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Author Topic: Please consider a fuse in the hv supply of your boat anchor.  (Read 1928 times)
N3DT
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Posts: 1792




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« Reply #15 on: October 08, 2018, 03:55:16 PM »

Recently I had a short on a filament choke on one of my R390A's. It took out the wiring to the RF section since the choke was at the end of the circuit in that module. It's a 7A filament line and it's not fused, other than from the input AC. But that was not enough to stop the wiring from smoking. I just happened to be there when I turned on the radio and watched the harness smoking and a wire melting in the harness right before my eyes. Apparently after 50 years the choke shorted to ground. It also took out the B+ choke that was right above it. Makes me want to fuse that 6.3V filament line right out of the transformer. It was quite a project to get it all back together, but it's working again as of yesterday. You can't have enough fuses.
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G3RZP
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Posts: 1319




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« Reply #16 on: October 10, 2018, 11:16:28 PM »

In a high current low voltage line, you  might need to be very careful in choice of fuse and fuse holder that you don't lose volts across the fuse because of resistance. All fuses and fuse holders of any given type are not necessarily equal!
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N2EY
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Posts: 5095




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« Reply #17 on: October 11, 2018, 07:48:43 AM »

Many of old BA rigs have two or more power transformers. Usually there's one multiwinding transformer for the low B+, bias, and heaters, and another big plate transformer for the final B+.

But for some reason, the rigmakers only included one primary fuse for the whole rig. That fuse has to be big enough that it doesn't blow even under full-yellow AM plate modulation conditions, so it's typically 8 or 10 amps, and often slow-blow.

Problem is, if there's a failure in the LV section, such as a bad filter cap or shorted rectifier, the LV transformer can cook nice and crispy without blowing the big primary fuse.

Solution is separate fuses of the right size for each transformer.

Yes, I learned that the hard way, with a Viking II many years ago.

73 de Jim, N2EY
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KOP
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Posts: 346




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« Reply #18 on: October 12, 2018, 10:47:11 AM »


Solution is separate fuses of the right size for each transformer. or winding

Yes, I learned that the hard way, with a Viking II many years ago.

73 de Jim, N2EY

Same here. A V II CDC sat on a shelf for years awaiting a salvaged transformer.

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I considered a microwave oven magnetron and a 4' dish as a drone-killer. The ERP would be on the order of a hundred thousand watts or so. ~anon

November 28, 2018, 09:16:04 AM
KM1H
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Posts: 5528




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« Reply #19 on: October 12, 2018, 11:26:15 AM »

Quote
Same here. A V II CDC sat on a shelf for years awaiting a salvaged transformer.

I caught the overheating oil filled cap in my CDC before anything bad happened.
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N0IOP
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Posts: 13




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« Reply #20 on: October 31, 2018, 03:10:28 PM »

Tight fusing in the primary works too, and allows you to use ordinary 250v fuses.  Usually a time delay fuse works best, allows you to fuse very close to the actual current drawn while operating. 
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