Fluke 732A excessivew drift

[Awesome14]

I purchased a Fluke 732A  precision DC standard that had been stored in a powered-down state for an extended period. The batteries were stone dead, and one of them had a hole in the side. I replaced the batteries and monitored the unit against a 732B for three weeks. During each sampling, measurements were taken both in serial with the 732B and directly on an Agilent 3458A. The 732A in question drifts 1.1ppm/week at a constant rate. Factory spec is <=0.6ppm/month for the 732A.

Fluke Calibration states that they have no experience other than to repair such a unit, and they have no idea whether or not the unit would ever stabilize again. Further, the unit is designed to be under power at all times, even during storage. Fluke no longer repairs the 732A.

I can send the unit back to the seller, but I was wondering if anyone has experience with a 732A that was stored cold and then powered up again. The oven thermistor varies by 1.4 ohms. There are several reasons I want to keep this unit. It is a 1988 (later) model. The oven, by the thermistor, appears to work very well. The unit is clean inside and out, and it appears to function properly in every respect other than the drift.

The unit appears to never have been opened (or tampered with) until I replaced the batteries. The calibration pots appear to never been adjusted. Is it likely the unit will eventually stabilize? My other 732A took weeks to stabilize. But the 30-day warranty runs only until tomorrow. I have already indicated I will return the unit as "not working". But testing one of these is a PITA!    I'm sure if I changed my mind the seller wouldn't force me to return the instrument.

[ltz2000]

I happen to know an Eastern Europen cal lab that bought eight used 732As from various USA second hand test equipment dealers. They had a limited budget and a surplus 732A cost only a couple of hundred dollars back them.

Finally only one of them turned out to be good and stable. If that ratio can be generalized, there are a lot of bad ones.

On the other hand I have seen so many 732As which have spent their whole life in the same lab and only get better when aging.

The 732A seems to be very sensitive to bad storage conditions. Or there is some failure mechanism triggered by the stress.


A good 732A drifts a fraction of a ppm per year. If even close to the 0.6 ppm/month specification, there is something wrong.

[ManateeMafia]

One of my 732A's had a broken trimmer pot where someone appeared to adjust it past its physical limits. It could be as simple as that or a bad zener/ref amp. Either way, most likely it will be a problem inside the oven.

I have been inside a few 732A's and they are not impossible to service, but there are several thin enameled wires that could break.

You could try turning the pots back and forth several times, but you would need more time to monitor it. Perhaps the seller would be willing to extend the 30 day warranty for another week to potentially save on return costs?
Also, there may be a note in the manual on how long these take to stabilize after being powered down.

[acbern]

I have had items that would just not be stable, and remaind that way. So in your case, I would just return it (unless turning th epot cahnges things substantially). Its not worth keeping one that is that unstable, in the end you need to rely on it, and how can you, even if it gets a little more stable. And if you try to repair it, you are not sure you can and in addition, the option to rertur it has gone.
These are very stable units, well below 1ppm/year drift usually, otherwise they are broken. Cal labs tell me they will keep their 732As instead of buying later and newer ones, because of their stability. And never turn the pots once stable.

[Awesome14]

I got another month to continue testing (and uptime). The unit has improved and is--near as I can test--somewhere near factory spec.. After reading the replies, and gauging by the other 732A, I'm inclined to send the unit back. The other 732A drifted roughly 0.5ppm in 18 months. Thank you for your input.