[Heathkit] Seneca problem fixed

[Jim Brannigan]

Good show, glad you got it fixed

Jim


> Thanks to all of the persons who offered advice and
> suggestions on the problem that I was having with low
> plate current on 2 meters on my Heath VHF-1 Seneca. 
> The problem turned out to be one of those "strange"
> things that "just happen".
> 
> There are several feed through capacitors (0.001 mfd)
> that go from the outside of the various cages to the
> circuitry inside.  Somehow the feed through that goes
> from the center tap of the grid coils through an r.f.
> choke (for 2 meters) or a 100 ohm resistor (6 meters)
> to the grid of the clamp tube isn't doing its job.  I
> added a disc ceramic in parallel with the feed through
> and things are acting much more like they should. 
> However, I now have to reneutralize the finals since
> there is a change in the grid circuitry of the 6146s. 
> But, the rig is now putting out about 55 watts on 2
> meters (about 47% efficiency) and 90 watts on 6 meters
> (about 64% efficiency) which is fine for a pair of
> 6146 tubes.  The finals now show slight signs of
> improper neutralization but that is easy to cure.
> 
> I believe that r.f. was upsetting the clamp tube
> making it "think" that the grid was operating
> correctly when it wasn't due to the improper
> bypassing.
> 
> I finally found the problem when checking the voltage
> on the grid of the clamp tube which comes directly
> from the "cold" end of the r.f. choke from the grid of
> the final amplifier tubes on 2 meters and the "cold"
> end of the 100 ohm resistor that comes from the 6
> meter grid coil.  On 6 meters the voltage was fine but
> on 2 meters my DMM acted like it does when trying to
> measure the voltage in a circuit where r.f. is
> present.
> 
> Don't have any idea as to what happened to the feed
> through, but obviously something happened!
> 
> Again, thanks for those who tried to help.
> 
> Glen, K9STH