Re: HP 8567A YTO Unlock message!

I don't remember well and I don't have a set of schematics in front of me, but a couple of suggestions:

It's unlikely that you are running without *any* 10 MHz reference. That reference goes into locking everything in the box and even if the instrument were in ext ref mode with that loop unlocked, the error is probably small enough that the other loops wouldn't have a problem. Normally the 10M tuning range is only a few ppm.

The YTO loop, (the "outer loop") probably needs several other properly locked references to operate correctly. In fact, it's "YTO unlock" error may only be a logical OR of inner loops. The idea being that if any of those other loops are not locked properly, the YTO signal cannot be trusted either. Oven Cold and Ext Ref are certainly affiliated with the precsion 10MHz reference and probably a detector circuit that looks for 10MHz external signal.

Are there rear panel BNCs on that instrument with adjacent "10M out" and "10M in" labels that should have a short jumper cable that is missing? That might almost explain all of your problems.

The slow refresh rate is probably related to error reporting of the unlocked condition. It will probably go to normal rate once all the loops are properly closed. I wouldn't be surprised if test mode ignores the warnings so that diagnosis can proceed more or less normally.

Glenn
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