HP-3478A multimeter maintenance

The HP-3478A 5-1/2 digit multimeter in all its '80s glory

Another auction score that needed some attention (don't they all?)... This time it's the venerable 5-1/2 digits benchtop multimeter from Hewlett-Packard, straight from the '80s (but not from Compton). This particular unit was 'only' one year out of calibration and thus was properly maintained. Until now... ;-)

While the device was fully functional it's always worth having a look inside, even if it means breaking some calibration seals. You never know which capacitor is ready to burst into flames...

Inside the HP3478A. This is an early version with ferrites being used for communications between the digital and analog sections (colorful cables just under the power transformer)

With nothing suspicious from the top side, let's have a look at the bottom of the PCB. It's usually easy to spot where repairs have been made from the solder side, and this device is no exception: can you spot the previous repairs?

U461 (upper left) and BT701 (lower right) have flux residue...

This device is no exception: three components have been changed, all visible thanks (?!) to flux residue on the board. First is U461 which is the voltage reference. Not so nice to see flux residue there, as this could contribute to leakage. Next is the battery BT701. Nice to see it was replaced recently, as replacing those little buggers can be a real pain. Hint: just desoldering and replacing won't work: in fact you'll completely mess your multimeter because the battery is used to keep the calibration parameters in RAM. Special procedures are needed for this job: backup battery in parallel (diode-OR'ed), power on at all times, and just to be safe backing up the RAM through HP-IB. But that's for another day... The last replaced component is a relay, not sure why but it's not a big deal.

A replaced relay, and no flux cleaning again...

The flux issues are minor and overall the meter is in great condition, especially for its age. Still, I can't give 100% kudos to NEC Management Partner Ltd for their maintenance and calibration work...

Next time please remove the flux residues!

Everything looks nice, but is everything NOICE? Well, let's check some caps shall we... The electrolytics are fine, but sure enough the mains filter caps are from the most evil type: RIFA! Those are prone to burn, explode, and generally ruin your day. Check this EEVBlog video for more details on those little buggers. Needless to say they have to go.

I spy with my little eye... Gaaa! RIFA caps!

There's a total of 4 caps: two near the HPIB connector and two in the mains filter. The replacement parts from Digikey are here and there. Getting to the mains filter caps is not easy so I recommend disassembling the mains switch to get better access:

A couple of screws and the mains switch is loose.

Once the caps are replaced things look much better and much safer! (just kidding, it looks exactly the same, but it is safer)

 
Fresh caps to the rescue...

Just to give you an idea of how bad those RIFA caps can be, check the pics below. It's actually pretty lucky that they didn't blow up earlier: they are cracked to the point of exposing the capacitor foil inside. And the measured value is not exactly close to the one on the label either. Whew, nice to see those gone for good...

 
 
Oh the horrors!