mixer sort of work, but mostly not, and when the spurious output is down, so is the RF output, using the recommended Sylvania tubes.  I could only find 12BY7As, so maybe that’s a problem (?).

One thing that isn’t right is the bias reading on the driver tube.  The cathode is supposed to read about .36v, but I’m at just around 0.2v (which is now down from where I started a few days ago, with a new driver tube).  Resting plate current on the 6550’s seems about right — around 60 on the output scale.  I have yet to actually track down the resistors in the driver/final bias string — I know they must be in there somewhere, but trying to find things in that morass under the band switch will leave most mortals in my spot — blithering and drooling ;)

Anyone have any bright ideas?  Count to 10?  Walk three times naked backward around an oak tree?  Eye of newt?  Toe of frog?

Grant NQ5T