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Author Topic: ACG Fuse Mystery  (Read 482 times)
W0FM
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Posts: 2052




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« on: December 11, 2006, 01:28:06 PM »

I wonder just how critical the voltage rating enbossed on an AGC type fuse really is.  I now suspect it means that the fuse is rated to handle "at least" XX volts and the current, obviously, is the important spec.  A recent encounter with an AGC fuse has me scratching my head.

I sent a friend's resurrected KWM2-A and 516F-2 power supply off for realignment, re-capping and new tubes.  We had never had occasion to inspect the fuse on the power supply as the rig worked quite well.  My friend and I had made numerous DX QSO's over the past several months with that rig and P/S, even with less than perfect antennas, but we both felt that it was getting a bit long in tooth and could use some new tubes, caps, etc.

The other night I got the rig back and fired it up.  I tuned around the bands for 30 minutes or so but never transmitted.  I shut the rig down, had some dinner, then went back to enjoy the warm Collins glow before bed.  When I turned the KWM2-A power switch on, the P/S popped and everything went out.  An immediate check of the house AC line showed normal voltage for my area.  I removed F1, the fuse in the power supply.  It was totally black inside and looked like a small explosion had occured inside the glass tube.  The squiggly little filament was plastered against the inside of the tube, not just separated as is often the case.

Closer examination of the fused revealed that it was a Buss AGC-5 (not the AGC-4 recommended for the rig) but what really caught my eye was the fact that it was labeled 32V!  I would have expected it to be labeled 250V.

I called the gentleman who did the realignment, etc and he said he never had occasion to look at the P/S fuse as it worked fine as he performed the final checkout.  He was as mystified as me.

How could this fuse support a 100W transceiver operation for all this time under mis-match conditions and all?  Is it simply the fact that the voltage stamped on the fuse is it's MINIMAL capability?  Or is it because the extra current capacity of this fuse (5amp rather than 4) saved it from destructing itself for all these years?

73,

Terry, WØFM
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W0FM
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« Reply #1 on: December 11, 2006, 01:34:00 PM »

Yep.  Got me!  The subject should read "AGC" not "ACG".  Fat fingers, old eyes.
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N3EF
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Posts: 247




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« Reply #2 on: December 11, 2006, 01:36:50 PM »

  The voltage rating is how much voltage can be across the fuse without it arcing over. In normal operation, a fuse is a short circuit and there is very little voltage across it. When the fuse opens due to overcurrent, there can be considerable voltage across it depending on it's application.

Eric N3EF
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KA5N
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Posts: 4380




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« Reply #3 on: December 11, 2006, 01:55:45 PM »

I suspect you somehow had a momentary dead short somewhere.  That is usually the problem when a fuse blackens the glass really bad.  I would open the rig up and look for blackened areas particularly around the power connections.  
GL Allen
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WD8PTB
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« Reply #4 on: December 11, 2006, 01:56:13 PM »

Eric has it correct. If you had unlimmitted current capacity the 32v fuse may have arced across and you would have had a real explosion! If you shorted a 250 v fuse directly across the ac line it would not be good but the fuse would blow and not continue to arc. The 32 v fuse might continue to arc. You should never replace a fuse with a voltage rating lower than the fused voltage for this reason. Don WD8PTB
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K4III
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« Reply #5 on: December 11, 2006, 01:57:03 PM »

Well, I guess you found the problem. However, I have been having some fuse problems with 120w VHF State "fleet" radios temporarily shutting down on mic keydown, even with perfect antenna and wiring. Issue? Bad 30A Spade fuses! No, they didn't pop or blow, but there was a batch of these older yellowish-green ones that just wouldn't pass more than about 15 amps after awhile. They didn't even pop or blow! Strange, but true... All I've done is replace them with the newer bright green 30A fuses and wala! Magic! Working radio! I've already replaced about 7 of these fuses on vehicles. Something to check for in automotive installations, if you have similar issues and don't want to spend endless hours of troubleshooting.
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W0FM
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« Reply #6 on: December 11, 2006, 02:06:50 PM »

<You should never replace a fuse with a voltage rating lower than the fused voltage for this reason>.

Cardinal Rule, Don.  I have always been a subscriber to the well known fact that a fuse lighter than specified will usually blow and a fuse of greater capacity than specified will fry your equipment.

My friend said he has had this rig since 1963 and never even looked at the fuse.  That being said, the repair tech found a number of "fascinating" modifications done by others that he had to rework.
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KI4HLB
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« Reply #7 on: December 11, 2006, 06:42:10 PM »

Hi all,

I have a round emblem KWM-2 with p/s and b4 console, I was wondering who you might have had do the service to the rig? can you share this info?

thanks


73  DE   KI4HLB

Hari
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W0FM
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Posts: 2052




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« Reply #8 on: December 12, 2006, 09:12:59 AM »

Hari,

Please email me directly and I'll give you the contact info on the person that I used for this. He was a real pleasure to deal with, but I'm a bit hesitant to post it as I'm not sure how much volume of work he wants to take on.

73,

Terry, WØFM
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