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

[Barry]

Hi Jacques,

Thanks so much for the tip about measuring between pins 5 and 6.  I hadn't thought of doing that.  Disregarding an ohm or so for the primary of T701 it's still around +10% from nominal.  It's one of the resistors that I've been considering of replacing in the PTO but I doubt it will make a great amount of difference.  R701 (68k control grid) is also a bit high so I may go ahead with the work on the PTO I'd planned.  I haven't checked between pin 5 and P715-A but plan to do that as well.

This morning I decided to check the transconductance of the tube I originally had in for V701 and it was on the low side.  I found a 5749 in my spares drawer that was significantly higher and putting it in place, increased the voltage at E210 to inside of the tolerance values in the manual.  It's still a bit on the low side but at least inside the tolerance.

Thanks again,
Barry - N4BUQ

> 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
> 
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