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FeelTech/FeelElec FY8300S Quick Tear Down
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edigi:

--- Quote from: grizewald on January 27, 2020, 11:42:53 pm ---If you want to wire it up so that you can feed it with a proper 10MHz reference from a GPSDO, just cut the legs on the clock chip. Then there's no risk of melting sockets and you're hardly going to use or need the internal reference again if you can connect a proper reference signal.

--- End quote ---

I'd not like to rely on GSPDO always and I'd rather have a stable than an absolutely accurate reference.
My aim for accuracy is somewhere in the 0.1 Hz range and the refurbished OCXO that I plan to use for this has voltage control with 10 Hz/V tuning sensitivity (because of this it's not good for any serious use) so if adjusted with a small trimmer it's probably good for a year or so. I have still not decided that if it worth the trouble of the external reference input as it would require some kind of signal detection and switchover.

With better stability references available nowadays the tendency is that even in case of more expensive devices the external reference input is the first victim of cost-cutting (a very unfortunate tendency btw.).
grizewald:
I think I'll probably just use a switch to select between external and internal references and replace the internal reference with an OCXO. It will probably need an add-on power supply for the OCXO as I don't think the built in one has enough reserve to drive the OCXO's heater.
nctnico:
Just best sure to add some ESD protection to the external input. A 100 OHm series resistor and a TVS diode for example.
edigi:

--- Quote from: grizewald on January 28, 2020, 09:22:37 am ---It will probably need an add-on power supply for the OCXO as I don't think the built in one has enough reserve to drive the OCXO's heater.

--- End quote ---

It's hard to tell without having detailed technical specification of the generator.
I've checked my OCXO and it heats up much faster than I've expected. It uses 0.5A for around 45 sec and then it drops quite fast to 0.2A before 1 min. is reached.  That 0.2A is probably less than what is used by the fan (that I cannot measure since it's faulty).
If the fan is sacrificed (it's probably anyhow not very efficient even if it works, especially for cooling the opamps since they are in the front side of the generator) and use some ventilation holes instead it has to survive the higher load only for 1 min. or less.
I've calculated with an extra 50mA for stabilizing the voltage for the tuning input but that can probably pushed down if really needed.
Unfortunately I don't have much time for this before the weekend, but I don't see it totally hopeless.

If I have to replace the PSU it needs a dedicated PCB due to the screw hole constraints and that means weeks at earliest...(not that I would be in a hurry)
edigi:
I just couldn't resist making a quick check of why the fan in my device is not working so I've desoldered it (not without surprise: I saw already earlier that one of the wires is soldered to a pin of a regulator, but I did not expect the other wire to be soldered to a small capacitor as it was hidden by the wire).

Checking with lab PSU the fan started to work from 6V up. I cannot really blame the vendor of that fan that it does not start less than half of its rated voltage...
Unfortunately I've also grossly overestimated the amount of current that can be gained by not using the fan. It probably does not use more than 30mA from 5V.
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