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 Post subject: SX-100 R37 too hot?
PostPosted: Feb Tue 23, 2021 5:09 am 
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Joined: Feb Sun 14, 2021 9:05 pm
Posts: 2
Good evening,

I was wondering if it's normal for R37 (WW 4.5K 10W) on a Hallicrafters SX-100 to get crazy hot? Some 200F according to my Flir One. See attachment.

I'm feeding the unit 120V from my variac at about .58A. That gets us about 70W which looks to be within spec.

I recapped the unit, replaced some bad resistors, and tested all the tubes - AOK. R37 still runs hot. Would the 150V rail sag under a partial short condition? I should probably get some more readings, but thought I would ask here real quick.

Once this checks out, I'll move on to alignment.

Thanks!


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 Post subject: Re: SX-100 R37 too hot?
PostPosted: Feb Tue 23, 2021 8:13 am 
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Joined: Dec Sat 28, 2019 4:18 pm
Posts: 672
Location: Corinth, TX
According to the schematic, the 4.5 Kohm resistor should be dropping 120 volts. That means about 3 ma. through it with a dissipation of about 3 watts. I do not think a 10 watt resistor should be running so hot.

John


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 Post subject: Re: SX-100 R37 too hot?
PostPosted: Feb Tue 23, 2021 2:14 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 7787
I wouldn't worry about 200F, check the voltage on the 150V rail and if it is good then the resistor is fine.

It is dissipating over 3 watts and think how hot a lowly #47 dial lamp gets with less than 1 watt of power input.

Wirewound resistors are generally specified for up to a 350C (NOT F) temperature rise above ambient. You really don't want one getting to its rated temperature near other electronic components but relatively speaking, 200F degrees is hot to humans but not at all for wirewound resistors. Even a carbon comp resistor is generally rated for an operating temperature of 250F.

Rodger WQ9E


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 Post subject: Re: SX-100 R37 too hot?
PostPosted: Feb Tue 23, 2021 3:02 pm 
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Joined: Dec Sat 28, 2019 4:18 pm
Posts: 672
Location: Corinth, TX
Better answer than mine, Rodger.

John


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 Post subject: Re: SX-100 R37 too hot?
PostPosted: Feb Tue 23, 2021 3:33 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
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John,

I remember an old timer told me three things never to touch even when the power is off:
1. Rectifier tubes
2. Audio output tubes
3. Wire wound resistors

The old rule of thumb for transformers and chokes was that if you could put your hand on one for a few seconds without extreme discomfort or burns if it was not overloaded but once they changed to newer spec wire coatings and insulating material between winding layers you wouldn't want to try that test with a modern transformer!

Rodger WQ9E


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 Post subject: Re: SX-100 R37 too hot?
PostPosted: Feb Tue 23, 2021 3:52 pm 
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Joined: Feb Sun 14, 2021 9:05 pm
Posts: 2
Thank you for the replies!

These old radios are a new exploration for me.

I have 20+ years working with modern electronics. So, its been a refreshing change.

I'll update the post when I have time to measure that 150V rail.

Paul


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