HP 3478s and 3468s

[jwm]

Through some fat fingering on ebay, I ended up with more of these than intended.

It looks like the 3468 is from 82, the second one down is from 83 and the bottom one is from 90 or so.

They all self test OK and seem reasonable, the only issue is that the 3478s have horrible measurement of 2-wire ohms. 3-4 ohms high, however in 4 wire mode they are spot on. Hmm... Perhaps the mechanical front/back terminal switch is cruddy? It would explain why the older 3468 with no back terminals seems to have better 2-wire sensing.

They all have calibration stickers so were part of a program at some point, but no papers, am I hurting the resale value at all if I break those calibration stickers to do something about the 3V ram batteries?

I was thinking of keeping one and then doing a supercap/battery mod to the others and selling them, do you think there is interest in supercap modded 3478a's on the resale market? By which I mean adding a 1F 5.5V supercap to the SRAM in addition to a socketed lithium cell, so you can just replace the battery without issue and use standard lithium coin cells.

Rigging up something with an IC clip to read the current RAM calibration values for hard storage (on a printed piece of paper paped inside the chasis if nothing else) is also something I'd like to do.

Something pretty cool I noticed about the 3468a is that it has a 'talk only' mode, where there is a dip switch you set on the back and it just spits out all its data on the HP-IL port on the back without waiting for any commands, which is a simple bit-bangable serial protocol. perhaps I can give it a nice VFD display via that route just displaying what it spits out and using the front panel for controls still. If only HP-IL cables didn't cost almost as much as the multimeters do i'd be experimenting with it by now....

    John

[alm]

The 3468A (and I think also the 3478A) can be calibrated in either 2-wire Ohms mode or 4-wire Ohms mode, not both, so the offset in 2-wire mode may very well be because it was calibrated in 4-wire mode. How do readings of a 100-300 Ohm resistor compare between 4-wire and 2-wire mode? Is it also 3-4 Ohms high in 2-wire mode?

I fixed a 3468A with an offset of about -0.3 Ohm (presumably because it was calibrated with a 0.3 Ohm 'short') in both 2-wire and 4-wire mode a couple of years ago. I used three 1 ft banana leads to make a 4-wire short as shown in the service manual and only calibrated the zero Ohm value on all resistance ranges; I skipped the other values. After this a 100 Ohm resistor measured in spec in both 2-wire and 4-wire mode (which failed before).

No idea what this would do to resale value: I probably wouldn't trust a random eBay seller to do this properly and not screw up the calibration.

[mmagin]

Yep, discovered this myself.  A real "trap for young players".  I have a 3478a which seems to be properly calibrated on DC volts.  However on ohms, it's a sizable fraction of an ohm off on two wire measurements on all ranges.

Then I discovered in the service manual, "4-79. The 3478A can be calibrated in either the 4-Wire or 2-Wire Ohms function.  It cannot be calibrated in both."  And sure enough, my 3478A seems to be correct in 4-Wire measurements.

[CustomEngineerer]

I would think the mod would hurt the resale value. Mainly because of what alm said. I'm pretty sure the kinds of people who would be interested in buying a HP3478 (or 3468) would be capable of doing the mod themselves, or just using a power supply to keep from loosing the calibration data.

Disclaimer: I do not own an HP3478, nor have I ever used one (that Im aware of), so may not know what I'm talking about.

[Neganur]

Doesn't hurt the resale value as long as you have the tools to cal it if you choose to mention that in the sale. If their only issue is the low ohm on 2-wire then I wouldn't worry too much. But if you just want to flog them again I wouldn't touch it.

Btw, everything is derived from the (DC) volts path, if that one drifted everything else will be crap too.