[time-nuts] Z3801A OCXO manual trimming

[gandalfg8 at aol.com]

Crazy thought.Could you just force a DC offset into the EFC assuming the
internal varicap is not out of range.
It would be simply adding a resistor to pull up or down to see if you can
get a bit of pull and allow the dac to move back in range a bit. Its a band
aid.
I know its sort of crazy. But ripping the oven apart really is no fun at
all.
Regards
Paul
WB8TSL

 

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Not really such a crazy thought, I've done this on a Trimble-Nortel NTGS50AA where the original 34310-T oscillator had "aged" beyond the 3 to 6 Volt EFC range.

On a unit that was refusing to lock the oscillator was removed and confirmed to require an EFC voltage of approx 6.5 Volts for an output frequency of 10 MHz. As other tested samples of this oscillator, although admittedly not many, required around 4.5 volts there does remain the possibility of an actual internal fault but it did test on the bench as otherwise ok.

Using a simple unity gain level shifter based on a few resistors and a TL071 op amp, just because there was one to hand, the EFC voltage from the control circuit was shifted 2 Volts high and the unit then behaved as expected.

The effects of temperature, supply variation, etc on the modified circuit were not investigated as this was only a short term test but it certainly looked to be a viable proposition, to the extent that rather than fit a replaccement 34310-T that board was fitted with SMB connectors using the pads provided to keep as a test bed.

Nigel GM8PZR