Vintage Vacuum Tube equipment

A few months ago I bought a Eico Model 950 Resistance-Capactiance-Comparator Bridge off of eBay.


The seller claimed it was restored IE recapped. When I got it the power cord look a little scary looking so I was going to change it out to a newer polarized cable but quickly found out only one of Capacitors were changed out. 95% of these caps are original from 1964.

Thing is paper based and electrolytic caps dry out and break down. Even if they were never used. When this happens they turn into a short or into a resistor like state.

So now I have to start on the quest of finding some modern caps for this unit.
Vacuum Tube equipment uses high voltages so high voltage caps are needed. Also they’re typically low values and can be a pain since a lot of capacitor values listed are not common anymore so you have to round up or down to the next value. The hardest part honestly is finding Axial type caps. My best bet is to find Mylar or Orange drop caps for this project.

There is also a trap waiting for newbies as well. Depending how old the device you’re working on the Schematics or Service Manual will call Capacitors something else such as Condensers. You’ll also see MFD or MMFD instead of uF, nF and pF. MFD = uF and MMFD = pF. They will list nF values as uF so a 100nF cap would be 0.1uF or in this case 0.1MFD. You can easily obtain a conversion chart from here.

Above was a couple of weeks ago. Since then I have been able to replace most of the caps except a few I don’t have in stock.

Rectifier Tube and two old caps from 1964
The only cap replaced by the Seller was the 4uF cap.
Old cap hiding, needs replaced.
Filter cap that needs to be changed ASAP.

Now the thing that kinda scares me is the third picture of that old wax cap. It’s going from the AC power switch to Ground.

It’s a 10nF cap. I might replace it with a XY style cap. If I use a typical cap and if it was to fail it would cause a short, make the chassis live or a fire hazard.

The new line cord will be easy. Live would go to the rectifier tube and Neutral would go to the switch. I might add a terminal strip for the Line cord so I can put the proper XY safety cap across the line.

I am aslo thinking to lightly wet sand the front of the unit to clean it up. The lettering is engraved and paint filled so it shouldn’t hurt anything. The back side of the case has some rust spots on the bottom but I can easily fix that.


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