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

Hi all,

Well the Z3815A with its E1938 OCXO is working again! It turned out that one or more of the internal thermistors in the sealed oscillator had failed causing the temperature control system  to switch off the MOSFETS supplying power to the heaters. Once I received a spare from a friend I replaced the sealed unit and the oscillator heated up at once and locked to the GPS within 10 minutes just as it always had done. It’s a shame the sealed oscillator unit can’t be repaired. It’s a piece of electronic art but I suppose 20 or more years of continuous operation at an elevated temperature was more than it was designed to do. Interestingly this is the second HP oscillator failure I have had to deal with. The 00105-6013 series OCXO in my 5061A failed about a year ago too and with help for a very kind member of the HP community I was able to obtain a 10811 replacement for which I am very grateful. OTOH the ancient OCXO in the 524B counter is still going strong and is  remarkably accurate and stable!

While I’m on the subject, if anyone knows of detailed information or a manual for the Z3815A I’d love to have a copy. There are quire a few headers, switches and LEDs on the motherboard that it would be good to know about.

Morris

VK3DOC in Melbourne, Australia

Hi all,

The Z3815A is working again as described in my recent post. I am now left with a complete but inoperative E1938 OCXO unit. AFAIK the circuit board with the microprocessor and control circuits is OK  but the fault lies in the sealed oscillator oven unit which probably has one or more non functioning thermistors in it. I rigged up a plug to test it on the bench and can confirm that it oscillates a few KHz below 10 MHz but the oven does not warm up so it never gets to 10 MHz proper.

I’m happy to send it to anyone who wants to play with it or needs a functioning board. I don’t want any $ other than the cost of shipping. I’m in Melbourne Australia and would be prepared to send it anywhere in the world to give it a good home.

Interested? You can email me at the address in the header.

Morris

Thanks to everyone who responded to my offer.  the E1938 has now been spoken for.

Morris