Quick question about HP 3468A

[c4757p]

OK, I have a question about my "new" HP 3468A multimeter - this should be easy. If I leave it in DC V mode with probes attached but floating, it constantly switches between ranges: on the two lowest, very high-impedance ranges, the probe wires quickly pick up enough charge to bump it into the higher range. Then the low impedance at that range discharges them, and the cycle repeats. This doesn't happen with the probes disconnected because charge pickup is much slower.

Should I be worried about wearing out the little relay with this happening, and disconnect the probes when not in use, or is it not a big deal? How long do the relays typically last under heavy use?

[BravoV]

Same feeling here when I bought it for the 1st time, but after few years, I adapted not to worry about it too much, beside it survives throughout those long years.

But most of the times it becomes a habit that I always shorted both probes clipped together when not in used but powered on, don't know, maybe its just me, I feel like I'm going to have a mental breakdown to see the least significant digits keep changing all the time when not in used. 

Btw, have you replace it's RAM backup battery BT551 ?

Edit: Posted this infos at other thread, attaching them here again to save you from drilling into the service manual just in case you have not done it. 

[c4757p]

Thank you. Good to know it's not a problem.

Btw, have you replace it's RAM backup battery ?

Not yet. I just got it a couple days ago and haven't had a chance to open it. I definitely plan to, though.


I wish it had individual range buttons like the similar Fluke models from the time period - I'd like to be able to just stick it in the 30V range as soon as I turn it on. There's hardly a point to autorange when there is a single range that covers most voltages I ever encounter with 100 uV resolution. The little up/down keys are a pain in the ass, though.

[BravoV]

Btw, have you replace it's RAM backup battery ?

Not yet. I just got it a couple days ago and haven't had a chance to open it. I definitely plan to, though.

One important thing to remember, don't let the RAM ever disconnected to the power source when replacing the battery or you will lose the calibration. 

I wish it had individual range buttons like the similar Fluke models from the time period - I'd like to be able to just stick it in the 30V range as soon as I turn it on. There's hardly a point to autorange when there is a single range that covers most voltages I ever encounter with 100 uV resolution. The little up/down keys are a pain in the ass, though.

I guess we will have to live with this old design, have to admit features wise it is beaten to death compared even to today cheapo dmm, one big feature I missed is nulling. 

But personally I think the biggest advantage of buying "working" old quality bench dmm is they're proven through time, and the best part is it's internal voltage reference is so well cooked that the drift is so small that re-calibration is not that demanding, at least at my unit.

My 3468A probably more than 20 years old and now  about 3 years since I bought it, with with last cal stamp dated about 8 years ago and still very accurate, fyi last year I checked it against my friend's 34401A (annually cert-ed) and its still on spot according to it's spec. 

Pics taken last year when I did measure low ohm resistor, and the resistor had been verified using 34401A.

[robrenz]

I am not trying to be a wise guy but the 34401A 1 year accuracy on 0.010 ohm is +/-0.004 ohm or +/-40% of value.  The 3468 is only slightly less accurate at 0.010 ohm than the 34401A at +/-0.005 ohm or +/-50% of value.  The 3468 is still a very nice piece of equipment and is way better than  worst case spec since it agrees with the 1% resistor.

[alm]

Yes, but the 34401A is on a yearly cal cycle. This gives confidence that it will meet its 1yr spec. The 3468A could have drifted out of cal in the 8 years since its last cal.

I will usually set DMMs to a higher range or resistance when the leads are open. I find the relay chatter annoying and it will eventually wear down the relay, though probably not with typical hobbyist usage unless you leave it powered on 24/7.

[robrenz]

Yes, but the 34401A is on a yearly cal cycle. This gives confidence that it will meet its 1yr spec. The 3468A could have drifted out of cal in the 8 years since its last cal.

I completely agree.  My point was neither meter even calibrated can verify that 1% 0.01 ohm resistor is in spec.  Even if the resistor was only within 2% it would be 20 times more accurate than the worst case accuracy of the 34401A at 0.01 ohm.

[c4757p]

Btw, have you replace it's RAM backup battery BT551 ?

Update: The battery measures 3.04V - should I bother replacing it, or is it good for a while longer?

[SeanB]

When was it made? That is a better measure than voltage, they get a little dicy after 15 years. You never know when it will be EOL and drop voltage over a period of a few days.

[c4757p]

Ah, OK. Forget 15 years, the most recent date code I can find in this thing is 83 (none visible on the battery itself) and no signs of a replacement ever having been done. I guess I will change it.

[alm]

I concur. These batteries generally last well beyond their expiration date, but how much is anyone's guess. Like Sean writes, the battery might be good for another couple of years or another couple of days.

[SeanB]

Probably best. I have a chime clock from Maplin ( bought 20 odd years ago and I took all they had ( 4) as spares) which was using a huge Eternacrll as chime drive which I put in 2 years ago to replace the AA cells that only did a year. Last monthit died, but still shows over 3V, just an internal impedance in the kilohm range. I had placed a 1000uF capacitor in parallel with it and this helped with it lasting. Hard to date the cells, but I suspect they were made in the 90's, date coded 152/7BA.

[c4757p]

I'm trying to figure out what kind of battery to use as a replacement. The original one seems to be pretty much unobtainium now. Forgive me, I'm clueless about batteries. Can I just divide the capacity by the load current to get the expected life (bounded by the specified shelf life, of course)? And what is the load current like for a 1980s SRAM chip?

