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 Post subject: Fusistor help
PostPosted: May Wed 16, 2018 6:38 pm 
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I just finished recapping my 10VT10R Motorola TV set. It has a 7.5 ohm fusistor which went bad. the house AC current goes through a Filament transformer which feeds filament voltage to the tubes and then another lead goes to the 7.5 Ohm fusistor in series with a 140MFD filter cap, the fusistor feeds the selenium rectifiers which connect to rest of the circuit, what wattage should I use to replace the fusistor, and I've heard that once they get an overload, they increase drastically in resistance (which happened to mine) but once the source of that overload is eliminated, they'll go back to their normal resistance value. Does that mean the fusistor shouldn't be replaced?

The max resistor wattage I have is 1, I'd hate to do all this work recapping it only to have to wait another week for a damn resistor to show up, I might be able to buy them somewhere though.
No component was obviously shorted, could a bad transformer cause the fusistor to overload?


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 Post subject: Re: Fusistor help
PostPosted: May Wed 16, 2018 6:47 pm 
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They do not go back to their normal resistance and need to be replaced just like a fuse. They serve two main functions.

One - limit the turn on surge current to protect the selenium rectifiers. Modern silicon power rectifiers are more robust and this not as much of an issue.

Two - act as a fuse and blow open (or go to a high resistance) if overloaded.

The original was probably rated somewhere from 5W to 10W. You can replace with a resistor in series with a fuse. You could put five 33 or 39 ohm 1 watt resistors in parallel to get something close to 7.5 ohms they can handle the power.


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 Post subject: Re: Fusistor help
PostPosted: May Wed 16, 2018 7:12 pm 
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bandersen wrote:
They do not go back to their normal resistance and need to be replaced just like a fuse. They serve two main functions.

One - limit the turn on surge current to protect the selenium rectifiers. Modern silicon power rectifiers are more robust and this not as much of an issue.

Two - act as a fuse and blow open (or go to a high resistance) if overloaded.

The original was probably rated somewhere from 5W to 10W. You can replace with a resistor in series with a fuse. You could put five 33 or 39 ohm 1 watt resistors in parallel to get something close to 7.5 ohms they can handle the power.



I'll do that, what amp fuse should I use?


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 Post subject: Re: Fusistor help
PostPosted: May Wed 16, 2018 7:49 pm 
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Something like 1/2 an amp to an amp. Depends on your B+ current. You can measure it for sure once the set is running by measuring the voltage drop across the resistor and using ohms law.


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 Post subject: Re: Fusistor help
PostPosted: May Wed 16, 2018 8:39 pm 
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Location: New Hampshire
The common fuseable resistors in newer tube B&W sets were 5.6 or 7.5 ohm
5 watt. You should be able to find them BUT they are different than the common
power resistors. If you use a plain power resistor be SURE you add a fuse in
series. I would go with a 1/2 amp slow blow for starters. If it blows randomly go up a little.
If you dont fuse it & get a short the resistor will overheat & actually turn red.
Of course if you like seeing fire trucks dont fuse it :D !

73 Zeno 8)
LFOD !


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 Post subject: Re: Fusistor help
PostPosted: May Wed 16, 2018 8:43 pm 
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zeno wrote:
The common fuseable resistors in newer tube B&W sets were 5.6 or 7.5 ohm
5 watt. You should be able to find them BUT they are different than the common
power resistors. If you use a plain power resistor be SURE you add a fuse in
series. I would go with a 1/2 amp slow blow for starters. If it blows randomly go up a little.
If you dont fuse it & get a short the resistor will overheat & actually turn red.
Of course if you like seeing fire trucks dont fuse it :D !

73 Zeno 8)
LFOD !


+1 - look for Workman "Fuzits" - the normal resistors do not fail safe like the fusistors do. Safety yes, but a continued burn is not good with any fusible. Oneida, GC, Belmoor and Philmore also made the fusibles. The originals as used by Motorola and RCA were IRC, long gone from their products list.

Never try to second guess the original engineering.

