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 Post subject: Fusistor Question
PostPosted: Dec Fri 07, 2007 1:55 am 
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I seem to remember my dad working on TVs and there were what he called Fusistors. Kinda like a sand resistor but a fuse as well. Is that what they really were, a resistor with a fuse? If so, can't you just replace it with a resistor of the correct value with a fuse in series with it?

Or, are they still available? I never really understood what they were for...

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PostPosted: Dec Fri 07, 2007 2:09 am 
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Don- I have often wondered about this myself. Back in the days when I was working on TV sets, I was told two things about them. One guy said they were a resistor that simply changed its value when it got hot to limit the current, and would resume their cold value when they cooled down. The other guy said they were a resistor with a fine wire that acted like a fuse and when they blew, they were done for.

So I am curious too. Maybe two of us can learn something here!
Curt

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 Post subject:
PostPosted: Dec Fri 07, 2007 2:34 am 
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When I worked in a TV service shop in the late 60s, I seem to remember large sand-type resistors that were called "fusible" resistors. They were generally under 50 ohms and rated at 5 or 10 watts.

They were usually used in series-string, transformerless TVs that often had a voltage doubler B+, made with selenium rectifiers.

I believe the main purpose was to reduce surge current when the filter caps first charged up, but also to act as a fuse if the set drew too much current for a significant period of time.

Rich


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PostPosted: Dec Fri 07, 2007 3:12 am 
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The fusible resistors I'm familiar with had lugs crimped on the wires and plugged into a small terminal strip so they were easy and quick to replace. IIRC the common values were 4.7, 5.6, and 6.8 ohms, we usually just used whatever ones we had on hand.

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 Post subject:
PostPosted: Dec Fri 07, 2007 3:21 am 
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Definitely a one-time thing. Sometimes you get lucky and NOS stuff from closed up shops or distributors turns up at swap meets or on eBay.

However, there's no problem replacing with a wirewound resistor of exact value in series with a fuse.

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PostPosted: Dec Fri 07, 2007 3:25 am 
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And they got real hot and smelled when one of our co-ops would put in a 150 ohm or so piece in the Philco portables. Look the same, are the same they thought.

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PostPosted: Dec Fri 07, 2007 3:35 am 
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Worst case I ever saw was when someone replaced one in an old metal cased Admiral portable with a wirewound resistor of the right value, but very much higher wattage. One day the rectifier shorted dead, putting the resistor directly across the power line in series with the electrolytic. After a very short time, the resistor was glowing bright orange and really smelled bad. Still can't figure why the electrolytic didn't explode, it certainly should have. It was bad though......

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PostPosted: Dec Fri 07, 2007 2:58 pm 
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I still use those things in late 50's and 60's vintage car radios. They are used in the emitter circuit of the output transistor, usually something between .33 and .68 ohms. Fortunately a local radio supply house still has some of the fusible resistors in stock. Harry


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PostPosted: Dec Fri 07, 2007 5:31 pm 
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In the 50s I had a small B&W TV in my room that just died. I went with my dad to the TV shop where the guy fixed it while we waited. I could see him plug something in. Dad was outraged by the repair bill.

It died again a month later so I opened it up, saw that new plug-in fusable resistor and rode my bike to that shop for a replacement. The service guy recognized me and gave me a "better one" for a buck.
Dad was amazed a kid could repair a TV for a dollar.


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 Post subject:
PostPosted: Dec Sat 08, 2007 1:35 am 
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Under the "Workman" brand a number of smallish fusable resistors were sold, and were color-coded. The units were plastic, as opposed to "cement", small, rectangular, and had two angled "legs" at the bottom. I never ran into a TV that used these, but saw a large display at my regular parts jobber in the '70s. Maybe for inexpensive to moderately priced B/W sets?
Kevin

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 Post subject:
PostPosted: Dec Sat 08, 2007 5:01 am 
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Those rectangular plastic colored fuses are Belfuses, and were used in many brands and models of TV's, both B&W and color. The socket and plug-in fuse tabs were designed so one could not use a drastically higher value of fuse than the original, it simply wouldn't plug into the socket. Of course the concept was to match the color, so you would have the correct value. But within a certain very limited range, other colors would plug in.


The original Belfuses were described as "chemical fuses" and left a white powder residue when they blew, but IIRC some of the aftermarket replacements like the Workman line may have just had a conventional fuse element.

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 Post subject:
PostPosted: Dec Sat 08, 2007 6:13 am 
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Zenith used belfuses (maroon, light green, and white were the most popular) on all the color tv tube chassis's from the mid to late 60s to the chromacolor II chassis in 74. They had a circuit breaker in the CC II chassis.

I think I have a few left in the basement.

Chuck D. KB9UMF


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 Post subject:
PostPosted: Dec Sat 08, 2007 2:43 pm 
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Thanks for all of the replies. Now that you say it, I remember some of them having ends on the wires that would plug into a socket for easy replacement.

Well, I guess they can be replaced with a wire wound resistor with a fuse in series. Now, if one can only find the specs of it, that would help. Did Sams only have a part number or the values.

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