[R-390] HP410B rectifier tubes

[n4buq at knology.net]

Many of you own the HP410B VTVM and use them on your R39* series radios.  I 
have one and some time ago, I noticed the AC probe stopped working.  
Normally, after an hour or so, the probe will become warm due to the 
rectifier tube's filament and mine had stopped warming up.

Figuring the diode in the AC probe had died, I recently started looking for 
a replacement diode.  As many of you also know, these tubes are becoming 
quite expensive.  Knowing this, I decided to check everything out as the 
tube itself may not really be the problem.

Some of the meters use a 2-01C diode, some use an EA53, and others may have 
a probe where either type can be used (there are supposedly slight physical 
differences between the two tubes making them not completely 
interchangeable).  Mine has an EA53 and I checked the specs for it.  I 
noticed that the 2-01C uses a 5.0VAC filament while the EA53 runs on 
6.3VAC.  Checking in my manual, it states to set the filament voltage to 
5.0VAC and that was what I did when I went through it a few years ago.

A little reading on the internet reveals that if your meter uses an EA53, 
you should set the heater voltage to 6.3VAC, not 5.0VAC and some manuals 
may even make mention of this (mine does not).

There is a variable resistor used with a ballast tube to set the heater 
voltage.  I set the heater voltage at the probe to 6.3VAC and reinserted 
the EA53.  I let it warm up for about an hour and, sure enough, the probe 
began to warm up again.  It is again working in the AC modes; however, it 
reads quite low (about 30VAC where it should be seeing 120VAC).  I think 
this is just a calibration thing and I can fix that.

I just thought I'd pass this along in case others may have experienced non-
working AC probes and not realizing it may just be an incorrect heater 
voltage setting.  Check the diode in your probe first, though!

Barry - N4BUQ