On Wed, 30 Jul 2003 17:21:51 -0700 (PDT), Dave Haupt
<emailw8nf@yahoo.com> wrote:
>I am not familiar with your model of VTVM. However,
>most VTVMs in that era had nearly identical
>schematics. The Heathkit IM-11, IM-18, IM-21 and
>IM-28 had a similar tube complement: a dual triode and
>dual diode.
_________________________________________________________
Would you like a little tip for improving your tube-type VTVM?
Back in my TV repair days in the '50s, '60s and '70s, I and everyone
in the shop used some kind of tube-type VTVM. One minor but constant
annoyance was the zero setting would constantly drift around,
necessitating constant re-zeroing.
One cure for this is to replace the 12AT7 or 12AX7 with a 6FQ7. The
pin connections are identical except for the filament, which is easily
rewired. The 6FQ7 is a much more stable tube, since it was designed
to be used as a horizontal oscillator in an RC type circuit where
changes in tube characteristics would cause changes in frequency.
Once the change is made minor recalibration is necessary, and then you
have a rock-solid VTVM.
--
73, Bill W7TI
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