[Collins] 75A-4 Resistor Identification HELP!

[Gerald Johnson]

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

---------- Original Message ----------------------------------
From: "Roger Shultz" <byorslf@earthlink.net>
Reply-To: "Roger Shultz" <byorslf@earthlink.net>
Date:  Fri, 15 Nov 2002 08:43:43 -0500

>I am trying to help a friend on the west coast identify a problem which
>keeps burning up a 2.2K 1/2 watt resistor.
>
>This resistor is on the B+ line and located to the front of the electrolytic
>C-94 and comes off the center section of this capacitor along with another
>2.2K, several red/white wires and a couple other resistors.
>
>The one in question goes to a terminal strip to the front of the
>electrolytic and is joined with a white wire with red and blue tracers. This
>wire then goes into a small but tightly laced bundle where it goes
>somewhere. The "somewhere" will lead me to resolving what is causing the
>extra current draw and burning up the resistor but I can't find the wire
>without cutting the lacing on my receiver which I really would like to
>avoid.
>
>Fro the schematic, there appear to be at least nine, 2.2 K resistors that
>are possibilities and I eliminated them I thought from the rather poor
>pictures in the book but I was wrong. I thought it was R100 but that is not
>it. Others seem to be R31, 26, 22, 21, 12, 52, 47, or 84. I may have missed
>one as well. I have a picture which I can send if that will help identify it
>but I need someone who has more information than in the manual to point me
>in the right direction.
>
>Thanks, Roger NJ2R
>
>
>Sponsored by the Collins Radio Association
>http://www.collinsra.com
>