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[TowerTalk] MFJ 259

To: <>
Subject: [TowerTalk] MFJ 259
From: (Ian White, G3SEK)
Date: Thu, 25 Jun 1998 08:25:01 +0100
Mike wrote:
>The major problem:
>It was reading erroniously, so we hooked it to a dummy load. Kinda funny
>the dummy load was showing 100 ohms on the meter and flat SWR. 
>How can that be?
>It ended up going back to where it was bought from at the advice of MFJ.
>So please someone run this thing through some serious paces and please try
>the dummy load things, maybe we are just idiots and a 50 ohm dummy load is
>supposed to show 100 ohms???

If the dummy loads were good, it sounds like the Resistance pot R31 was
not adjusted correctly.

My MFJ-259 was pretty accurate right out of the box, but if you want to
experiment there are four trimpots accessible through the rear of the PC
board, in a layout something like this:

2  3  4

1. R17, SWR meter. 
BEFORE you adjust this pot, check for a 3:1 SWR reading with a 150-ohm
load and a 16.7-ohm dummy load; then check for a 2:1 reading with
100/25-ohm loads. Be prepared to accept some compromises at various
frequencies, but if you aren't satisfied or can't resist, try adjusting
this pot.

2. R32, counter bias. 
If the frequency counter is working OK, leave this alone.

3. R31, resistance meter.
BEFORE you adjust this pot, check with the various dummy loads.
Again, be prepared to accept compromises at various frequencies (more so
than with the VSWR reading).

4. R18, oscillator ALC.
This probably requires no adjustment, unless the Resistance meter is
reading significantly low at 146MHz (you can expect it to read
progressively lower above this frequency). It is possible to adjust R18
to lift the response above 146MHz, and increase the output in "signal
generator" mode at all frequencies, but only at the expense of increased
harmonic production - and therefore R/VSWR inaccuracy - at lower
frequencies. Once again, be prepared to compromise.

(This information was remembered from a posting by
G3SKR from W8JI, which I carelessly deleted - hope I got it right, Tom.)

Bottom line: the MFJ-259 is a fine instrument, but don't expect Hewlett-
Packard performance - you didn't pay a HP price.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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