My SB-200 Adventures...

[The Bisset Family]

I have been "ressurrecting" and SB-200 Linear that I picked up at a
reasonable cost, and have thought of the idea of "Brain Dumps" for various
equipment. There is quite alot of info regarding the SB-200 on the net, but
I found almost all of it no use to me when it comes to fault finding. And an
organised and maintained listing of various cures could be a very powerful
tool to any Heathkit nutter (I have an almost complete SB-102 station).

I thought I would share my findings. And I also have the complete original
manual - which is invaluable.

Anyhow, the most obvious fault was a blown filament on one tube - tried the
old soldering iron on the filament pins but it didn't come to life - new set
of tubes. (Lucky as hell - our local Ham Branch has a number of matched
pairs).

Fired it up, but virtually no added power out- fully tuned and fully loaded.
It was still 100 watts.

I attacked the schematics and the operating procedures. Essentially, this
Amplifier can be broken down into four parts. 1. The HV PSU, the Input Assy,
The tubes and the Output assembly, and the Keying/Grid bias Circuit.Other
than the metering circuit, it is almost as simple and straight forward as
one can get in an Amplifier - or so I thought.

The meter showed NO grid standing current in transmit with no RF input. And
even with full 100Watts in, there was still no grid current.

In receive, the grid bias cct puts about -130 Volts onto the grids to bias
the tubes into cut-off. This function was working.

When the Amp is keyed, the change-over relay kicks in when the relay imput
is put to ground. This worked fine. This same ground is fed through a couple
of fairly low resisters (1.5 ohm and a 33 ohm) and an inductor to the grid
equilizer resisters (both 33 ohm). It allows the grids to sink to
around -2Volts. Careful measurement on the grid connections of the tubes
showed one at 65 Volts and one all over the place.

AXIOM NUMBER TWO  - Heathkit resisters (as originally supplied) seem to be
of pretty poor quality. Of the three 33 ohm ohm resisters, one was 3.3k, one
was 48 ohms and one was 16.4k). I replaced the lot, except for the 1.5 ohm
resister (used by the metering cct to monitor grid current). Bingo - in
transmit, both grids now at about -2.2 Volts.

Also discovered the 20uf (150Volt) Cap that filters the 110Volt DC supply
was fried - replaced.

Still not much gain (very dissappointing) - approaching about 200 watts
maximum output on 20 meters.

The meter indicated only 1400 Volts HT.....

Remembering AXIOM Number Two, the HT is measured by the metering cct by a
voltage divider made up of three 4.7 megohm resisters in series and a 15K
resister to ground. The 15k was at about 16k (not a prob for a 10%
resister). The 4.7M resisters were 8.2M, 7.8M and ~9.5M (my Fluke 77 was
pushing it) . Changed all three, and now indicate about 2000 Volts - still
abit low, but functional. A quick check of the diodes (all 16) showed one a
dead short, but that won't affect it much. Am picking that the caps are
probably due for changing and I believe RF Parts have direct replacements.

AXIOM NUMBER ONE - a bit late. The HT in these Amplifiers is lethal. NEVER
try to measure anything in the anode Tank or the output directly. I keep one
hand in my pocket when ever I am in anyway near it.

So to the input circuit. Straight up and down, just a series of tuned
circuits to match the 50 ohm TX impedance to the Cathode input. I just
monitored the SWR with my IC-730 - they are VERY broad-banded and unlikely
to cause trouble. The only thing I have noticed is the capacitors in
parallel with the inductors are showing discolouration on some bands
(particularly 15 and 10 meters) - So I will resolve to change them.

Lastly, with new tubes, there are many sources out there that recommend
"burning in" the tubes for between 80 and 100 hours. I have had mine burning
for 50 hours and the output has considerably increased. It is now somewhere
in excess of 450 Watts on 20 Meters.

Work is progressing and I will hook up the SB-102 tomorrow to try it as
well.

To run the Amp on the IC-730, I have made up a small external relay cct,
capable of keying the SB-200's 110Volt relay, and have a 12Volt plug pack
supplying this - works a treat and no active componants -just need as fast a
switching relay as you can get your hands on.

Interested in hooking up with other SB-200 owners, and maybe starting an
SB-200 specific web site. What a great little amp!

Regards

Kevin Bisset
ZL2KC
Ashhurst, New Zealand

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