Hickok 209A VTVM Restoration Part 6
This is the sixth part of an “N” part series documenting my restoration of the Hickok 209A VTVM, SN 1333254.
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 replacees. 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.
Thanks for reading.