EDIT: Here's two on eBay (NOT me, no affiliation, caveat emptor...)

https://www.ebay.com/itm/2-NOS-Vintage- ... SwzaJX9uC9

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Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Fusistor help
PostPosted: May Wed 16, 2018 9:17 pm 
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They are still made by several manufactures like Bourns. The closest DigiKey has in stock is 10 ohms at 5 watts

https://www.digikey.com/catalog/en/part ... vendor=118


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 Post subject: Re: Fusistor help
PostPosted: May Wed 16, 2018 10:01 pm 
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Thanks for all the help guys, I'll probably end up using the 5 1 watt resistors. Kinda a stupid question, I know, but does it matter if I wire the fuse before or after the resistor?


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 Post subject: Re: Fusistor help
PostPosted: May Wed 16, 2018 11:16 pm 
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No, it doesn't matter. Good luck.


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 Post subject: Re: Fusistor help
PostPosted: May Thu 17, 2018 3:47 am 
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Thanks, I'll let you all know how it goes... Keep your fingers crossed for me :lol: :lol:


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 Post subject: Re: Fusistor help
PostPosted: May Sat 19, 2018 7:09 pm 
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Ok so, the set powered up and seems to work very well except some problems with the pots etc. Theres a few concerns I have now, taking the set up with a variac over the course of 5 hours really seemed to pay off as theres very little hum. However, after running the TV for a few minutes, I went to turn it off, and the 1/2A fuse blew. I was able to turn the TV on and off many times, but after running the TV for a bit it. The fuse itself didn't even blow, the wire inside the fuse was bubbled but still intact, but my DMM said the fuse was open. After putting the 3/4thA fuse, I was able to run the TV for longer than I was the 1/2A fuse. But I turned it off and after a few minutes I went to turn it back on and the TV tube wouldn't come on. Now, I have a 1A fuse in it, and I see no problems yet,

You guys have any idea what might be causing this? could this somehow be the capacitance in the TV tube trying to discharge and somethings going wrong? I did hear some crackling but I believe i sourced this to the electrical tape i used on the leads of the 5x39 Ohm 1W resistors melting due to the heat. Which makes me think maybe the heat from the resistors was causing the fuse to blow while not really being overloaded. Because of the bulkiness of the fuse holder im using, I had to have everything really close together. I'll probably end up using a 10 ohm 10W resistor and see if this fixes it, if nothing else seems to be the cause of the fuses blowing

Also, cable (I forgot the actual name of it :oops: ) that goes to the choke to the metal coating on the body of the picture tube, where that cable rests on the body has been scratched away a little so i put a tiny piece of tin foil under it to connect it to the body, I'm not sure if this really made a difference or of it matters, but the fuse hasn't blown yet, Just thinking what else could be causing this

Thanks again for your continued help with this project!


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 Post subject: Re: Fusistor help
PostPosted: May Sun 20, 2018 12:16 am 
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What size fuse is it supposed to be? If you put in something smaller than that there's a good chance it will blow. On the other hand, the fuse is there to protect something. If you put in a larger size to prevent it from blowing, you will probably blow whatever was supposed to be protected.

Use the right size fuse and find the problem.

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 Post subject: Re: Fusistor help
PostPosted: May Sun 20, 2018 2:42 am 
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Jim Mueller wrote:
What size fuse is it supposed to be? If you put in something smaller than that there's a good chance it will blow. On the other hand, the fuse is there to protect something. If you put in a larger size to prevent it from blowing, you will probably blow whatever was supposed to be protected.

Use the right size fuse and find the problem.


I dont know, schematics dont tell me anything.... only that its a 7.5 ohm resistor, which is now at about 11 ohms with a 1A fuse in series


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 Post subject: Re: Fusistor help
PostPosted: May Sun 20, 2018 3:59 am 
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Jim Mueller wrote:
What size fuse is it supposed to be? If you put in something smaller than that there's a good chance it will blow. On the other hand, the fuse is there to protect something. If you put in a larger size to prevent it from blowing, you will probably blow whatever was supposed to be protected.

Use the right size fuse and find the problem.

The set did not originally have a fuse. The fusistor acts both as a resistor and a fuse. I have never seen a specification as to what current they are supposed to blow at. The manufacturer would have had a target in mind, but I don't think that it was tightly specified or controlled. In any case I think that it was to blow if there was some catastrophic failure such as a selenium rectifier shorting.

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 Post subject: Re: Fusistor help
PostPosted: May Mon 21, 2018 1:24 am 
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Yup, I forgot what this thread was about. Sorry about that. Do you have a schematic? If so, you can make a semi-educated guess based on where the fusistor is located in the circuit.

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