[Heathkit] SB-401 Tune-up problems

[wb8jkr@juno.com]

  I think your tubes are ok, power output on the
lower bands is ok. I would triple check the 
alignment on the upper bands by using
a scope and a low C probe on the final
grid until you can get enough drive for
proper grid current indications.

73,  Mark  WB8JKR


On Tue, 19 Feb 2002 10:27:31 -0700 Shawn Duley <sduley@if.rmci.net>
writes:
> After reviewing all the Tempe and QTH archives for this rig, I need 
> to
> bounce some ideas off you all.  I haven't worked on a tube 
> transmitter
> for a long time.
> 
> I am in the process of final tune-up before testing this rig on the 
> air
> (recent purchase).  I have been faithfully following the manual
> procedures.  Disconnected screen and plate connections before 
> starting.
> Everything was looking good until I was attempting the final coil 
> tweaks
> with the finals hooked up.  All coil adjustments were made on the
> shallow slope of  each coils voltage curve.  After final tweaking of 
> the
> Driver, Grid and Het coils I have the following readings:
> 
> On 80, 40, and 20, full power out with normal (about 220-250ma) 
> plate
> current and grid indications
> On 15, about 70 watts with normal plate and grid indications.
> On all bands 28 and above, I get 50 watts, normal plate and NO grid
> indications.
> 
> On all bands, I get above power output with the level control around 
> 12
> o'clock postion.  On the higher bands, extra drive has no effect, 
> but
> the level control will reduce power to zero on all bands.
> 
> So far, my thinking is weak tubes or improper neutralization.  My rf
> probe is homemade using the circuit in the ARRL handbook and it may 
> be
> loading down the circuit giving me a false neutral point.  I know it
> loaded the coils because there was quite a bit of difference between 
> max
> voltage indication with the meter and max relative power when 
> adjusting
> the coils at the end of the procedure.
> 
> Any and all thoughts would be appreciated before I attempt more
> troubleshooting.
> TNX, Shawn, N7PRP
> 
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