Fixing an HP3455A multimeter

[(*steve*)]

Some of you may be aware of my huge stash of test equipment bought for a song and transported half way around the world at great expense...

Well, among that stash is a couple of HP3455A multimeters. Because I had 2 of them I decided to take one down to my local hackerspace to equip them with a 6 1/2 digit meter.

After switching the power supply to local voltage it all worked fine.

Pulling out the second one I was distressed to find written on it "parts unit". :-(

Powering it up revealed that it didn't work.

Opening it up, it all seemed to be there (no major parts missing) and the was an additional note written on a large shield: "5V not present. p.s. capacitor A10C65 12V 4200uf missing".

At this point I didn't really have time to try anything, but before going to sleep I located the service manual. I will admit to getting out of bed, then chastising myself and going back to bed

The next day powered it up, and sure enough the 5V rail measured 3.8V, and the input to the 5V regulator was on the very low side of the acceptable range of voltages. Removing the regulator from the circuit improved things slightly, but the voltage still read well below the upper limit.

My thoughts were that maybe something was pulling down the unregulated rail, but something didn't seem right. Looking at the circuit again, I realised that the missing cap was the main filter cap for this rail!

Adding the cap brought the DC voltage back up to the expected maximum, and plugging in the regulator gave me a solid 5V rail.

And it seemed to be working

Further testing asked it to be well calibrated and far more stable than the other unit.

So it's really weird. Why was the cap missing? Surely it must have happened before the unit failed? Is it possible that it was purchased as a parts unit? Seems a very expensive way of doing things.

 

[HellasTechn]

Probably some ignorant person was bored enough to go buy a new cap and decided to terminate a well working unit.

I would also check the same cap on the other unit also just out of curiosity.

 

[shrtrnd]

This is the way my company does it:
Some production-line will shut down. The test instruments involved will be 'surplused', meaning anybody else anywhere in the company can ask for any of the instruments if any of them are something they also use. Anything
not claimed in-house, gets sent to a liquidator.
The up-and-running production-lines also occasionally will shut-down one line of production, usually keeping the
un-used instruments for their other operational product lines. If I need a part, I can try to buy a new one, OR, I
can go into those unused instruments and salvage parts out of them. Depending on company rules, I MAY be able to just replaced the broken unit with one of the working ones from the shut-down production line; or I may be required
to salvage parts from the instruments from the shut-down production line. Why? Because my company owns some
of the instruments used in production, and some other company or the gov't may own other instruments, and there
are rules about 'sharing' the instruments or parts. I don't make the rules, but you can bet your life I gotta follow them.
Some test equipment cannot be salvaged (rules), some can be 'kept for parts' units, and some can have their ownership 'transferred' (more rules).
You got, by one rule or another, an instrument that was 'kept for parts', which was apparently originally working, but
could not be placed back into service. Somebody like me salvaged the shield and the cap to repair another instrument, and when we ran out of room in our 'kept for parts' salvage yard, surplused it to make room for other
incoming production-line-shut-down instruments.
Just buying the shield and cap is a bugaboo all buy itself. There are rules for purchasing parts too. If somebody has a production-line shut down and it's critical to get it back up running again ASAP,
we have to follow the rules to do whatever we have to do to put the instrument back into service ASAP.
It doesn't makes sense sometimes, but the rules is the rules.

 

[HellasTechn]

Ok. I think i understand.

 

[shrtrnd]

Hellas Techn, YOU DO?????
Please explain it to me. What they make me do at my job has NEVER made any sense to me.
They tell me I'm just a tech, and management sees the 'big picture', where everything is clear to them(?)

 

[hevans1944]

but the rules is the rules.

Or, as they say in the McChicken business: parts is parts.

 

[(*steve*)]

I'm just really happy that the are people out there who are obviously willing to sell working test equipment at scrap prices.

One piece of equipment was used by keithley as a sales tool. It has clearly been opened up many times, and was a bit bumped up, but has almost pristine controls, terminals, etc.

 

[shrtrnd]

I'm sure management at the companies that helped supply your needs, were happy to rid themselves of their
surplus items. The 'big picture' after all for management, is NEW sales.