sb401

[Mark Johnson/Ontario/IBM]

You may have more than one problem....Audio/Audio drive circuit may be no
good, and RF circuits no good. I'd start with low RF problem with CW first.

I had similar problems with an SB-400. Drove me nuts. In the end, I
replaced most tubes including the finals. Some things I found...Tubes that
tested GOOD on my Tube tester, once in the transmitter, were no good. Only
sure way was to sub in new (known good) tubes and go forward.  Also, My RF
wattmeter was very stingy. I was using the dummy load and Wattmeter on a KW
Super Tuner. When I put my Heath Wattmeter in the line, the output was
higher.

Also, I found on 10 meters, I could not get the same output on all 4 of the
10 meter bands. The tuned circuits did not seem to be broad enough. I then
picked the middle part of the band (hi end of second position on 10 meter
band sw) and did my alignments from that spot. I then got about 40 to 50
watts RF output across 10 meters.  No matter what I did or replaced or
checked, I could not get anymore than 40 to 50 watts RF output on any of
the 10 meter bands on the SB-400 I'm referring to. This also goes for my
Sb-100, My SB401, and my SB-101.

Also, alignment. I've been doing these a long time and I still screwed up.
A kind member of this reflector pointed out to me that I had made a mistake
during the process. Once I realized that, my alignment went OK and things
improved.

Start with the obvious...Check the voltages as per the manual and anything
out of reasonable spec, hunt that down first. My SB-101 had low audio drive
and it turned out that a resistor in the plate circuit of the first audio
driver circuit was out of spec, causing the plate voltage to be too low.
The resistor was supposed to be 100K. Actual measurement was 125K. Enough
to bring the voltage down too low. Once I replaced that, my audio was very
very good!!! Who would think an extra 25K would cause that......



Mark V. Johnson
VE3DJU


Dennis Ka9Dzr <Ka9dzr at AOL.COM>@LISTSERV.TEMPE.GOV> on 11/19/2000 10:38:03
AM

Please respond to Ka9dzr at AOL.COM

Sent by:  Heathkit Owners and Collectors List <HEATH at LISTSERV.TEMPE.GOV>


To:   HEATH at LISTSERV.TEMPE.GOV
cc:
Subject:  sb401



  need help on a sb401 tx.
   when i first turn on the 401 i get abt 75-100 wats in tune.then it drops
in power.
   can only get abt 150 ma at 50 wats out.have low audio out.no alc
indition
on the meter.10 wats out on voice peaks on ssb. cw only 50 wats output.any
type of help .tnx dennis

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