I suggest fitting a fixed capacitor to replace the variable you added, the variable ones tend to drift off with temperature and over time/age, using fixed values would be better.
Kerry - if you tinker with it again I think I'd replace the added trimmer either with a ceramic or piston trimmer but probably better to use a NP0 cap of an equivalent value that allows you to fine tune the first cap correctly. If all you want is a ball park figure then this is ok...of course that's why we network all the gear in the lab with a GPSDO or Rb standard anyway. Thanks for sharing!
I would drill a hole to adjust the frequency from the outside of the meter if possible and cover the hole with an aluminum sticker. And i would replace the second trimmer with a fixed one or glue it with special adesive so it dont drift with temperature.
Sorry, that's been no good repair, as the oscillator lost for sure its temperature compensation feature by the additional capacitor with its unknown and undetermined own T.C. The circuit for sure was faulty, probably not the XTAL itself, maybe the oscillator/temperature compensation circuit, which was not analysed properly, but bodged in an unqualified manner only.
Agreed that modding a TCXO like this will likely break the TC part. It could be that the crystal has been damaged by shock or something, but my first guess was maybe the temperature sensor wasn't working right. Still, 2 kHz off is a huge error. I've had very good luck using eBay TCXOs in the DIP-14 4 pin package, all the ones I've ordered were well within spec and seem to be stable long and short term. For occasional use equipment like this I wouldn't bother doing a OCXO, a TCXO will be much more accurate and convenient for a device that's moved around a lot and not left alone and powered all the time. A big equipment rack like that should have a system 10 MHz that's used when readings must be accurate and consistent between gear.
For any precision instrument the moment you open it you technically "ruined" the calibration and added some unknown factor to the characteristics. The main point of this video is that a crystal oscillator's oscillation frequency can be fine tuned via external capacitors. Yes, most likely the temperature compensation wouldn't work properly as it was tailored to the original crystal's curve.
@@KerryWongBlog Only today I discover this video. I agree with other gentlemen who stated that you did not repaired the TCXO. Sorry to be rude but this is the truth!! Adding a capacitor to the xtal modifies the Q factor. You tuned the frequency widening the bell shaped response, let me say worsening the sharpness of resonance curve. Furtheremore, when you tune variable capacitor it is better to use a PLASTIC screwdriver not a metal one...Do not be offended, a sincere criticism is better than a false praise. Regards.
A further demonstration is that adding a variable capacitor you compensate 1800Hz. On the contrary, the TCXO manufacturer allows only 4.7 Hz...this is not possible, you see. Another proof could be obtained from a Phase Noise measurement. You will see a big degradation on the performance due to a lower Q. But measuring xtal oscillator Phase Noise requires very expensive equipment.
I suggest fitting a fixed capacitor to replace the variable you added, the variable ones tend to drift off with temperature and over time/age, using fixed values would be better.
Kerry - if you tinker with it again I think I'd replace the added trimmer either with a ceramic or piston trimmer but probably better to use a NP0 cap of an equivalent value that allows you to fine tune the first cap correctly. If all you want is a ball park figure then this is ok...of course that's why we network all the gear in the lab with a GPSDO or Rb standard anyway. Thanks for sharing!
Thanks for the suggestion. I would probably eventually replace it with another TCXO or a low-powered OCXO (since the onboard PS is pretty weak)...
I would drill a hole to adjust the frequency from the outside of the meter if possible and cover the hole with an aluminum sticker. And i would replace the second trimmer with a fixed one or glue it with special adesive so it dont drift with temperature.
Thankfully you keep the crystal, because according to EEVblog aged stuff is more stable.
Nice job ! TNX 4 the upload n
73 N8AUM
I wouldn't trust anything that has drifted this far - maybe you got it corrected for now but I doubt it will still be accurate in a few months...
I probably wouldn't had bothered replacing the 'tickso' and replaced it with an OCXO.
The power supply on the 775A is pretty weak though, so probably wouldn't provide enough juice for the initial current needed by the OCXO...
Sorry, that's been no good repair, as the oscillator lost for sure its temperature compensation feature by the additional capacitor with its unknown and undetermined own T.C.
The circuit for sure was faulty, probably not the XTAL itself, maybe the oscillator/temperature compensation circuit, which was not analysed properly, but bodged in an unqualified manner only.
Agreed that modding a TCXO like this will likely break the TC part. It could be that the crystal has been damaged by shock or something, but my first guess was maybe the temperature sensor wasn't working right. Still, 2 kHz off is a huge error.
I've had very good luck using eBay TCXOs in the DIP-14 4 pin package, all the ones I've ordered were well within spec and seem to be stable long and short term.
For occasional use equipment like this I wouldn't bother doing a OCXO, a TCXO will be much more accurate and convenient for a device that's moved around a lot and not left alone and powered all the time.
A big equipment rack like that should have a system 10 MHz that's used when readings must be accurate and consistent between gear.
I agree, it is better as nothing but this way you degrade it. Besides that, it is so far of there must be some other problem.
For any precision instrument the moment you open it you technically "ruined" the calibration and added some unknown factor to the characteristics. The main point of this video is that a crystal oscillator's oscillation frequency can be fine tuned via external capacitors. Yes, most likely the temperature compensation wouldn't work properly as it was tailored to the original crystal's curve.
@@KerryWongBlog Only today I discover this video. I agree with other gentlemen who stated that you did not repaired the TCXO. Sorry to be rude but this is the truth!! Adding a capacitor to the xtal modifies the Q factor. You tuned the frequency widening the bell shaped response, let me say worsening the sharpness of resonance curve. Furtheremore, when you tune variable capacitor it is better to use a PLASTIC screwdriver not a metal one...Do not be offended, a sincere criticism is better than a false praise. Regards.
A further demonstration is that adding a variable capacitor you compensate 1800Hz. On the contrary, the TCXO manufacturer allows only 4.7 Hz...this is not possible, you see. Another proof could be obtained from a Phase Noise measurement. You will see a big degradation on the performance due to a lower Q. But measuring xtal oscillator Phase Noise requires very expensive equipment.