SB 200

[Kevin Bisset]

Just a quick suggestion John,

With the SB-200 switched off, measure the output SWR using the SB-200's
metering circuit (metering still works).

I have recently had lots of adventures with my SB-200, and I just love my
wee amp! Personally, I would say that the input is probably not the problem.
I tried using a tuner on the input to neutralise the input SWR. My Icom
indicated 100W out, but I ended up with less anode current, and even less
power out! So even with perhaps a less than ideal input SWR, the amp still
develops the high input drive need for grounded grid operation.

4 ft input cable? Don't know why, but yes, it is important (same as
Collins). measuring SWR on the input is farly tricky, and I just used the
Icom's power output meter as my indicator to adjust the input tuning - it is
VERY broadbanded, and the peak is very subtle!

In the grid path, there are the two 33 ohm resisters - check those (if you
haven't already done so). In the same path is the 1.5 ohm metering resister
and a further 33 ohm resister (grounded when you earth the relay input)
Check that 33 ohm resister. I found by slightly adjusting this 33 ohm
resister, I was able to stabilise the anode standing current to 90-120 ma -
check the tube specs, as it can be different for different makes. Watch for
the retro-fitted mod to stop Standing Anode Current from indicating! ie
key-down but no signal.

I have found that Antenna SWR/cabling affects the amp the most - be careful
not to get tricked.

Kevin
ZL2KC

----- Original Message -----
From: "John Barkow" <KD6PT at AOL.COM>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Monday, October 22, 2001 7:15 AM
Subject: SB 200


> Hi Gang.  Progress okey so far.  The grid resisstors were okey and looks
like
> they had been changed once before.  I tried tuning the 10 meter input
circuit
> but changing the position of the slug has no effect.  Capacitor checks
okey
> but SWR is 3:1 at 28.8 MHZ and 1:1 at 28.12  MHZ and I cannot ballance it
> out.  It is still 2:1 at 18.5, which is where I want to center it.  The
> manual calls for a 4 foot cable between the exciter and the amp and I am
> wondering if this length is critical.  I am using a much shorter cable,
about
> 18 inches.  Anly thoughts on this?
> John kd6pt
>
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