"paolo67" wrote in message
...
This receiver has a tube based PLL for the LO. This was completely off
on all bands due to the fact that all the cores of the Ls loosened,
they were kept firm with some sort of glue across the thread that
pulverized over the years. The vibrations of the drum did the rest.
After the alignment, it is now rock stable, better than the 8640!
However, I did not consider the IF filters when I did this, I tuned
the LO for *exactly* 3.3MHz IF that then become 300kHz after another
mixing with a fixed 3MHz quartz oscillator. It is not a good idea to
tweak this oscillator too, as it also generates the harmonics used by
the PLL. This is actually 3 to 5 Hertz off.

I am *very* worried of upsetting the filters. I like the idea of
identifying which trimmer changes which part of the curve before
attempting the real job and preparing a reference chart. I am
convinced that some previous owner tried the alignment before and not
necessarily did a terrible job. The manual just has charts with the
expected shapes of all six, the number of peaks, at which freq they
should be, the maximum ripple. The representation on the scope is
linear, I suspect it will not be accurate enough. Today I tried with
the digital scope, set the persistence to 10sec so that I can see a
curve produced by ten sweeps, this seems to be an improvement. Also,
-3dB=0.707x linear, the digital scope has cursors that also help.

Another thing I wanted to ask and you already mentioned: which filter
to start with? The narrowest, this one is easier to center at exactly
the right freq. You make a difference between "crystal" and "tunable",
however this thing must be quite odd, the crystal filter is also
tunable, has its own ten trimmers as the non-crystal types.

Thank you again for your help
Best regards
Paolo

One of the biggest issues of a radio with variable selectivity and passband
tuning is figuring what is being tuned and how the IF and BFO might be
shifting and what oscillators are being shifted in and out for various
modes, then determine what filters are the least adjustable and see what
shape you can get from them. Typically the monolithic filters, be they
crystal, mechanical or ceramic can only be adjusted for input and output
matching.

PLL cores are adjusted so that the VCO can be reliably tuned across it's
range. The steering voltage is usually specified either at some frequency
or at the extremes.

Offset oscillators for various modes also need to be accounted for.

Transceivers are even more critical because you will wind up poor fidelity
and/or spurious. Loading changes can make a shift between transmit and
receive too. I can't figure out how the High Fidelity SSB guys deal with
phase noise and carrier rejections as they approach response below 200 Hz.
Especially the ones who do a base boost to try to get fidelity out of the
cheap 500-4000 Hz speakers in mobiles, but sound like a craphouse studio on
a good quality station speaker.

3-5 Hz is pretty good but it just makes it more noticeable when the hum on
the incoming signal is out of phase with the hum modulation in the receiver