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Author Topic: Henry 1KD5 blows Ip fuse  (Read 2789 times)
AE7AE
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Posts: 17


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« on: October 12, 2005, 04:39:05 AM »

Got a 1KD5 with odd fault. When turned on, the plate volts is OK at 3,000v and the filaments look lite correctly.  When keyed-up with no drive, it blows the Ip 1A fuse(F3) after a few seconds.  Before blowing, the Ip idle current is correct at 100ma...then POW!!! it blows. Ig at idle is about 5ma. A resistance check to ground at the fuse holder indicates correctly at 10K. With the F3-fuse removed, the 'hot' side measures 'infinity' so there's no short to ground on the fil xfmr.  I put in a new 3-500ZG, with same result. When keyed, the only change to the Ip(Cathode) circuit is that the 10K 2w resistor in series with the fuse is shorted across.  All affected resistors measure correct values and do not look discolored.  HELP!!!!!!!!!!!!!
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retired but not expired... glass is usually half FULL, not because I didn't fill it... Just can't keep it full!
WB2WIK
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Posts: 21836




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« Reply #1 on: October 12, 2005, 09:11:47 AM »

I don't have the 1KD5 schematic, but it sounds like F3 is a Cathode fuse, based on the 1A rating (which would be about right).  Obviously, any short anywhere along the path would blow this fuse, and of course it needn't be a direct short at all.   A 3000 Ohm short to ground, at 3000V, will blow a 1A fuse.

The "short" (or fault, I should call it, because it may not be anything like a short) is likely in the HV section, not near the cathode.  Cathode current is the sum of plate current and grid current, and obviously the larger contributor is plate current.  Anything that causes a lot of plate current to flow will blow the cathode fuse.

Sounds by your description that it takes a while to happen, so the fault may be related to something "charging up" and then arching.  Take the 1KD5 out of its outer case (the RF plate compartment will still be shielded), defeat the cover interlock if there is one (probably is), and turn on the amp in a darkened room so you can look through the openings in the RF cage to see if anything obvious happens, like a flash between active wiring or components and chassis ground.  At least, this is where I'd likely start.

WB2WIK/6
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KA5N
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Posts: 4380




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« Reply #2 on: October 12, 2005, 09:15:15 AM »

From the symptoms and the checks you make I suspect some problem in the filament windings (alternate path to ground).  Does the plate current jump when the fuse blows?  If so then the tube must be arcing or there is a parasitic.  If not, then the current blowing the fuse is outside the high voltage circuit.  There may be some funny business with the ALC circuit.
Allen
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