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 Post subject: Zenith 9s262 question
PostPosted: Jul Thu 11, 2019 1:58 am 
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Joined: Jan Wed 11, 2012 4:21 pm
Posts: 770
Location: Elko, MN.55020
Hi guy's,
Have a resistor question for the guys in the know.
My chassis has a rather large dog bone that I want to replace .
I am guessing that is probably a 3 watt by the size of it.
Schematic shows a 15k and a 10k, both lusted as 1 watt resistors on the parts list.
Is this a typo or did someone replace a 1 watt with a 3 watt?
I put a 2 watt 15k in its place and it seems to get pretty hot.
Should I get a 3 watt to replace the 2 watt?
Just happens that I don 't have any 3 watt resistors.
Murf


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Thu 11, 2019 2:15 am 
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Location: Aurora, CO
Somewhere I've seen a chart that shows the wattage of a dog bone resistor based on size. Anyway, if you have to order resistors, go with 5 watts for those 2. They are B+ voltage dividers. Higher line voltages mean higher B+.

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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Thu 11, 2019 2:19 am 
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Murf

Dog bone were larger than same wattage later resistors. To determine wattage measure voltage across your resistor in question. From there knowing resistance and voltage drop wattage can be calculated.

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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Thu 11, 2019 2:23 am 
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Location: Austin, Texas
According to the Zenith voltage chart, the power dissipation should be 1.25W in the 10K and 1.88W in the 15K. Either use two 2W resistors for each location or get some 5W resistors. For the 10K you could put two 20K, 2W, in parallel. The 15K would be two 30K, 2W, in parallel.

Jay


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Thu 11, 2019 2:26 am 
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Joined: Jan Mon 16, 2012 4:15 pm
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Location: Near Brandon, Iowa
There's no reason to disbelieve the parts list power rating for any given part.

Your two-watt part may get hot but a three-watt part of the same resistance won't run any cooler. They will both radiate the same amount of waste heat energy.

You can measure the voltage drop across the resistor, calculate the current flowing through it (I=V/R), and from this can calculate the power being dissipated in it (P=I*I*R). If the calculated power is less than 70% of the part's power rating, no worries. But if it exceeds the power rating of the originally-spec'ed part (from the Parts List), I'd suspect that there is a fault in the radio that is causing excess current draw through that part.


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Thu 11, 2019 4:30 pm 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
lorenz200w wrote:
Your two-watt part may get hot but a three-watt part of the same resistance won't run any cooler. They will both radiate the same amount of waste heat energy.

It is true that the heat produced is the same, but the three watt will be cooler because it has a greater surface area to get rid of the heat.

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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Thu 11, 2019 8:36 pm 
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Notimetolooz wrote:
lorenz200w wrote:
Your two-watt part may get hot but a three-watt part of the same resistance won't run any cooler. They will both radiate the same amount of waste heat energy.

It is true that the heat produced is the same, but the three watt will be cooler because it has a greater surface area to get rid of the heat.

Assuming that both parts are convection-cooled types, the difference will not be noticeable, even using an index finger that is calibrated/traceable to the NBS. Hot is hot.


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Fri 12, 2019 1:13 am 
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Location: Dallas, TX
True there is not a lot of difference between 2 and 3 watt.

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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Fri 12, 2019 6:57 am 
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Joined: May Wed 23, 2018 6:28 am
Posts: 398
This chart shows resistor surface temperature vs power applied for a range of ceramic resistor wattages:

Attachment:
TE ceramic-silica T vs W.jpg
TE ceramic-silica T vs W.jpg [ 84.9 KiB | Viewed 257 times ]


Temperatures are degrees C. Ratings are for open air, which is not necessarily what you have under a chassis. There is also derating to consider, which is 0.4% per degree C above 25. The same amount of heat is created no matter what wattage rating is used. Without proper ventilation it will raise the ambient temperature, so placement is important.

John


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Fri 12, 2019 8:27 pm 
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Joined: Jan Wed 11, 2012 4:21 pm
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Location: Elko, MN.55020
Some people recommend just replacing with 5 watt resistors.
Thoughts?
Murf


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Sat 13, 2019 1:12 am 
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I have used 5 watt or bigger resistors for Candohms before. Some will say overkill and they would be right. It’s really your choice as long as they are properly rated or over it’s no big deal. I liked the way it looked under the chassis when all mounted on strips.

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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Sat 13, 2019 1:47 am 
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Quote:
...For the 10K you could put two 20K, 2W, in parallel. The 15K would be two 30K, 2W, in parallel.


+1


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Sun 14, 2019 2:33 am 
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Joined: Jan Mon 16, 2012 4:15 pm
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Location: Near Brandon, Iowa
murf wrote:
Some people recommend just replacing with 5 watt resistors.
Thoughts?
Murf

Depends if you subscribe to the belief that radio engineers selected resistor power levels in part so that they could function as fuses in the event of a catastrophic power-bus failure.


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Sun 14, 2019 3:03 am 
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Joined: Jan Wed 11, 2012 4:21 pm
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Location: Elko, MN.55020
Best i can come up with right now is 3watt resistors.
Think i will just put them in and call it good.
Thanks for all the input guy's.
Murf


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Tue 16, 2019 6:07 am 
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Location: 253 Blanche St. Plymouth, MI USA
JnTX wrote:
According to the Zenith voltage chart, the power dissipation should be 1.25W in the 10K and 1.88W in the 15K. Either use two 2W resistors for each location or get some 5W resistors. For the 10K you could put two 20K, 2W, in parallel. The 15K would be two 30K, 2W, in parallel.

Jay



only the common values today are called 22K ohm and 33K ohm so in parallel you get 11K and 16.5K which is fine anyway., use 2 watt of each and you get 4 watts per combo.
Mark Oppat


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 Post subject: Re: Zenith 9s262 question
PostPosted: Jul Tue 16, 2019 8:17 pm 
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Joined: Jan Wed 11, 2012 4:21 pm
Posts: 770
Location: Elko, MN.55020
Ok, thanks for all the advice again guys.
Greatly appreciated.
Will dig out some 2 watt resistors and install as suggested.
Thanks,
Murf


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