Re: HP 8567A YTO Unlock message!

Sotris wrote: 

Switched on external reference and checked with a +3dbm 10MHz sinewave
. When i applied the +3dbm signal it works like with the switch on
internal position. When i removed the signal from input, all system
freezed... So the internal ref must at least works. Dont know about
the freq accuracy of it yet (I'll measure it), but its working. So
thats good.

Correct, sounds like it is using either the internal or external 10 MHz reference.


Do you have the service manual of the 8567? If you have it, can you
make some copies of specific parts of it (I'll pay all the cost for
them of course)?
Sorry, I don't. I'm doing all this from very old memory! 


Yes thats should be true. However I still cannot locate a 10MHz TCXO
inside that instrument. Have download some service (verification &
Given the symptoms, I wouldn't worry about that at the moment. Since it works in "Int 10 MHz" position, it's finding something.


Oven Cold and all other messages except the "YTO unlock" are only come
on during the error correction routine. At normal operation mode, only
"YTO unlock" & VERY slow refresh rate of sweep are the symptoms. Even
at that refresh rate can make measurements of an input signal... I
understand the YTO loop as you explain it. The strange thing is that
during the test mode almost all measurements of inputs signals are
correct both for frequency and amplitude. I have also checked an AM &
an FM modulated carrier up to 1250MHz. And I say almost because I've
noticed that when I set a Span <1.000MHz cannot find the input
signal. Increasing again the span at >1.001MHz signal appears again...

I think that may be a clue. Narrow spans probably use somewhat different phaselock circuits, in order to improve phasenoise characteristics close to the carrier. The 8568 was very different from the 8566, as someone (John Miles? Hi John de n6gn) has already noted.

The 8568 used a "pilot channel" which was sort of a twin of the main signal path but there only to allow improving the phasenoise of the LO. I wouldn't be at all surprised if things are switched differently for narrow spans where narrow RBW filters mean that the improved phase noise was important.
However, the fact that it sweeps slowly and issues an unlocked error on wider sweeps leads me to suspicion that there is still something more general wrong with the YO loop. 
There is a BNC for 1st Local out and another one for frequency
refference IN/OUT depending on an INT/EXT switch aside of it. I've set
it on Internal position under normal operation.

Judging from what you've written, I'm shifting focus away from suspecting the 10M reference.

I agree with you that you really need at least an accurate functional block diagram so you can unravel things.

Sorry I don't have any documentation and that I can't remember more detail.

Glenn
.

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