Originally posted in December, 2018

This post continues my restoration of the Hickok 209A VTVM, SN 1333254, Part 2

Hickok 209A VTVM Restoration Part 6: Component replacement decisions

In this Part 6 of the series I will document the condition of the passive circuit components in the Hickok 209A VTVM, and justify why I decided to replace (or not replace) them.

Starting with the resistors, I measured 8 initially, and of those 5 were out of marked specification on the high side. One more was right at the high side specification and two were within specification. These were all 5% and 10% tolerance carbon composition resistors. Based on these data, I made the decision to replace all the 5% and 10% carbon composition resistors. I used 1% metal oxide resistors with equal or (where space allowed) greater power rating than the one replaced. I did not see any carbon composition resistors which had visible signs of overheating. I did not intend to replace any of the precision resistors on the range switch nor any of the wire wound resistors. It is significant, and confirms the accepted wisdom on the subject, that the carbon composition resistors which were near or out of specification were so on the high side.

Considering the capacitors, all of the ones I measured initially were within specification. However these were all mica capacitors save one which was a paper capacitor. None was leaky when tested at its rated working voltage. That said, in equipment of this age, I normally replace paper capacitors as a matter of course. And two of the original mica capacitors were in the bridge circuit, so I decided to replace those with modern, closely matched silver mica capacitors. A mica capacitor was also used in what we now call the line to ground safety position (see below), so I decided to replace it with a proper X1/Y2 Safety Capacitor. I used my HP4253A LCR meter to check the capacitor values and my Sencore LC53 to check leakage current.

In summary then, I intend to replace all the carbon composition resistors and all the capacitors in the unit.

I have completed the electrical restoration of one Hickok 209A VTVM. This included:

• Replacing all the paper and electrolytic capacitors
• Replacing all the carbon composition resistors (most were out of tolerance)
• Installing a polarized AC plug, line fuse and proper safety capacitor
• Replacing the meter movement (original was mechanically damaged)
• Rebuilding and restoring the battery holder
• Balancing the bridge tube resistors
• Giving the unit a full check on all modes and ranges

With all of the above complete I can declare the unit ready to use. The cabinet still needs to be cleaned and refinished, but that will have to wait until warmer weather arrives and I can work outside.

The results of my testing were very interesting. I used my lab equipment (see below) to check the 209A on all of its ranges and modes (with the exception of the decibel range). The testing revealed that the 209A is a 5% accuracy instrument in most modes and ranges and perfectly usable. Resistance, +vdc,-vdc, vac (rms >3v) and zero center vdc are all 5% or better accuracy when compared with modern equipment. DC current, AC peak to peak voltage and capacitance modes are much less accurate, and I probably would not use them.

Where the 209A is most accurate and useful to me is in the zero-center vdc mode where my tests showed it to be within +-3% of my Keysight U1253B from 1.5 to 600 vdc. This, combined with the huge and easy to read meter will be very useful in aligning tube radios.

Equipment used:

• Keysight U1253B DMM
• Keithley 245 HV DC Supply
• HP 3245A Universal Source
• HP 4263A LCR Meter
• GR Resistance Decade Boxes
• GR Capacitance Decade Box
• Type P40106 High Resistance Decade Box
• Siglent SPD3303S DC Power Supply

 What remains to be done:

• Clean and repaint the case
• Fit a new carrying handle
• Investigate why the DC current ranges are not very accurate while I know the FSD on the meter is within 2% of specification