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Author Topic: HELP! Which fuse should I use? (fuse blew on pow  (Read 997 times)
KA3ZMN
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Posts: 35




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« on: November 30, 2001, 09:33:04 AM »

I have a Drake "twin pair" T4XB transmitter and R4XB receiver.  After 10 years of me using the rig (which I purchased used way back), the fuse on its AC-4 power supply blew.

My question is, what fuse rating should I use?  I ask because the fuse I removed was a 4 amp 250 v fuse.  However, the back of the powersupply says 5 amp (near the fuse holder), and the power supply schematics from the transmitter's instruction manual shows the schematic for the fuse indicating  6 1/4 AMP SLO BLO for 120 VAC  and 3 AMP SLO BLO for 240 VAC.  So, I'm confused about a couple of items.

Note that I'm using 120VAC.

Oh, and the powersupply is not in the transmitter itself, but in a seperate case.

Of course, I wonder why the fuse blew in the first place.  However, it was just after I tuned up in 80 meters at full power.  And since I only had a 4-amp fuse in there, perhaps it just couldn't take the load (although it's worked ok for 10 years).  I have made some antenna adjustments prior in the past couple of days which has gotten the antenna tuner working better with my Butternut vertical... so perhaps the transmitter was now able to get more power out (more efficient?) and therefore surpassing the fuse rating.  Does that make sense?  Still, I will check out antenna and tuner to see if they are the cause of any problems, but I have found, so far, no signs of arcing, and the SWR seemed good.

Anyway, I need to replace the fuse on the power supply.  And I assume I should NOT be using a 4-amp one since it doesn't match what's written on the powersupply itself or the schematics.

However, which do I use?  Since the powersupply says one thing and the schematics say another (see what I wrote earlier), do I get a 5-AMP?  or 6 1/4-AMP?  And should I still go with SLO BLO fuses?  (I assume so, since that's what the schematics indicate.)

Yet another question is the V rating on the fuse.  My 4-amp fuse (that blew) says 250 V on it.  What does that mean? And should I be looking for the same on whichever correct amperage I get for a new fuse?

I do not see any mention of a V (voltage?) rating on the back of the power supply or in the schematics (or anywhere in the transmitter/powersupply's documentation.)  I only see 5-amp (again, on the back of the powersupply) and 6 1/4-amp (in the schematic), no mention of 250 V or any other "V" ratings for a fuse.

And the final question is... the 4-amp fuse that I pulled out looks like a regular glass tube fuse...it has a thin metallic strip inside the tube... but the strip ends in a small spring-like thing.  Is that a SLO BLO?

So, in summary...

1) Which amp rating should the fuse I get have?

2) Which V rating should the fuse I get have?

3) Should I be using SLO BLO fuses?

4) And is the blown fuse I pulled out last night a SLO BLO according to my description.

I wonder why the original owner had a 4-amp fuse in there in the first place.

Thanks so much for any help you can provide.

Steve Rifkin
KA3ZMN
steve410@erols.com
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W9GB
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Posts: 2611




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« Reply #1 on: November 30, 2001, 10:12:09 AM »

Steve -

Replace the fuse with the mfg. (Drake) recommended size and type.  A "Slow-blow" dows have a spring like device inside the glass, a normal fuse is a single filament.

"Slow blows" are typical where initial current (when powering) is high for a very short period.  

Based on your comments (documentation and power supply markings), I would get a 5 amp and a 6 amp "slo-blow" if you are using 120 VAC.
 
First, I would look for possible causes for the fuse failure (shorts, failed components, tube shorted or going bad)

Then, I would install the 5 amp device and run the transmitter.  Only if it also "blew" fairly quickly would I consider using the 6 amp fuse.
 
greg
w9gb
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W9GB
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« Reply #2 on: November 30, 2001, 10:15:22 AM »

I forgot your voltage question.

Most of these fuses (the Buss brand that I purchase) have a 250V rating, but always ask - I have seen a few imports with different stampings

w9gb
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KA3ZMN
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Posts: 35




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« Reply #3 on: November 30, 2001, 10:35:17 AM »

Note also that after the 4A  250V written on the fuse, it says 313.

Is that a series of some sort?  Radio shack has 5A 250V Slow Blow fuses, but they say MDL/3AG.  Does that matter?

Steve
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NH7S
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Posts: 11




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« Reply #4 on: November 30, 2001, 10:48:23 AM »

   I suppose the text book answer would be to use the type of fuse recommended by the manufacturer.  This means you’re down to either a regular 5 amp (as the power supply marking doesn’t indicate S.B. - - Slow Blow) or  6 ¼ Slow Blow when on 120 VAC or 3 Amp Slow Blow on 240 VAC.

   Confused?  So I’m I with all the information in your post. Since you’re using 120 VAC, then the 240 VAC amperage ratings don’t apply.  FYI - You’ll need less current at a higher voltage for the same wattage consumption, that’s why the current rating is half at double the voltage supplied, simple Ohm’s Law.

   With fuses, the voltage rating only applies when your fuse blows, the gap that’s left won’t arc at that voltage rating.  So, the higher the voltage rating the better the fuse.  But it doesn’t really matter if your main supply voltage doesn’t exceed the voltage on the fuse. So for the same amperage rating, a higher voltage rating can be substituted for a fuse of lower voltage rating.

