[time-nuts] Z3801 10 MHz reference help

Jim Sanford wb4gcs at wb4gcs.org
Sun Mar 3 15:06:54 EST 2013

Seeking help with my Z3801. I was referred here by some folks on the 
Microwave reflector.

Background:  It ran very well for several years.  Then, when 
spot-checked on another counter, was found a few Hz away from 
10.00000MHz.  Looking at GPSCon, it showed the output to not be enabled, 
and a status or offset word that somebody told me indicated the 
"correction" was at a maximum extreme, and suggested I looked at 
temperature control circuitry.

I let it sit for a year or two, and have recently gotten back into it.

I have read all the notes I can find on the internet, including KO4BB 
site.  I have the user manual, but have not been able to find a service 
manual, including from the helpful people at HP/Agilent who will dig 
through their paper archives.  Struck out at Symmetricom as well.

Upon power up, a reow of LEDs near the power connection flash red, and 
then the one toward the middle of the board begins flashing green at 
about a 1Hz rate.

Check of all power supply voltages against the diagram on the 
realhamradio.com web site is all good.

Running GPSCon results in serial port timeouts -- no communications 
to/from the unit.  I have verified the serial port settings.  Given the 
vagaries of USB<==> serial converters these days, I have verified that 
the serial communications does work, using other hardware and programs, 
but the same laptop with which I'm trying to talk to the unit.

I have read the discussion about how the temperature controller is 
supposed to work.  It includes a discussion about the controller getting 
"confused" and breaking one lead to inject +5 volts through 10K to 
regain control.  The fact that the voltage at TP 104 remains at 16 volts 
(for days) suggests that this may be what is going on. Unfortunately, to 
verify this problem and attempt the fix, I need clarity on the fix, and 
to restore the serial communications, so that I can see the status words.

Any suggestions??

Thanks & 73,
wb4gcs at amsat.org

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