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Old 27th March 2011, 06:51 PM   #1
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Default heathkit im18 fuse

I have a German version (220V 50Hz) Heathkit im18 VTVM.
It has a fuse type F 0.1A 250V that has blown. I tried exchanging with a F0.25A fuse I had. But it blew as well. Before experimenting anymore I wonder if anyone knows what the correct value is?

The tubes light and the ohm meter works, but the voltmeter does not work. There are no apparent problems, except old capacitors.

If anyone has a digitized assembly manual and/or operating manual I hope you can give me a copy.
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Last edited by trondareo; 27th March 2011 at 07:08 PM. Reason: added question
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Old 28th March 2011, 12:16 PM   #2
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You will find a schematic at Heathkit Schematic Diagram Archive
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Old 28th March 2011, 01:02 PM   #3
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Thank you.
I have found the schematic, but I cant find a value for the fuse. I have checked several sources and values range from 0.1 to 1 amp. I dont want to fry anything. Thats why I am hesitant to go higher in values. I know this seems like a minor thing, but its easier than to start searching for a fault that may not exist.
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Old 28th March 2011, 02:45 PM   #4
cliffyk is offline cliffyk  United States
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The schematic shows the voltage and current at the power transformer's secondary windings as 6.3VAC at 0.8A, and 120VAC at 10mA.

Ignoring transformer efficiency and power factor to make this a "back-of-a-napkin" exercise that works out to 5.0W and 1.2W respectively, a total load of 6.2W.

240VAC at 0.1A = 24W so unless the transformer is grossly inefficient a 0.1A fuse should do the trick. I'd check that big cap (C1) first thing...
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Old 28th March 2011, 08:35 PM   #5
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So: The tubes "light up", I assume then the filaments get power. That means that the transformer is OK. Where in the circuit is the fuse? Check D1 and C1, also for chaffed wiring. The rest of the circuitry is high-impedance. Also get the tubes tested for performance.
E
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Old 29th March 2011, 11:37 AM   #6
cliffyk is offline cliffyk  United States
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Quote:
Originally Posted by mickeymoose View Post
So: The tubes "light up", I assume then the filaments get power. That means that the transformer is OK. Where in the circuit is the fuse? Check D1 and C1, also for chaffed wiring. The rest of the circuitry is high-impedance. Also get the tubes tested for performance.
E
Good question, I had mistakenly got it in my head that it was a power line fuse--obviously the filaments would not power up if that were the case...
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Old 29th March 2011, 07:47 PM   #7
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Unfortunately this was a combination of errors on my side and a faulty wire on the measuring probe. Its incredible how confused one can be when interpreting things wrong.
The fuse is inline with the primary of the transformer as usual. 0.1A is the right value.
I wont go into the details of my stupidity
Thanks for helping out every one.
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