[R-390] What gives here!?

[n4buq at knology.net]

What got me started on this was the fact I wasn't getting the -0.4V on 
V506, pin 2.  Resistance checked around 400K instead of the listed 500K; 
however, looking at the schematics revealed that there is about 1.7M in 
parallel with that pin via the RF deck.  If pin E on the RF deck did indeed 
have INF resistance, then the 500K value would be correct; however, it is 
not INF and the 1.8M it really measures pulls that 500K reading down (as it 
should).

Perhaps these resistance values were checked on the RF deck with everything 
disconnected, but I doubt it as there are references to different readings 
on some pins depending the setting of certain other controls that are not 
part of the IF deck which means it would have to have been connected to the 
harness for at least some of these readings.

I'm just glad I found the problem, albeit I stumbled on it.  At the very 
minimun, I should have checked all tubes for grid emmission, etc.  I chose 
5814's from the calibrator as my first choice for subs, but those were 
apparently a little weak as well.  Since I swapped with another IF deck 
5814, I probably just moved a problem to a different part of the radio.  
Looks like I may be looking for some good tubes as I don't think I have 
many (if any) replacement 5814's.

Thanks!

Barry - N4BUQ
 

> Might be as simple as an outright error.  That was the initial motivation 
> behind doing the Y2K book - errors in the originals.
> 
> One thing that seems to cause inconsistencies in resistance charts is the 
> use of unused tube or connector terminals as convenient tie-points.  In 
> addition, because these were arbitrary, they might vary from one batch of 
> radios to the next.
> 
> Then there's the question of how the resistance and voltage charts were 
> compiled -- from a careful read of the (usually early) schematic? -- or 
live 
> readings -- from a prototype?  Either way, there can be errors.
> 
> Barry
> 
> ----- Original Message ----- 
> From: "Barry" <n4buq at knology.net>
> To: "R-390-List" <R-390 at mailman.qth.net>
> Sent: Tuesday, March 07, 2006 10:14 PM
> Subject: [R-390] What gives here!?
> 
> 
> > Paragraph 56 (page 96) of TM 11-856A shows various table for the 
> > resistance
> > values for each pin on the various subchassis connectors.  Table "a" is 
> > for
> > the RF Subchassis and shows the resistance for pin "E" to ground to be
> > Inf(inite); however, if I trace the schematic, there are two resistors, 
> > R201
> > (270K) and R234 (1.5M) in series to GND.  I'm getting about 1.8M so my
> > reading agrees with the schematic.
> >
> > I wonder where they came up with "Inf" for this pin??
> >
> > Barry - N4BUQ
> >
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