Is My GPSDO DOA?

[NT0Z]

Hi, gang,

Last year, a friend gave me an older BG7TBL-style (a rare generic variant/clone with Trimble innards) GPSDO, and I went through the learning experience of figuring out how to set up and use Lady Heather, get the unit talking on the serial ports, tweak the cfg file, and connect the 10-MHz ref output to my older HP 5328A counter.

For several months it all worked perfectly.    But recently, when I switched on the counter, it showed "all 8s" on the display when the external reference was selected (works fine on the internal reference). I looked at the schematics and consulted a few online helpers, who suggested that the HP may have a bad diode or two (CR5 and CR6), or a bad IC (SN7413N) near where the external ref is connected and switched between int/ext.

Tonight I popped the covers and used a scope to see where my external ref signals might be going south. To my surprise there was no 10-MHz signal at all, and a test of the GPSDO showed no output on either 10-MHz output (although the 1PPS is still active).

I connected a separate 10-MHz XTAL Reference oscillator (1.8 Hz low at 10-MHz at last check) to the external ref input, and the counter still shows all 8s when switched to the external reference and still works normally when using its internal oscillator. Using the scope I traced the temporary ref oscillator input to the junction of CR5 and CR6, and to the input pin of U16. I didn't trace farther because I'm not ACTUALLY sure what I'm doing.

NOTE: I had to remove the HPIB board (option 11) to access the main board and the parts mentioned earlier. I didn't reinstall because I will never need the functionality, and the HP manual suggests that it won't affect anything whether it's in or out. I guess this could be incorrect, but the behavior of the counter seems identical either way.

So, now I have a working counter that can no longer use an external ref, and a GPSDO the works fine from the PC and satellite side of the house (Lady Heather shows "lock" and strong signals from 7-10 birds), but has no 10-MHz outputs.

Suggestions?

As always, thanks,

Kirk, NT0Z

[GigaJoe]

gps blow up your counter's input

gps:
10M out usually started at OCXO,  so some pin should generate 10M output , if it "locked" and 10M out exist,  you may tap from that pin, over cap and some resistor  ( to be safe) ; or follow where it stops ...

for a counter
no clue ,  but follow the input , maybe something fail  ;  service manual usually help:
https://www.keysight.com/us/en/assets/9018-05738/service-manuals/9018-05738.pdf

[ddosegov]

Suggestions?

Service manual, scope, inputs and outputs of U16 and U17...
Diodes are there to limit input signal to TTL levels. In one switch position CR5 should be shorted...