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

[Roy Morgan]

On Jan 4, 2009, at 1:55 PM, 2002tii wrote:

> 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. ...  Don't forget, there were also  
> non-buffered VOMs in use, too.

<Roy on Soap Box - ON>

Tisha's being a fan of VTVM's may well be based in part on an aspect  
not mentioned yet in this thread:

DVM's of the modern sort have no resistor at the *end* of the test  
leads.  VTVM's of the kind common at the time of the R-390's did.  The  
non-isolated test  probes, whether having test prods or clip leads,  
can load the grid circuit of an R-390 or R-390A with respect to RF or  
IF signals there.  The mixer grids have the local oscillator signal  
present.  If that signal gets "sucked out" by the capacitance of the  
meter the results will be confusing at the least.  It isn't the  
buffering of the meter that matters when measuring oscillator or mixer  
grids, but the isolation provided to the circuit under test.   
Certainly the VOM's common in the time of the R-390A would cause  
trouble measuring some points in them.

<Roy on Soap Box - OFF>

Roy

Roy Morgan
k1lky at earthlink.net
529 Cobb St.
Groton NY, 13073