[R-390] R-390/URR - Resistance Checks

[Jacques Fortin]

Hi Barry,

Well, I wrote you before to be cautious about the information contained in
both the TM11-856 and the TM11-5820-357...
So many errors in there, coming from nobody knows where and never corrected,
like the tuning racks cams reference positions...

As your PTO is a 70H-2 1st version (no Z702 top adjustment) you should
measure 33K + 2.2K + the 180V supply line value to GND, whatever what that
last value is in your set.
"Readings with P715 disconnected" makes no sense to me as you should measure
infinity from both pins 5 and 6 of V701 to GND that way, whatever the actual
values of R702 and R703.
However, if you measure BETWEEN pins 5 and 6, there should be a value near
to 33K ohms (R702 value + the primary winding of Z702 which should be very
small)
And between V701 pin 5 and P715-A, that should be close to 2.2K ohms.

I do not know where the "40K" value came from: it is written as 44K in the
1st R-390 Collins manual (the 1953 one that pre-dates the TM11-856).
But even there, all depend on the value "seen" in the power supply line WRT
GND.

73, Jacques, VE2JFE in Montreal

I've been measuring the resistance values at the various pins at V701 and I
am wondering about the proper way to do this.  There's a note at the bottom
of Figure 82 and Figure 83 that states:

"RECEPTACLE RESISTANCE READINGS ARE TAKEN WITH PLUG DISCONNECTED."

Is this supposed to mean that if I'm measuring the resistance between the
tube pin and GND, that the plug for a given module (e.g. IF chassis, PTO
chassis, etc.) should be disconnected?  If so, I have always gotten values
that are very different for some/most of the pins that way.

Of particular interest is V701, Pin 6.  Figure 83 shows that to be 40k.
Without P715 inserted, that value is 0 on my PTO.  If I reinsert P715, then
I get 59k.  From what I can tell, an additional 19k is at J155A to GND which
(I believe) comes from R623 and R618 in the power supply.  If that's the
case, then the resistance at V701 Pin 6 should be R702 (33k) + R703 (2.2k) +
19k from the power supply which totals 54.2k so I don't know how Figure 83
shows that as 40k.

Is it possible some changes were made in the circuits that were not updated
in Figure 83?  If not, then I'm pretty puzzled.

Thanks,
Barry - N4BUQ