SB-400 Low Power and alignment

[Ed Richards]

Mark,

I am not home to check but if my memory is right, you need to do the
following (from the book):

1. Before attempting alignment, remove the 6CL6 and shield, remove the
top and rear covers from the PA cage and disconnect the 800 volts between
the feed-through capacitor and the bottom of the RF choke, disconnect the
680 ohm resistor from the PA screens. The book will tell you to install
these parts after alignment. Otherwise you are running full power during
the alignment procedure.

2. A RF choke will detune the tuned circuits. I remember the book saying
to use the RF probe only to get in the ball park then disconnect it and
tune for max PA grid current. I have done several and it works for me.

73,
Ed Richards

On Fri, 25 Aug 2000 09:29:52 -0400 Mark Johnson/Ontario/IBM
<mjohnson at CA.IBM.COM> writes:
> Hello everyone,
>
> My sb400 (part of sb300/400 setup) started to show low power
> recently so
> thought I'd dig into it and see what's going on. During a quick
> tune-up,
> (re-peak the driver and plate coils on each band for maximum power
> out) I
> got 100 watts on 80 & 40, 90 watts on 20, 65 watts on 15, and 50 to
> 15
> watts on the 4, 10 meter bands.
>
> On spec I installed new finals (Westinghouse 6146B's, NOS), a new
> 6CL6
> driver, a new mixer (V4) and a new iso amp (V5). No real change so
> thought
> I'd actually get the book out and follow the proper procedure for
> the
> alignment. After doing that,  the results are worse...I suspect that
> using
> the prescribed method of using an RF probe and VTVM, is causing the
> problem. I suspect  the probe/VTVM is loading the tuned circuits
> such that
> the alignment is faulty. I've got some work to do, but has anyone
> used the
> prescribed method in the SB400 manual with good results??
>
> Anyone have any starting point suggestions for my low power
> problem??
>
> Thanks Team!!
>
> Mark V. Johnson
> VE3DJU
>
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