[time-nuts] help reviving Trimble UCCM-LPS GPSDO

[Wilko Bulte]

hi,

I am currently trying to revive a Trimble UCCM-LPS ex-telco GPSDO.
The model is a 57963-D according to the PCB silkscreen.

Got the board without its OCXO but luckily my stash of surplus I have
discovered a Trimble OCXO labeled 72345-86. Given that that OCXO also has a
label UCCM-LPS it "should work".

Joining the two resulted in:

UCCM >syst:status?

-------------------------------------------------------------------------------
57964-86     serial number  S35DB68332   firmware ver  1.0.0.1-01 Ext
mode
-------------------------------------------------------------------------------
Reference Status __________________________   Reference Outputs
_______________
XX Ext Ref   : [LOS]
                                              TFOM     2            FFOM
3
                                              UCCM A Status[OCXO WARMUP]

>> GPS: [phase:-2.8E-09, settling]
ACQUISITION ...................................................[GPS 1PPS
Valid]
Tracking: 8 ____   Not Tracking: 3 ________   Time
____________________________
PRN  El  Az  C/N   PRN  El  Az                GPS      13:28:51     23 Dec
2021
 15  71 212   37     5   6 195
 24  45 272   43    30   7  85                ANT DLY  77 ns
 17  28  97   37    19  19 124                Position
________________________
 13  55 140   38                              MODE     Hold
 14  39  58   39
 23  33 291   44                              LAT      N  51:xxx
 10  17 324   40                              LON      E   5:xxx
 12   7 212   34                              HGT               +27.54 m
(MSL)




ELEV MASK  5 deg
-------------------------------------------------------------------------------

Command complete

Unfortunately it never went GPS locked. Longer story short: the pin of the
OCXO that (I think) should produce a Vref for the EFC circuitry produced
some random ~ 266mV. 

A quick experiment learned that the OCXO freq responds to the EFC voltage. 
So, looks like the Vref circuit in the OCXO has died.

Now the question: has anyone a working unit that they can measure the
correct Vref voltage from?  

For now I have fitted a 2.5V LM336 vref and I
see the EFC voltage go up and down a bit, with the output freq hovering
around 10MHz (+/- 4mHz roughly).

tia,
Wilko