HP 403B Restoration

..well, that would be a bit of braggadocio on my part because my original eBay unit was in immaculate condition. It just had some problems, that's all. And to say I solved the problems would make me a lying sack of crap. So I will forthwith present videos, pictures, and excerpts of the incredible help received form some fine hams who helped me nurse this thing back to health. Firstly, a couple of "silent videos" I posted on You Tube showing the problems with the unit -- and why I dubbed this "Shimmy Shimmy Shake II".

Here's part I, comparing it to the HP 3400A measuring the same voltage. Note that after the meter "warms up" (this "mambo" is typical of HP AC voltmeters), the 403B continues to wiggle.

Here is part II and the wiggling continues..

The three main concerns about this meter were the wiggling, the wild jumping when the range was switched, and the almost-moribund battery pack. I initially went out to the Yahoo HP group and got the following advice from Chuck Harris, a regular there (emphasis added):

Hi,
Your 403B should achieve a reading in a fraction of a second, and should be rock stable. It should never glitch, jump or waver.
So, you have a few of possible causes:

1) your voltage source is intermittent.
2) your meter is broken.
3) there is a transmitter that is occasionally transmitting nearby.

The 403 is very simple, and doesn't have a lot of possible fault areas. Knowing HP equipment from that vintage quite well, I would say that its problems are likely one of two things:

1) bad electrolytic capacitors
2) dirty switch contacts.

As I recall, the 403 is littered with Sprague TE style axial leaded electrolytic capacitors. They are marked either TE, or The most notable characteristic is the body is bright aluminum color, covered with a sleeve of clear heat shrink tubing. I have found a large number of each style that have gone open circuit...the rubber end seal becomes brittle, and allows the electrolyte to dry up. They can be replaced with any high quality modern axial lead electrolytic. I prefer to use Nichicon, or Panasonic 105C rated capacitors. Pick a long life variety. ESR is not important. Mouser and DigiKey catalogs are littered with suitable replacements. The exact values are not critical (or they wouldn't be aluminum electrolytic capacitors). There exists a tantalum style capacitor in this same appearing case, that has a glass ceramic end piece, that is very reliable, and should be ignored...I don't think there are any in the 403.
Dirty switch contacts should get a drop of Caig DeOxit on the metal contact area, and the switch should be worked around. DO NOT HOSE THE SWITCH DOWN WITH CONTACT CLEANER!
Dirty pots should get a drop of Caig Slider-lube on their contact and element area.
~Chuck Harris

Following Chuck's advice, I checked the ESR of all the caps and the only one found wanting was the huge C20 cap which I replaced with an almost identical value orange Sprague as shown below:

Still the wobbling needle persisted so I poked around and noticed that C7 was not stock (had been changed) and replaced that with an original cap that tested good for ESR. For reference, I excerpted some pages from the manual (available all over the internet) and made a checklist of caps should you want to do a restoration. Also, here's a couple of pages of schematics with all the little buggers highlighted.

"Click to Embiggen"

"Click to embiggen"

"Click to embiggen"

Then I turned my attention to Chuck Harris's (and other's) advice to apply some cleaner to the range switch contacts -- which helped immeasurably. But the needle still had the shakes so I thought a total re-cap job was necessary.

As I prepared for that, for some strange reason I thought I would  first proceed with rebuilding the battery pack. Dunno why. Anyway, I'll lave that as the subject of my next post and drop some pictures in here of the 403B's battery stock battery compartment for reference.

-73-