[R-390] Re: HP410B rectifier tubes

[Steve Hobensack]

   I had a similar problem with my ME-26D/U (a 410b clone) AC probe. The 
measured voltage was way off. It was a shorted capacitor just after the 
needle and in front of the 2-01c diode tube. A replacement cap can be 
fashioned from a disc ceramic cap and a dremmel tool. If you need that 
special cap, dremmel off the coating on a suitable capacitor plus the leads 
until you end up with a pill shaped ceramic cap. It was tricky to install. 
The cap is easily ruined by trying to measure over 300 vac.
73...Steve...N8YE

Date: Sun, 26 Mar 2006 21:42:33 -0600
From: "Barry" <n4buq at knology.net>
Subject: Re: [R-390] HP410B rectifier tubes
To: <r-390 at mailman.qth.net>
Message-ID: <001f01c65150$7b7a8b60$6401a8c0 at knology.net>
Content-Type: text/plain;	charset="iso-8859-1"

Did some more digging.? With the cover off, I can measure the filament
voltage for the EA53 while it is in circuit.? I had to crank the
resistor
quite a bit farther "up" to get 6.3VAC across the filament.? This did
cause
the probe to get really warm like it used to; however, the probe still
doesn't check AC properly (low reading).

I did some checking in the manual.? It states to check the resistance
"below" the ballast tube with the ballast tube removed from the circuit.
This should produce 4 to 6 ohms.? If it doesn't, most likely something
has
been "shorted" (I assume it means something has damaged the rectifier).
Mine checks a little over 2 ohms, regardless of the setting of the
variable
resistor.? Looks like I probably do have a damaged EA53.? I checked the
rest
of the wiring and some of the test point voltages and nothing was
particularly too badly awry.

Looks like I'm back to looking for an EA53 :-(

Brry - N4BUQ