The General Radio 1806-A Electronic Voltmeter was one of the finest vacuum tube voltmeters ever produced. It had a minimum 100 Meg input resistance, could make DC measurements from 0.005 V to 1500 V, AC measurements from 0.1 V to 1500 V at several GHz, resistance measurements from 0.2 ohms to 1 gigohm....It had a retractable AC probe and a large, easy to read meter. Today, it has a cult-like following and examples sell on eBay for exorbitant prices. Anyway, this is how I repaired one.
It's here!
The problem with this meter was that the 6.3V regulated filament supply was at 9.5V, rendering the instrument inoperable. So, who were the culprits? Well, it turned out that in that supply, Q501 (2N1544), Q502 (2N1304), and CR506 (1N750) were all bad. I replaced Q502 and Q503 with a matched pair of 2N1304s. While I was in there, I also replaced the B+ bypass capacitor, a 0.22 uF unit. To do all of this, I had to take the face of the meter off which was not exactly a fun time.
Performing an autopsy of the instrument
It's dead, Jim
Repaired filament supply calibrated
While I was testing the meter, the B+ filter capacitor let out a cloud of smoke and blew one of the fuses, so I replaced both of those as well.
Cremated Capacitor
A quick calibration check shows that everything is how it should be.
Resistance check
DC check
All buttoned up and ready to go
