[Heathkit] Problem neutralizing an SB-401

[Shawn Duley]

I have a an SB-401 that I am having a problem with neutralizing the
finals, coupled with low output.  Hoping someone can help.

Unit was non-working when I got it.  Final rf choke and several burnt
resistors.  Also had arcing damage near the antenna connector.  I
replaced the rf choke, replaced all resistors in the finals, checked all
the bypass caps on a Sprague cap checker.  Replaced the damaged
components in the relative power detector, and the charred caps on the
back of the switch by the antenna connector.  While I was at it,
replaced about 20-30 other out-of-spec resistors, which has returned all
voltages to within specs.

Proceeding with the alignment, (by the book), everything goes according
to plan until testing, after hooking up the grid and plate, after
neutralization.  Initially, I see over a 100 watts (CW) on all bands
except 15 meters which is about 75 watts.  This sounds pretty good, so
to check the neutralization, I compare max power out with minimum plate
current (since at resonance, maximum power transfer should occur with
the plate dipped to minimum current).  On 80m thru 20m, max power and
minimum plate current occur about the same setting of the final
control.  But on 15m and above, the two diverge and max power and
minimum plate current occur at considerably different settings.  When I
"tweak" the neutralizing capacitor on 28.5mhz (similar to the procedure
in the HW-100 manual) so that max power and minimum current occur
together on all bands, I get 100 watts on 80m thru 20m, 75 watts on 15m
and 25w on 10 meters.

I have tried different driver tubes and finals (both 6146's and 6146A's)
with the same results.  (I actually have the same problem with another
SB-401).  I used an HP VTVM with an 11 meg impedance rf probe for
everything except setting the 8mhz trap.  For some reason, with the HP
hooked up to pins 5 of the finals, I read over 100 volts, but with a
digital meter and homemade rf probe (about 6k impedance) I can I read a
couple volts and can adjust the trap down to about .8 volts.

In either case, above, output appears clean on a monitor scope in both
cw and ssb with a two -tone applied.  I know improper neutralization
will cause low output, but in the first case above, I am concerned about
operating out of resonance and overheating.

Could the trap be incorrectly adjusted, which might give the above
symptoms?  I'm wondering if the low impedance probe might be loading the
circuit giving a false minimum reading.

Or, am I just chasing a ghost?

Any thoughts or pointers would be appreciated.

73's
Shawn, N7PRP