[Heathkit] SB-401 Neutralization, Low power and parts

[Shawn Duley]

Thanks to those who took time to respond!

I've pretty much solved the neutralization problem.  Original builder
took some liberties with lead and wire routing.  While probing around
the driver and bandswitch wires at power, I saw downward swings of 20-30
watts on 10 meters.  After rerouting in accordance with the manual, I
can get a stable neutralization.

I still need help from the collective Heath human knowledge base on the
low power issue.  I have some useful observations that may help point me
to the next level.   All tests done in CW or Tune.  I have 105w/80m,
100w/40m, 89w/20m, 65w/15m, 45w/10m.  Have used 3 different wattmeters
and measured the rf voltage to verify.

I may have a thermal related, or power supply issue that is contributing
to the problem.  From a cold start, and after a minute warmup, power is
low, but stable.  On second and subsequent key downs, grid current (also
plate current) will slowly drop about 1/2 to one unit on the installed
meter.  At one point, I saw a 100 ma drop in plate current, but traced
that to poor driver tube shield contact.  In fact, have re-cleaned all
tube shield ground connections.  I've tested 5 driver tubes (GE and RCA)
and 3 combinations of finals (all RCA) and see the same drop.  Probably
not the tubes.

I'm pretty sure the 8.6mhz trap is set correctly, as I have experimented
by adjusting at power and observing the results.  Even tried sweeping
the trap and observing on an O'scope to get the best setting.  Didn't
work.  The VTVM is the best way.  Still haven't proved whether or not
the 21 mhz trap is needed for stand alone operation.  Tried to align the
rig with an SB303, but the reciever had some hetrodyne coil problems
(that's another issue, tho).  Still had low power out with the 303
connected and the 401 unlocked.  I have observed the various voltages as
close to the driver/final section and the Hi and Low voltages at the
final and driver swing with the power change (I believe an effect, not
the cause).  Full load plate voltage is about 720 at 250 ma (right on),
but the driver no-load plate voltage is about 15v low, about 240 instead
of 256 v (same at the power supply).  The RCA TransmittingTube manual
and Radiotron Handbook mention that class AB1 operation is less (though
not totally) sensitive to power supply regulation than other amplifier
classes.

When I was in the power supply, the electolytics tested good with no
appreciable leakage at working voltage (using a Sprague TO-5 tester).
The load resistors were way out, and were replaced.  I've actually
replaced about 50 resistors, so far.  All the 1/2 watts are carbon
composition, since readily available.  All the 1 and 2 watts that have
been replaced have been with flameproof metal oxide, since carbon comps
are harder to find.  This includes the two 1 watts in the driver
circuit.  Now, I thought that metal oxide resistors were supposed to be
non-inductive.  Does anyone have and experience with metal oxide
resistors at RF?

Short of measuring the plate current simultaneously on each final, is
there any other way to ensure each tube is pulling an equal load?
Neither final shows an abnormal color after extended key down.

Heath used bazillions of the Z5U caps.  I haven't been able to find a
ready source for the .005uf 500v Z5U caps.  What are the rest of you
using for replacements?  (haven't done any replacements, yet)

Would appreciate any suggestions or observations that might help.

73's
Shawn, N7PRP