There are 950 mAh button cells available - could these really source 10 uA for 10 years? I like the idea of making a custom battery holder that holds two in parallel for painless replacement in the future.

[BravoV]

That is long life, low drain and with very flat discharge Lithium battery, the only known reliable manufacturers that I'm aware of are :

- Panasonic BR series -> http://www.panasonic.com/industrial/batteries-oem/oem/primary-coin-cylindrical/cylindrical-type-br.aspx
- Maxell ER series (Lithium Thionyl Chloride Battery) -> http://biz.maxell.com/en/product_primary/?pci=9&pn=pb0004
- Tadiran

Quote discharge curve from Maxell ER series, see how long it's life, but the voltage will drop like a stone once its time to die. 


Personally mine is on "emergency" state too  , and planning to buy one of these above.

[BravoV]

And what is the load current like for a 1980s SRAM chip?

That is not a SRAM, its an ancient 256x4 bit static cmos ram with part no 5101L (google it for it's datasheet), when in low voltage data retention/standby mode, the minimum Vcc is at +2 volt and consumes only 10uA@2 volt, while working the max Vcc = 7 volt, so even with 3.6 volt battery should be fine instead of 3 volt.

[EEVblog]

OK, I have a question about my "new" HP 3468A multimeter - this should be easy. If I leave it in DC V mode with probes attached but floating, it constantly switches between ranges: on the two lowest, very high-impedance ranges, the probe wires quickly pick up enough charge to bump it into the higher range. Then the low impedance at that range discharges them, and the cycle repeats. This doesn't happen with the probes disconnected because charge pickup is much slower.

Should I be worried about wearing out the little relay with this happening, and disconnect the probes when not in use, or is it not a big deal? How long do the relays typically last under heavy use?

That is a known wear issue on the HP3458A.

Dave.

[c4757p]

There - replaced the battery (Tadiran TL-5104/P). The original battery has an impedance around 120 Ohm, which seems very high to me. No problems at all - sure feels like delicate surgery, though, when there's no way in hell you can afford to have it calibrated if you screw up...

[BravoV]

There - replaced the battery (Tadiran TL-5104/P). The original battery has an impedance around 120 Ohm, which seems very high to me. No problems at all - sure feels like delicate surgery, though, when there's no way in hell you can afford to have it calibrated if you screw up...

Please share the detail process what you've done in that surgery, and appreciate if you could share some photos.

[c4757p]

Well, the process was simple in theory. I found the point where the two power sources are connected with diodes, and soldered in a third diode, then soldered wires to the diode and a nearby ground point. I used a power supply plugged into a UPS to provide power - I didn't have any batteries with terminals I could solder to, and I did have a UPS available. I slowly turned up the voltage to make sure nothing went wrong, and made sure the voltage at the diodes rose, so I knew it was powered by the supply. Kept an eye on the power supply's ammeter (with the current limit set to 10mA) and a voltmeter connected across that rail the whole time. Desoldered the battery from the PCB, then soldered in the Tadiran. Realized it didn't fit, so I swore profusely, then bent the leads around to make it fit. Then I disconnected the power supply, put the can back on, and realized that even though the battery looked like it fit, it was still blocking the damn can. Swore profusely again, this time in two languages  , because I had to very carefully bend the leads and tape over them to keep them from shorting to anything, and finally got it to fit. I put a spacer of hot glue under it and taped it down to the PCB. (Making sure all the glue and tape were only over the digital section - I don't know if the glue has any electrical effects that might interfere with measurements at high precision. Who knows what the dielectric constant of El Cheapo hot glue sticks is..) Then I put it all back together and started it up to verify I didn't ruin it.

No pictures because I'm not particularly proud of how it looks...  It's in there pretty securely, but looks pretty redneck. Just imagine a battery about 50% too long with its leads bent all over the place, taped down to a PCB...

[SeanB]

You have to do it at times, though I am a fan of double sided foam tape for those battery packs.

[BravoV]

Thanks, btw just curious on the battery replacement, did you deliberately buy an oversized one ? Also could you close the metal shield ?

[c4757p]

I measured to make sure it fit. It did fit, I just didn't really think about how hard it would be to fit it in properly and anchor it so it wouldn't move, and also fit whatever I used to do that. The shield closed fine after a few repositionings of the battery.

[BravoV]

The shield closed fine after a few repositionings of the battery.

I can imagine that must really tight fit, anyway glad to hear its done, and prolly you don't need to worry for the next 20 years or even more with that oversized battery. 

[c4757p]

Good, that's the plan. I do not like messing with backup batteries - I am happy only when the battery long outlasts the instrument itself...

[willb]

I just replaced my battery in my 3478A today. I wasn't sure on the age of the battery, so I did it to be on the safe side.

Here is the battery I used :

http://www.alliedelec.com/search/productdetail.aspx?SKU=70197006

It was the only place and only battery I found in stock that would work and ship to Canada. Anyhow, I powered up the meter and used a Weller Pyropen to solder the new battery. The whole process was painless. I soldered pieces of 22ga wire on the battery tabs and fixed the battery in place with some double stick tape and a zip tie. I turned the meter around once the battery was in place and soldered the wires to the PCB. 20 minutes later I had my meter back in service.