[Collins] 75A-4 Resistor Identification HELP!

[Ian Webb]

And if you don't want to mess around on a live receiver if you have even 
the cheapest of digital ohmmeters or at least a relatively decent analog 
one, just put one lead on the end of the resistor that connects to the 
wire and probe all possible destination wires and see if you can 
identify one with a few tenths of an ohm of resistance.  I'd suspect 
you'll find out where those wires go without much trouble.

I've often wondered if my phone line tester that generates a tone would 
work in this kind of situation and have never tried it.  Someday I will 
but it's usually easy to use the old familiar tools.  I suppose I could 
run the amplifier with probe along the wire bundle and see if you can 
follow the strongest signal but I'd suspect there's so much coupling in 
a confined chassis that it would be mighty confusing.

Ian, K6SDE

Gerald Johnson wrote:

>That white red/blue wire should be fairly unique to its circuit. Though there could be several in the radio. Collins used white wires with one or two stripes of all RETMA colors allowing some 99 unique wires if they only used two stripes, 999 if they used three stripes which they sometimes did.
>
>If you haven't replaced all the black beauty capacitors (if there were any) they are all suspect in having leakage to over heat the 2.2K B+ isolating resistors. Its possible that some disc ceramic bypass capacitor has gone leaky though its far less likely. The oiled paper black beauties are guaranteed leaky at their age and should all be replaced with mylar or polyester capacitors such as Sprague Orange Drops. They don't look original but they will probably work for the next 50 years with no problems, I don't have test data out to 100 years yet.
>
>There could be another leaky old electrolytic or a shorted cathode resistor bypass causing a tube to draw excess current, or an IF or RF tube with grid emission driving the AGC line positive and causing several tubes to draw excess current.
>
>One way to identify the wire is to check the voltage at the low voltage side of the resistor and look for that same voltage on all other wires in the radio of the same color. To make that voltage hold steady, you might bridge the 2.2K with a 1K 1 watt that won't go up in smoke so fast. Or a metal film resistor that may not go up in smoke at all, though it might damage things around it from the radiated heat.
>
>Another way to check with the radio power off is to look for the white red/blue wire that has zero resistance to the terminal where that wire connects to the 2.2K resistor. Then unhook it from the 2.2K resistor and check for leakage to ground.
>
>You should be able to slide the wire gently in the harness to find where it comes out. That's another tracing technique.
>
>
>73, Jerry, K0CQ, Technical Advisor to the CRA
>  
>