> There are several causes, the diagrams and calibration guide is
> available on the net.
>
> http://www.cqham.ru/ftp2/mfj259-cal.pdf
>
I hate to tell you this but that calibration outline for setting bias is
wrong. He probably copied that from MFJ, who off and on for years have
calibrated antenna analyzer bias incorrectly. You must NOT set the bias to a
certain value, you should always set the bias for minimum harmonic content
in the output at lowest battery voltage and lowest expected load impedance.
I had a bias pot put inside these units because the pot was needed, NOT
because it could be set to a fixed value with a meter. If it could be set to
a fixed voltage value, I would have used a fixed divider!!! Think about that
and it makes sense. Why would you use a screwdriver to set a bias point in a
fixed divider from a fixed supply to a fixed voltage? Anyone with more than
half-a-brain would just use a fixed resistor divider, and no one would ever
have to adjust it!
The pot is there so the harmonics of the signal amplifier can be nulled out.
The required bias voltage varies from unit to unit because of all the
component tolerances. What you are doing is setting quiescent current to
best overall linearity in the amplifiers. You have to do that at lowest
expected load impedance and lowest battery voltage.
The correct procedure for the 259B is on my web site. Although only for the
259B the procedure for amplifier bias by measuring harmonics applies to all
units.
If the counter reads zero, the oscillator or amplifiers are probably
toasted. It is possible to lose a detector diode and have drop out because
AGC is applied to the oscillator, but it is not normally what happens. It is
also possible someone applied too much RF to the frequency counter input.
73 Tom
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