[Milsurplus] 2X2 question

[jmfranke]

Jack,

The manual, AN 16-40SCR718-3,  for the SCR-718 radio altimeter has a warning 
on page 5-13 concerning some 2X2 rectifiers.

"Some JAN-2X2 tubes made by the National Union Company will become gassy 
(indicated by a bluish glow) after several minutes of operation.  Do not use 
these tubes as they will shortly cause the high-voltage power transformer to 
fail.  National Union JAN-2X2 tubes made in Canada (indicated by a maple 
leaf marking) have been found free of this trouble)."

I believe the power transformer fails due to excessive current.

The 2X2 was in the display indicator.

Good luck with your project which sounds like ARQ-5 receivers.  I have two 
and was thinking of getting them operational.

John WA4WDL


----- Original Message ----- 
From: "Jack Antonio" <scr287 at sbcglobal.net>
To: <boatandhors at mailman.qth.net>; <milsurplus at mailman.qth.net>
Sent: Friday, April 07, 2006 1:36 AM
Subject: [Milsurplus] 2X2 question


> Hi all
>
> Recently I've been working on a WWII countermeasures
> receiver, that uses a 5" scope for display, sort of
> an early "spectrum analyzer". And it was starting
> to work, the scope had a nice bright sharp trace,
> and the signal circuits were working, but needed
> a little troubleshooting to solve a calibration
> problem.
>
> High voltage for the scope tube is generated by about
> 1250 VAC delivered to the cathode of a 2X2 rectifier,
> about -1500 taken from the anode.
>
> After a disassembly/re-assembly trying to troubleshoot
> the IF, when I fired it up, the 2X2 glowed bright blue and
> the line fuse instantly blew. Of course I thought it was
> something I did in the re-assembly, and spent a couple
> of days getting nowhere. Then I pulled the 2X2 plate cap and
> applied -1100 VDC from an external supply  to the circuit with no 
> problems(thinking a capacitor was breaking down).
>
> Then I put the -1100 on the plate cap of the tube, and
> the tube drew lots of current, and glowed a nice blue.
> (No filament voltage)
>
> I pulled the tube and put a variable DC supply across the
> tube, reverse biased, and the tube breaks down at about 290 volts,
> at that voltage it has a nice blue glow inside, the higher the voltage,
> the more intense the glow. The manual says it should withstand 12500.
>
> My question is, anyone have an idea of what the failure mode is?
> (I have some replacement 2X2s coming).
>
> TIA
>
> Jack
>
> Jack Antonio WA7DIA
> scr287 at sbcglobal.net
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