Hi all,

I have had a Z3815A GPSDO working flawlessly here for many years but just now it has lost lock. Using HP Satstat software the self test and all the status messages are OK and the 1 PPS is valid. Querying the EFC:REL returns +639156  and the EFC:ABS returns 12.19E001.  It has been displaying OCXO warmup for days. There are no error messages and it runs in unlocked mode but the 10 MHz output is about 2 KHz low. I suspect the oscillator has drifted out of range of the EFC or there is some problem with the EFC. Before delving into it I was wondering whether anyone has had experience of this or can advise me whether there is a coarse frequency adjustment for the HP E1938 oscillator or there is some way to check the EFC despite the status being reported as OK.

Thanks in advance,

Morris VK3DOC

Thanks Bob,

I was thinking the same thing myself. It looks like I will have to open it up and see if I can find some obvious fault. Otherwise it will be trash.

Morris

I’m pretty sure the heating system of the OCXO is the problem and I’ve been looking into it with the aid of the schematics found on the web. There are 3 heating elements in the sealed “hockey puck” oscillator and they are all continuous with similar resistances. They are fed from three P channel MOSFETS that are in turn have their gates fed by opamps that have analog feedback from sensors in the sealed unit and digital inputs from the microcontroller or DAC. The power supply voltages are all present but the MOSFETS do not get enough gate drive to turn on. That’s about as far as I can get in the absence of more detailed information. There’s obviously a little bit of power getting to the heaters, the unit feels slightly warm after a while and the frequency climbs a bit but stops at about 1.3 KHz short of 10 MHz.

I have a replacement unit coming courtesy of a friend and I’ll report back when it’s fitted. Hopefully that will fix the problem. However I’d value any advice or experiences from time-nuts.

Morris