Good information. The fact that you see the REF UNLOCK suggests that
the 10MHz freq ref may need adjustment. This will mask any mis
adjustment of the 100 MH REF OSC.
Jinxie said that the Oven and REF unlock disappear after 6-7 minutes,
so it's just an oven warmup issue. This is completely normal behavior;
the reference is fine. Also, I would expect all lock error messages to
appear if the 100 MHz is faulty. From the service manual:
A malfunction of the 100 MHz VCXO is indicated by all phase lock error messages (PLl UNLOCK, PL2 UNLOCK, REF UNLOCK, YTO UNLOCK, MIN UNLOCK, and HET UNLOCK) being displayed on the 8566B CRT.
The only indication of fault I've seen so far is the YTO unlock
message. I'd focus on the YTO loop and the things that affect it, such
as the DAC pretune. If you look at the adjustment section for the DAC,
there are some tests you can do that only require an oscilloscope and
get you pretty far. You don't even need to adjust anything yet, just
do some of the checks and you'll get an indication as to whether
things need tuning.
Matt
On Tue, Mar 8, 2022 at 1:19 PM Bruce <bruce@...> wrote:
OK -
Good information. The fact that you see the REF UNLOCK suggests that
the 10MHz freq ref may need adjustment. This will mask any mis
adjustment of the 100 MH REF OSC.
This suggests that ref problem COULD be the problem with the YTO but
that there are MORE LIKELY other issues. Let me review the 8566
diagrams and come up with a set of steps to better identify the issue.
Two things to try:
1) Measure the LO output at the front panel - be careful, it could be
as much as +8 dBm
2) Measure the voltage at the 100 MHz oscillator (A7A2) control
voltage (volt meter and scope)- this is available at the test point on
the top of A7A2 (Right next to the "pull handle") - you can measure
the 100 MHz without pulling the A7A2 module - signal is available at
J3 on the top panel - removing this cable will mess up the 10/20 PLLs,
but you don't care in this case.
The voltage at the test point should be a (relatively ) stable DC
voltage (there may be some noise which should be small compared to the
DC level). The expected voltage at this test point is -8V Let me know
what you measure.
The power at the LO output should be (roughly) 4 - 8 dBm. The power
at J3 should be (roughly) 0 dBm If possible, it would also be a good
idea to check the power levels at J1 (-10 dBm 400 MHz) and J2 (0 dBm
at 100 MHz).
Question - Which of the green LEDs is not lit (on which module)? If
the measurements above ar reasonable, we whoould next look at that
module's PLL. No point in chasing the YTO unlock, till all the other
PLLs are locked.
Cheers!
Bruce
Also, can you measure the power at the LO
Quoting Jinxie <paul666@...>:
Bruce, further up the thread you asked what error messages I'm
seeing. I have just re-checked and my earlier comment was correct.
At switch on, I get these flashing:
Oven
REF Unlock
YTO Unlock
Battery
After 6-7 minutes, the Oven and REF Unlock errors disappear. The YTO
Unlock and Battery ones persist.
I would like to check for the presence of the 100Mhz reference
signal, but there doesn't seem to be any facility for doing so
without removing the board with some form of extender. In any case,
both the 500kHz signal and the 500kHz divided signal are so
catastrophically crippled that I believe the most likely explanation
is intermittent power failure in that section. I don't see how an
issue with the 100MHz reference oscillator would produce the trace
I'm seeing. I know it's extremely difficult for you and everyone
else to visualize the trace from my description, but it's not
something a single picture can adequately convey and I can't post
video clips of my scope's screen here, unfortunately.
