[R-390] R-390 C101 and C103 Caps

[2002tii]

Tisha wrote:

>I am a big fan of VTVM meters for high Z measurements where the 
>loading on a circuit by a DVM really throws off a reading

I hesitate to say "all" without checking, but I believe all of my 
DVMs have 10 or 11 Megohm inputs (VTVMs are generally 10 Mohms).  I 
know the ones I use regularly do.

> From an aesthetic standpoint a tube meter with a mechanical 
> movement is what the R-390 was originally designed to work with 
> back in the 50's and 60's.

I'm not sure the radios were really "designed to work with" 
d'Arsonval VTVMs, so much as those were what was available at the 
time.  Don't forget, there were also non-buffered VOMs in use, 
too.  Indeed, most field-level maintenance on R390s was almost 
certainly carried out by radiomen with VOMs.  Sometimes one finds a 
designated test point that the manual notes must be measured with a 
high-impedance meter to avoid loading error, or a note regarding what 
kind of meter was used to obtain the listed voltages, but often it 
was left to the tech to know.

Note that there were two basic designs of VTVM.  Both used a 10 Mohm 
or so divider string on the input with the selected tap driving one 
grid of a twin triode, the other grid of which was grounded.  In the 
preferred embodiment, the tubes were deployed as cathode followers, 
with the meter between the cathodes (with appropriate scaling and 
zeroing circuitry).  A schematic of a typical cathode-follower VTVM 
can be found 
here:  http://www.mcmlv.org/Archive/TestEquipment/Heath_IM-18.gif. 
(Note 7 indicates that the indicated voltages were measured "with a 
VTVM or 20,000 ohms/volt meter.")

Other designs (the RCA Senior Volt-Ohmyst comes to mind) put the 
meter in the plate circuits.  See 
http://bama.edebris.com/download/rca/wv98c/rca_wv-98c.pdf.  This 
doesn't do any useful amplification of the measurement input, but it 
does amplify the DC errors in the tube circuitry 
itself.  Consequently, range and zero drift all over the place on 
these meters.  If you have a plate-circuit VTVM, you can cure this by 
converting it to cathode-follower operation.  The same is true of the 
analogous FET-VOMs.

Best regards,

Don