Hewlett Packard 400C VTVM, Replacment Capacitor Locations Critical?

[Mr. Coffee]

Hello, all,
I'm trying to refurbish an HP 400C, 20Hz - 2MHz AC volt/millivolt meter--circa 1950.
I plan on replacing all of the old waxy capacitors and the big ones in the metal cans, too, because I think they are too old. I was thinking about leaving the cans on the top of the metal chassis in place and just disconnecting them. I want to put the new electrolytic capacitors under the old cans on the bottom side of the chassis (maybe on terminal strips).

Would that work or would they pick up hum voltages and mess everything up? The cans that are in there now are grounded to the chassis.

Thanks!
Rob

[Vgkid]

Are you planning on putting the new caps inside the cases of the old metal canned caps?  People do that in old audio restorations very often.

[Mr. Coffee]

No, I want to put them underneath, but not in the cans. Some of the cans are 4 section caps, and I don't think I can fit 4 axial caps in them, even though the modern ones are smaller for their ratings.

[N2IXK]

The locations of those caps is not critical. They are just power supply filters and such.

As was mentioned above, it is quite common to install the new caps inside the empty shell of the old one, if you want to retain the original appearance. If you want to go this route, plenty of info available here:

http://antiqueradio.org/recap.htm

[Mr. Coffee]

Thanks for the info. The originality will probably have to be outside the cabinet. I want to replace the huge selenium(?) recifier with a silicon bridge ditto, too. Some of the new yellow caps that replace the old ones will be visible with the chassis out. It would be cool to make it look totally retro inside and out if I had the time and was clever with it.

Still, it will be 90% original inside and all outside. I want it to work so I can use it to measure radio frequency voltages in parts of old radios and maybe low audio voltages in vintage audio equipment. I like fixing them up and sometimes I have to find where a signal gets lost. I think this will be an easy way to do that.   

[Mr. Coffee]

 
All done...this instrument is dead nuts accurate and flat responding! Thanks again for the hints.

I replaced all the  paper caps and 'lytics. One 6AK5 tube was noisy so I put in a new one. There were several out of spec resistors replaced, too. The 2W, 25K B+ potentiometer was frozen and I had to replace that. Other than all that and much cleaning, a new handle and power cord, I just set the regulated B+ and DC filament voltages and went through the calibration procedure.

I also took out the big, finned (selenium, I guess) filament power rectifier and put in silicon bridge and a 2 Ohm, 20W dropping resistor between the bridge and the filter.

This will be a welcome addition to the family.

Cheers,
Rob

[GreyWoolfe]

Two thumbs up!!  Good job on the repair.

[N2IXK]

Nice!  Great to see that old gear still running....

Good call on getting rid of the selenium.  If one of those fails in operation, the stench can linger in your lab for days. Fairly toxic, as well...

[Vgkid]

Good job on your fix, in that locationyou would have to turn the unit over to notice some of them. 

[Mr. Coffee]

Thanks fellas! I'm going to use it for a working meter for some audio and medium wave RF work.
I was glad to be able to keep it mostly original..especially that cool 104C regulator tube in the second photo.

[N2IXK]

I would set a search on eBay for a spare for that 10-4C tube if you plan on keeping this meter for a long time. Those Amperite regulators are very hard to find nowadays.
The tube consists of a simple iron wire filament used as a ballast resistor, likely to regulate current through other tube filaments to enhance stability with line voltage variations.

BTW, those tubes have a somewhat hazardous failure mode if they happen to get a crack in them while operating.  To increase heat dissipation, the internal atmosphere of many types is not a vacuum, but hydrogen! If a small crack develops, air leaks in until the H2/O2 ratio hits the explosive range, then * BOOM *. 

[Mr. Coffee]

That's a good idea...I did find some and the prices are pretty high. Yes, it is used to regulate filament current. It was an upgrade from the previous serial number series, I believe. This is a late serial number and has all the bells and whistles. The reg tube is not essential, but enhances accuracy in case of varying line voltage.

If it poops out, I may buy a NOS tube or leave it in for looks, (it makes no visible glow in this application, anyway) and hide a modern regulator under the chassis with the other new parts.