SB 200 Loading

[John Farrington]

At 12:51 PM 09/14/1999 EDT, you wrote:

>I recently purchased a SB 200 which seems to be in pretty good
>shape, although the HV is only 1100V.

John kd6pt:

If you are getting near 500 W out then the high voltage has to be
near 2300-2400 V. The low voltage reading you are getting is
typical of SB-200's that have been used a while - what happens is
that the old-style composition resistors that Heath used in the
meter voltage divider string deteriorate over time because they
have too much voltage (800V) across each one. The string has to
be replaced by modern high-quality resistors, and more of them so
that the voltage drop across each one is not so great.

The string is under the chassis, beside the filter board, and
is designated "R3" (three 4.7-meg 1-watt), and "R4" (one 15K
1/2-watt). The three 4.7 megs total 14.1 meg, which can be
replaced, for example,  by 14 1-meg 1/2-watt 1% or 5% (plus a
100K if you want to try to be exact); the 15K at the meter
pick-off point can be replaced by a like value high-quality
resistor. You could also opt to replace the whole rectifier-
filter board with an up-to-date one that incorporates the
meter string, from Harbach Electronics (address below).
By the way, if the electrolytics go bad it's a lot more cost-
effective to replace the whole board with Harbach's kit than
it is to look for and buy just new caps from other parts
outlets (would you believe up to $30 apiece, times 6 for $180
total just for caps, vs $90 something for the whole kit?).
      --------------------------------------------------------

          Harbach Electronics        ( E-mail: wa4dru at iu.net )
          2318 S. Country Club Road
          Melbourne, FL  32901-5809

          You can look at his Heathkit stuff at:
             http//www.harbach.com
      --------------------------------------------------------

To check your voltage as is, take the SB-200 out of its cabinet,
lay it on its side or top, turn it on (with or without the tubes
installed), and locate the bottoms of the 6 electrolytic caps
on the filter board;  use a high-impedance meter to measure the
voltage drop across the terminals of each one (around 400 V),
then add up the 6 readings. Should total around 2400 V.

If you have the original paper-case electrolytics with twist-
lock bases, then you first need to trace out the filter board
foil pattern to determine which prongs are voltage leads, and
which are just locking prongs. It's not clear by just looking.

>It seems to load well, but I have a problem with my exciter.
>I have never used an auto-tune exciter with a linear before
>and don't know whether I have a problem or not.  When loading
>straight through, I get low SWR but, after tuning the linear,
>I get a high SWR on the exciter.

The SB-200 and newer solid-state transceiver exciters do not
always get along so well because the linear's input impedance
does not always exactly match the exciter (this seemed to not
be a problem with Heath exciters). I don't have an auto-tune
exciter, so I solved the problem by putting a small manually-
tuned transmatch between the exciter and the SB-200; then, of
course, unless you have a perfectly-matched antenna with zero
swr, you also need another transmatch between the SB-200 and
the antenna. Makes for a lot of knobs to twiddle and a lot of
settings to write down, but it reduces the exciter-to-linear
swr to zero on all bands. I don't know how well exciter auto-
tuners will do - I would think they would do a lot of hunting
and get confused by complications from the linear's output swr.

For the 17 and 12-meter WARC bands I presume you are setting
the SB-200's band switch to 15 meters; the linear should not
be used to full power on 30 meters because of the 200 W band
power limitation.

By the way, watch out for ALC incompatiblities between the
linear and the exciter: Kenwood transceivers have been
especially noted as not matching Heathkit linear ALC outputs.

73

John Farrington  KE5ZB

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