Resistor reliability question - Videokarma.org TV - Video

RetroHacker · #1 01-13-2008, 10:38 AM

I'm currently working on a GE console set, 40's vintage. I still have a few more capacitors to replace, but I don't have them on hand. I've got B+ and heater voltage, and the audio output section (push-pull 6Y6) seems to be working. At the moment the set is still silent, although I can get crackles by wiggleing the bandswitch, and a faint buzz if the volume is turned all the way up. This set uses the old style "body-end-dot" resistors. The ones I have checked seem within spec, but what is the general reliability on these things? Are these better or worse than the carbon composite banded ones? What's the makeup of these, anyway - wirewound? Carbon? I know with carbon composite, they tend to drift up in value, especially the ones that were larger values to begin with. Any "gotcha's" with these?

-Ian

Old1625 · #2 01-13-2008, 01:14 PM

If you are looking at what I think you are IIRC the resistor in question has a length of resistance wire coiled around a ceramic cylinder, with leadoffs coiled a few turns at each end, creating a spool-shaped device. Then the whole mess is glazed over in colors representing RMA code of its value.

If it measures to spec, and it is not showing visible signs of overheat then it should be good to remain.

As for the set being silent it has always been with me the game of starting at the first audio stage near the volume control and then working through the second detector and up the line. If I didn't have a signal generator on hand to produce the intermediate frequency I would take a working radio, tune it to a strong station, and tap off the final IF transformer and inject that signal into the various points along the IF chain while peaking the transformers. This would also unearth any defects along the way. Always worked for me....

Chad Hauris · #3 01-13-2008, 07:51 PM

I haven't had many problems with bad resistors in this age of set, but I would check all of the voltages at each element of each tube...problems I have seen with resistors often relate to those which have been damaged by overloads due to bad bypass capacitors like at a screen grid. The voltage check will lead you to the bad resistors if there are any because you will have an off value or no voltage.

The RF generator has been such a help to me with problems such as this.
One helpful thing is that you can make the signal very strong and this can help get a signal through a misaligned set so you can align it. have seen several cases where an alignment needed to be done to pick up any stations and I think it was just due to component aging.

Old1625 · #4 01-14-2008, 07:42 AM

Quote:

Originally Posted by Chad Hauris

.....The RF generator has been such a help to me with problems such as this.
One helpful thing is that you can make the signal very strong and this can help get a signal through a misaligned set so you can align it. have seen several cases where an alignment needed to be done to pick up any stations and I think it was just due to component aging.

'Tis true, but one of the worst alignment jobs I had was because a set had been hit by lightning and rendered inoperable about 30 years before I got to it. Apparently over the ensuing years after the lightning hit a few people in the family that owned the old Stromberg-Carlson radio thought they could get it going again. One of them found a whole mess of loose screws--and tightened them!

radiotvnut · #5 01-17-2008, 11:31 PM

I just tore into a circa '39 Coronado table radio. Of course, all the caps were shot. It has the old style resistors like the ones mentioned above. All the ones that I have checked so far read at least twice their marked value. What I have replaced so far has helped the performance of the radio.

blue_lateral · #6 01-20-2008, 03:19 AM

These old resistors are usually referred to as "dogbones", and are carbon. They are a different manufacturing process than carbon comps, but treat them the same. If they drift in value, it's usually up.

Many people have reported the same experience as radiotvnut, so check them all. I have had much better luck with them, however. I dont ever remember having to change all of them in a radio.

In the radio I am currently working on, all the dogbones were still well within tolerance, and most of the striped carbon comps were shot. Oh, and by the way, if there is no tolerance mark, a dogbone is a 20% tolerance part, just like a normal striped carbon comp. The tolerance mark, if it's there, will be a blob of paint on the normally unmarked end of the resistor. Silver for 10%, gold for 5%, just like a striped one.

The smallest, most common size of dogbone is 1/4 watt (and those are physically a little bigger than a 1/2 watt carbon comp. After the change to the striped resistors, 1/2 watt became the smallest common size. This is because 1/4 watt carbon comps are physically tiny, and usually have a farily low voltage rating.

John