Originally posted in December, 2018

This post continues my restoration of the Hickok 209A VTVM, SN 1333254, Part 1

Recall that the D-cell battery holder was badly corroded. Here is  process I used to restore the battery holder:

• • Desolder the two terminal wires
  • Remove the holder from the VTVM
  • Completely disassemble the holder
  • Separate out all the metal pieces
  • Remove the loose corrosion from the metal pieces using a brass brush
  • Degrease the metal pieces
  • Soak the metal pieces in Evapo-Rust until the rust was gone
  • Treat the copper contacts with household vinegar and a brass brush to remove the corrosion product
  • Remove the corrosion product from the insulator plate (phenolic?)

Once all the corrosion product was removed and neutralized I decided to prime and paint the battery clamp parts and clear coat the insulator base and spring clips (excluding the battery contact areas). I used rust primer and rust paint for the clamp and hardware and Nyalic for the insulator and battery clips. Nyalic is a clear coat product which adheres to nearly any metal or plastic and is extremely resistant to UV, acids, salts and chlorides.

The battery clamps were originally lined with cloth reinforced rubber, which I replicated using EPDM sheet material. I had EPDM sheet on hand, and it is very resistant to zinc chloride and potassium hydroxide which are common battery electrolytes. I intend to use batteries in this device.

The battery holder was restored and ready to re-install.

This piece of test gear is all about the large, beautiful meter movement, so I needed to determine if this one worked properly. I connected the meter to a precision power supply and good ammeter through a current limiting resistor in order to determine how the meter responded. The meter is marked as a 500uA full scale movement.

Here is the test setup:

A 10k ohm limiting resistor was connected in series with the power supply. The power supply was a Tektronix PS5004 and the meter was a Tektronix DM5120.

Full Scale Deflection on the meter was about 497 uA:

I recorded the meter current at intervals of 1/6 scale deflection:

The additional columns in the spreadsheet are the Ideal Current which is the current through a perfect 500uA movement at a given deflection. And the Real Current, which is the current that should flow through the meter, based on the measured FSD current.

I plotted the data in Excel as follows:

Plot of Measured, Ideal, and Real current of the Hickok meter movement

Although the resulting measured current plot is very close to being a straight line (Trendline R2 = 0.9994), I am not sure what that means in terms of the meter linearity once it is installed into the meter circuit. However I have verified that the meter movement is smooth, readings are repeatable, and it appears to be reasonably linear.

The restoration of the Hickok 209A VTVM will be continued in Part 3.

Part 1

Part 3