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| HP 5335A Timer / Counter - Anything i should know? |
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| tkamiya:
It's odd. I tested the oscillator by itself. It did consume 0.42A and declined. It got slightly warm. Cleaned contact and reinstalled. Now it's QUITE warm. So it might have been a case of oxidation. I didn't do anything at all. I'll have to keep an eye on it. It *may* have some intermittent problems. |
| Dr. Frank:
all contacts are gold plated, so that's not that probable. This thermo fuse might have a problem, as well as the oven regulator circuit. I once repaired a 10811 inside a 5370B, which has an oven indicator. So I could quickly find out, that its NTC intermittently failing to open, which gives no heating. Was a mechanical / vibrational error also. |
| tkamiya:
Oxidation may be a wrong word. More like contamination. I had plenty of times gold plated sockets and connectors failed me. I believe I'm having an intermittent issue. One time, it's quite warm to touch. Another time ambient. |
| tkamiya:
I have some data now. The frequency drift appears to be a result of cooling by fan. To reach this conclusion, I stuck a thermocouple on side of OCXO and charted time, temperature, and frequency. With case top open, it measured 34C and last two digit of frequency was 98. (last digit is 10^9) With case top open, 30 minutes later, the temp went up to 41C and kept going. At that time, frequency was 94. Now, close the case temp start to goes down. 14 minutes later, back at 34C and frequency was 98. So temperature co-effecient is somewhere near 1 x 10^9 / C Rate of drift is 1 x 10^9 / C (obviously...) This should stabilize at some point. Tricky part is adjustment. It may take few hours once case is opened and closed. Once it reaches equilibrium, I expect drift to be less. Looks like HP has designed a very efficient duct system. I looked at the temp fuse. It's socket-ed. So I cleaned both and reinserted. I think this is working. Perhaps when we tune, we leave the case open, so we have impression that OCXO runs that hot all the time? I don't think this is over-cooling because some chips run quite hot and they are closer to the fan. Opinions, please. |
| Dr. Frank:
The 10811As specified T.C. is < 2.5x10-9 over an environmental temperature range of 0..71°C. (see page 1-2 in manual of 10811A). So your observations might give too high a T.C., but maybe can not be distinguished so well from other influences. Also, the specification is given as a 'box method', not directly as ppm/°C. Maybe the oven control is not ok. A monitoring of the oven power consumption over temperature may give more hints. Its timely drift is specified < 5x10-10 /day. So your observations are inside specification., I suppose, that you let the counter plugged in all the time, so that the 10811A is continuously powered. When I switch my 5335A on, I also observe a slight drift of <1x10-9 after a few hours of internal heating up. I use the counter in a room at 21°C +/- 2°C over the whole year. The timely drift of my 5335A is less than 1x10-9 over a year, meanwhile the 10811A is so old, that it has settled. Again, whenever I unplug the 5335A, it takes > 48h to reach the old value within < 2X10-9. I recommend to let the 5335A warm up for several hours, with the lid unscrewed, but closed. By briefly shifting the lid open, trim the 10811A as close as possible to 1x10-9 deviation and observe again with closed lid, and repeat adjustment 1-2 times. I think, my counter meanwhile displays about 2x10-9 deviation, several years after last adjustment. That's totally fine, because that is really at the limit of what is possible with an OCXO. You need a Rb reference to get even lower deviation, about 1/100 of that ballpark. Frank |
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