synx508 wrote: ↑Sat Sep 02, 2023 5:28 pm
Zenith wrote: ↑Sat Sep 02, 2023 4:15 pm
That looks a bit like the OCXOs Marconi put in various bits of kit including the 2018 and 2019 sig gens and the 2955 receiver test set. There's a circuit diagram for them on the WWW, which someone worked out.
That's exactly what it is, I have a 2019 and a 2019A, the latter has a OCXO with a defective oven and uses a cheap random 10MHz crystal, I use them both from an external oscillator so it's not very important to have it working properly but now I could replace it if I needed to. The 2019A's OCXO was so badly contaminated that both leads of the lovely glass encapsulated crystal had been eaten. The crystal seems clean in this oscillator, so it's a big improvement.
I already have the schematic somewhere but I searched for it and found BD139's post about it on the other place.
Further down in the parts cabinet I found several ITT, Decca OCXO and a CQE DOC1478 that appears to be unused, so I might build myself a GPSDO with that so I'm prepared for when Droitwich closes down.
My 2019 wouldn't calibrate when I got it. As high as it could be tweaked, it was still way under by many ppm. I opened it up and tweaked the preset cap by about 1/6th turn. When I tried again, it easily came on frequency, with problems as described above.
At least those Marconi OCXOs can be opened easily. Some of the Racal ones are soldered shut. They can be opened, but it's a bit of a mission.
OCXOs seem to be a bit variable as they age. I've got one in a Racal 9915 which seems as good as new and it's the basic 04A option. I've also got a 1992 with the better OCXO option, and that's pretty much up to spec as described in the manual. Others are well out of kilter with the spec and take overnight to stop drifting.
This is the circuit diagram I found for the Marconi OCXO.
PM3 OCXO.jpg
R5 is a 2R4 not a 2K4 as in the diagram.
BD139 put me onto the surplus Aliexpress OCXOs. They're cheap at two or three quid each, and I've found they are remarkably good, and that's taking the reference voltage from a fairly noisy 5V supply line, without using a voltage reference chip or taking much trouble over the multiturn preset.
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