Hi Hank,

I just got through repairing the unit. Once I analyzed the schematic,
working between it and the block diagram of the PLL, I was able to label all
of the stages and their functions.
The problem with my unit was that one of the connections from the bifilar
wound output transormer had broken loose from its PC board connection. Once
I resoldered the wire, everything started working.
The PLL is actually pretty simple; a 1MHz crystal oscillator is divided down
by 2. There is some kind of step recovery circuit that gives this signal
very rich harmonics. This is your comb generator.
This signal goes into one port of a mixer. The second port of this mixer has
the tunable HF oscillator applied to it. This oscillator operates 4.875MHz
below the expected 500kHz harmonic that you will be tuning to, thus giving a
4.875 I.F. signal at the output of this mixer.
After being amplified, the I.F. signal is applied to one input of a phase
detector, while the other input receives a signal from a 4.875MHz crystal
oscillator. The filtered correction signal (DC) is applied to a Varactor
diode in the HF oscillator circuit to warp it on frequency if it drifts.
Now, the final part......... double tuned transformers T702 and T703 on the
top board are you 4.875MHz I.F. transformers, while tht pair of T704 and
C745 set the low and high frequency endpoints of the HF oscillator. In like
manner, the pair of T705 and C750 are adjusted for maximum amplitude for the
low and high frequency endpoints of the buffer amplifier.
I haven't figured out how to adjust T701 and C745 yet. This tuned circuit is
at the output of the 500kHz comb generator.
I will hook a frequency counter, RF voltmeter, and spectrum analyzer to the
LO output of the PLL later today.
The reason that it took me so many years to figure out this circuit is
because of the haphazard explanation that was given in the receiver's
manual. There was so much microdetail that I was unable to see the forest.

Pete

"Henry Kolesnik" wrote in message
et...
Hey Pete
Let me know if you find it as I need to do the same.
I gave up several years ago and need to get it going..
Good winter project.

--

73
Hank WD5JFR
"Pete KE9OA" wrote in message
. ..
A few years back, I ran into a fellow who was familiar (he figured it out)
with this procedure. I think he was from Texas.
Is there anybody who knows how to do this? Unfortunately, the service
manual doesn't explain the adjustment sequence.
Thanks!

Pete