[time-nuts] GPSDO won't lock: OCXO aged out?

[paul swed]

Newell
Thois is a different Paul.
Can you see what the xtal unit actually is.
I suspect there is not a hole to a trimmer cap. But potentially if you can
open it you could get to a cap or at least see the xtal and what caps are
around it.
Other more difficult approach is to insert a purposeful offset. Maybe 6 V
very current limited. Opamp would be the best way to do that. Just to see
if you can pull it to 10 MHz.
But it would also mean you have traced the efc control pin to some extent.
Good luck
Paul
WB8TSL

On Mon, Jan 11, 2021 at 9:06 PM Scott Newell <newell+timenuts at n5tnl.com>
wrote:

> TL, DR: Trimble 73090 5V OCXO won't tune to 10 MHz (9.999 989 MHz at
> 0 VDC EFC, 9.999 997 MHz at 4.88 VDC EFC). Worth fixing? Maybe an
> internal mechanical trim?
>
>
> I've got one of those China special GPSDOs (thanks again, Paul!)
> built around a surplus Trimble UCCM 57964 (or 57963?) board. PPS
> driving NTP went wonky the other day--turns out the GPSDO had lost
> lock. Due to WFH I didn't have LH running, so I can't see what the
> DAC was at before it unlocked. I did examine the Trimble logs after
> the fact and noticed this:
>
> L73:20/12/22.19:05:24 Phase over 10 microsecond
> L74:20/12/22.19:16:39 Phase over 100 microsecond
> L75:20/12/22.20:32:11 Phase over 1 millisecond
>
> Late December is when I noticed the problem, so the log looks reasonable.
>
> I now have my hands on it and can test. DC supply current and 10 MHz
> output from cold shows that the oven is working and the output
> frequency rises during warm-up (9.999 668 MHZ cold, 9.999 993 MHz
> hot), so SC cut rock, right? (Tbolt 10 MHz used as counter ref.)
>
> With no antenna connected, LH lets me fiddle the DAC voltage:
> 0 DAC = -100% = 9.999 989 MHz = 0 VDC EFC
> 524287 DAC = 0% = 9.999 994 MHz = 2.50 VDC EFC
> 1048575 DAC = 100% = 9.999 997 MHz = 4.88 VDC EFC
>
> I thought these 5V GPSDOs were using +/- 5 volt tuning, but that's
> not what I'm seeing. Not that it would help this unit... Tuning range
> from 0-5 V is about 8 Hz, or +/- 0.4 PPM. Does that seem reasonable?
>
> UCCM-P >*IDN?
> TRIMBLE,57964-80,60498844,V2.0.1.6-01
>
> Datecode on the OCXO is 0948. Paul gave it to me back in March of
> 2016, and it's run pretty much continuously since then. LH shows
> phase error accumulating, which makes sense for an off-frequency clock.
>
> Thought or suggestions?
>
> thanks!
> newell  N5TNL
>
>
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