Jim,
Thanks for the info. This SPM-19 does have the audio output circuit
which I suspect should produce a 1750 Hz audio tone when the unit is
tuned to an exact frequency. I believe that was the standard BFO tone
for the SPM 16 and 17. I did not know there was AGC in this circuit. I
am presently using the HP-3686 for the HGB reception. I am using the
demodulated audio on it which raises another question. Perhaps it also
has some AGC at this point which could distort the level measurements.
I will investigate that.
What I mean by this SPM-19 being off frequency is that it almost
produces a zero audio tone when the display is tuned to a signal instead
of the 1750 tone. The 1.792 KHz offset is what the unit displays when
the AFC is activated. Also when in the narrowest IF mode there is no
signal unless the unit is tuned up 1.792 KHz from the desired
frequency. So wouldn't the frequency error be ahead of the IF filters?
Another problem just developed. The IF mode is stuck in the 1.74 KHz
mode. Moving the electronic selector to any other IF filter instantly
causes it to return to 1.74 KHz when the lever is released. The unit
does not power up like I remember it doing when it was new. It seems to
be searching for lock while producing noise in the audio output. After
about 4 or 5 one second sweeps it will settle down but most of the time
an error code is displayed, 1--002. Operating most any function switch
after that usually restores normal level and frequency display. I see
several 2716 EPROMS on one board. Could it be these chips are like me
..... loosing memory?
Dexter
James Moritz wrote:
Dear Dex,
There are only 2 frequency conversions in the SPM19 - one to the 40MHz
first IF (which is what the 40-65MHz LO signal does), and the second to the
10kHz final IF. You could include a third conversion to get the demod audio
out at 2kHz as I recall - but I don't think you could be using the audio
output for level measurement because it has permanently-on AGC. So I guess
the problem you have must lie in the 40MHz to 10kHz conversion oscillator
I have the circuit diagrams for the SPM19, so I will take a look and can
send the relevant diagrams if neccessary. BTW, when you say the frequency
is wrong by 1.792kHz, how are you measuring this? is it high or low? One of
the manuals also contains a list of error messages, but they usually are
not very helpful - 100 ways to tell you it isn't working!
Cheers, Jim Moritz
73 de M0BMU
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