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knight kit t 60 help

J

jer19541

Member
Dec 7, 2011
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used this transmitter for about a year worked very good. and then, recently turned it on standby and the 10 watt ceramic resistor gets very hot and will smoke. very noticeable hum from the transformer. any help or suggestions will be apprecieated. n4tbu
 

Failed electrolytic filter caps. If they are original, it's a miracle they ran for as long as they did. That kind of part is not meant to last more than 10 or 20 years, let alone 50 or 60. Makes it sound as if this transmitter's mileage was really low when you began using it.

The question becomes, how many other parts in the power supply suffered damage besides the resistor? If the correct size fuse was in place, the transformer might still be okay. And if someone substituted a larger-rated fuse it probably is not.

73
 
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nomadradio said:
Failed electrolytic filter caps. If they are original, it's a miracle they ran for as long as they did. That kind of part is not meant to last more than 10 or 20 years, let alone 50 or 60. Makes it sound as if this transmitter's mileage was really low when you began using it.

The question becomes, how many other parts in the power supply suffered damage besides the resistor? If the correct size fuse was in place, the transformer might still be okay. And if someone substituted a larger-rated fuse it probably is ..... does not blow the 3 amp fuse
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jer19541 said:
used this transmitter for about a year worked very good. and then, recently turned it on standby and the 10 watt ceramic resistor gets very hot and will smoke. very noticeable hum from the transformer. any help or suggestions will be apprecieated. n4tbu
Click to expand...
these problems started when i changed the power cord n4tbu
 
The smoking resistor tells us that there is a short to ground on the B+ output of the power supply. Likeliest cause would be C29 if it's the original 60 year-old part. Next-most would be the 6DQ6 final tube. Unplugging the tube would clear the short if that's the cause.

But if the electrolytic capacitors are original, they are the most-likely failure risk.

And having it start after replacing the cord was just a happy coincidence. Don't see how to connect that hot resistor with a replacement power cord.

73
 
nomadradio said:
The smoking resistor tells us that there is a short to ground on the B+ output of the power supply. Likeliest cause would be C29 if it's the original 60 year-old part. Next-most would be the 6DQ6 final tube. Unplugging the tube would clear the short if that's the cause.

But if the electrolytic capacitors are original, they are the most-likely failure risk.

And having it start after replacing the cord was just a happy coincidence. Don't see how to connect that hot resistor with a replacement power cord.

73
Click to expand...
I remember put some strain on the neutral wire. Stripping it
 
Are you absolutely, positively sure that none of the new electrolytic caps are installed with the polarity backwards?

When that happens, the capacitor will usually break down internally and become a dead short. A resistor feeding into that capacitor will smoke after the capacitor fails.

Back in the bad old days, we would troubleshoot a short in an automotive wiring harness by pulling the fuse that keeps blowing and wiring a sealed-beam headlamp in place of the fuse. If there is a short on that circuit, the headlamp will have full brightness. Once you clear the short to ground in the wiring harness, the headlamp will go dark.

To isolate a short to ground in this transmitter I rely on the schematic. A multimeter to check for shorts on each side of the smoking resistor would be the starting point. The meter should reveal either a short to ground, or a low resistance on one side or the other of that resistor.

The wires feeding off the "short" side of that resistor get pulled off the tie point, to test each one individually for a short to ground.

73
 
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nomadradio said:
Are you absolutely, positively sure that none of the new electrolytic caps are installed with the polarity backwards?

When that happens, the capacitor will usually break down internally and become a dead short. A resistor feeding into that capacitor will smoke after the capacitor fails.

Back in the bad old days, we would troubleshoot a short in an automotive wiring harness by pulling the fuse that keeps blowing and wiring a sealed-beam headlamp in place of the fuse. If there is a short on that circuit, the headlamp will have full brightness. Once you clear the short to ground in the wiring harness, the headlamp will go dark.

To isolate a short to ground in this transmitter I rely on the schematic. A multimeter to check for shorts on each side of the smoking resistor would be the starting point. The meter should reveal either a short to ground, or a low resistance on one side or the other of that resistor.

The wires feeding off the "short" side of that resistor get pulled off the tie point, to test each one individually for a short to ground.

73
Click to expand...
Thanks. Will check. N4tbu .
 
Have a VERY close look at that switch. Odds are it's built from wafers of brown bakelite plastic. If you saw where on the switch the arc came from, get a bright light onto that section and see if a black bridge has burned/formed between adjacent contacts. This happens a lot in other radios, but I'm just guessing about seeing that fault in this one.

73
 
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