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| OCXO tuning screws not working on old equipment |
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| tverbeure:
The HP 5334A mystery is solved! I tried to trim the OCXO with the trim pot that's available in the back. Well, it turns out, it's not a trim pot, but a trim cap. And while its purpose is indeed to adjust the oscillator, it's there to adjust the XTAL oscillator... which is only present when you don't have the OCXO model! Here's the schematic, the trim cap is C1. And here's trim cap C1 on the PCB: Right below (and under the white cable) is the socket for XTAL Y1... which isn't populated. Was it really *that* hard to also remove C1 to avoid confusion? My unit has indeed the 10811 OCXO, with its own frequency adjust screw. When turning that, I'm able to fine tune the output frequency as expected. It's a bit tricker than other trim pots: the act of inserting a metal screwdriver in the trim hole is sometimes (but not always) enough to influence the output frequency. Still, the 5334A is now calibrated well enough for ~10-9 precision. One final mystery was the EFC (electronic frequency control) input of the 10811. It turns that there's an additional trim pot on the 10811 assembly PCB, and that you're supposed to calibrate the OCXO that way. This second trim input is accessible through the back panel, but it was covered with a calibration sticker. ::) And the other problem is: you need a really long and thin screw driver to reach it. |
| Wrenches of Death:
--- Quote from: tverbeure on June 28, 2023, 05:33:48 am --- It's a bit tricker than other trim pots: the act of inserting a metal screwdriver in the trim hole is sometimes (but not always) enough to influence the output frequency. --- End quote --- It's not a good idea to use a metal screwdriver. Not only can it detune the circuit while it's in the hole, it can short the oscillator circuitry to ground. A number of the HP canned oscillators are specifically labeled "USE INSULATED TUNING TOOL ONLY". WoD |
| rhb:
A wooden dowel with a 16 gauge aluminum blade epoxied in a slot in the end is easy to make and does a great job. |
| BillyO:
Yes, yes. A wooden stake through it's heart, that will liven it up! :-DD |
| trobbins:
Some of the earlier HP OCXO modules (such as the HP 00105-6013) used a fibre/poly rod to interface between a users adjustment screwdriver and the actual oven assembly - and over time that rod can easily snap, leaving no easy way to provide the standard coarse adjustment of frequency. |
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