   Now a word on substitution, while it can be done, it leads to the confusion you’re in.  I suspect that the reason the fuse you removed is different from what other indications you have is because the proper fuse wasn’t available.  So you should not substitute fuses of different values as this leads to confusion.

If it was my rig and only my property involved, I’d use what the schematic or manual says.  (disclamer:  I’m not telling you what to do as I don’t want to be liable for anything that could happen).  The markings on the chassis could be the rating for the fuse holder or the chassis markings could be generic, that it is the same chassis used on another equipment.

   Regarding the fuse with a spring inside and not labeled Slow Blow.  I don’t think it’s a Slow Blow if  it’s not labeled Slow Blow.  The spring probably just ensures a wider gap when the fuse element blows.

   Hopefully someone with better knowledge of the Drake T4XB/R4XB can give you a better answer.

   73 – Cal NH7S.

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WB2WIK
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« Reply #5 on: November 30, 2001, 10:54:44 AM »

Use a 6A S/B fuse.  I'm surprised the 4A fuse lasted as long as it did, it was probably getting warm and eventually opened; it probably should have opened years ago, if you used the T4XB frequently.  

The T4XB actually draws as much as 375Wdc from the AC-4 power supply.  Assuming 70% efficiency in the power supply (it's an old-fashioned, linear type supply whose efficiency actually gets worse at lower power levels), it would need to draw ~535VA from the 120Vac line.  That's ~4.5A, considering a power factor of 1.0

That a 4A fuse survived this application is amazing.  I would agree with the manual's rating of 6.25A S/B; but a 6A S/B should work fine.  The "313" is part of a Littlefuse product designation, I believe.  Fuses of other manufacture should work okay.

73 de Steve WB2WIK/6
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AC5E
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« Reply #6 on: November 30, 2001, 12:17:09 PM »

HI: Steve's right on - but let me add that the 3AG marking indicates case style and size. A 3AG fuse won't fit any other style of fuse holder properly, and vice versa.

  73  Pete Allen  AC5E
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KA3ZMN
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Posts: 35




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« Reply #7 on: November 30, 2001, 01:17:36 PM »

Well,  as of a few minutes ago,  I had decided to go with the 6-amp fuse.  I received lots of comments (and one from someone who sounded like they knew Drake equipment).  And, the consensus was to follow the schematic, even though the fuse holder on the back of the power supply said 5 amps.

Someone else, though, mentioned that on the side of caution, I should try the 5 amp first to make sure that my powersupply doesn't blow right away (i.e., that nothing is wrong with the powersupply..  If I did that, and things worked ok for a session, then I could move to 6amps to then be on the safe side.

Does that sound reasonable?  Or should I just go with the 6amp and be done with it?

Thanks to everyone for your advice!

Steve


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AC5E
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« Reply #8 on: November 30, 2001, 03:02:32 PM »

Hi again Steve: I don't blame you for getting confused. Personally, I'd go with a 5.0 Amp SLO-BLO
fuse.

   Why? First, because six amps are a bit harder to find. Second, because 5 amps at 117 volts is just under 600 watts. That's plenty for your Drake. And the extra peak capacity of a SloBlo will let your filters charge without blowing a fuse. And if you have something shorted, either size, fast or slow, is small enough to blow "instantly" and protect your power transformer.

  73  Pete Allen  AC5E
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KL7IPV
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Posts: 984




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« Reply #9 on: November 30, 2001, 04:53:20 PM »

Steve,
    I think what I failed to see is if the transmitter itself has a fuse too. Normally the power supply is fused at a higher amperage than the transmitter so if the transmitter fuse doesn't blow it wom't take the power supply to radio heaven with the transmitter. Before you put the Slo-Blo in (that is what you should us), check the transmitter for a fuse and be sure it is the correct size as well. The transmitter fuse should blow first and be a fast acting fuse not a slo-blo type. But follow the book for each type of fuse for voltage and current. The reason the 4 amp lasted so long is maybe because you hadn't used the full output of the transmitter which also means lower current draw. When you finmally loaded the tranmsitter to full power, it took the 4 amp fuse out. It should have also taken the tranmsitter fuse too unless the transmitter is fused correctly. Good luck
73
Frank
KL7IPV.
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N8FVJ
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« Reply #10 on: December 08, 2001, 09:38:31 AM »

I am surprised at the 6.25 ampere 'slow-blow' rating and the 5 ampere 'normal-blow' rating. Usually the ratings would be the other way around, that is 6.25 ampere 'normal-blow' and 5 ampere 'slow-blow'. A 'slow-blow' fuse will pass a greater current vs time as compared to a 'normal-blow'. I use a 5 ampere 'slow-blow' fuse.
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KL7IPV
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« Reply #11 on: January 16, 2002, 03:15:08 PM »

The slo-blow and fast blow ratings are correct. The Slo-blo is used in the power supply and a higher rating than the fast blow in the radio itself. The fast blow fuse is blown if the radio has a temprary and immediate problem and allows for easier access to the blown fuse and protects the power supply as well. If the slo-blo is blown at the same time as the radio fuse, it could indicate a more serious problem or an improper amperage used. The schematic should call for a 5A fast blow in the radio and 6 1/4A slo-blo in the power supply. The 250V rating is the voltage carrying capacity and is okay to use in a 120V circuit. If you tune the radio to a harmonic rather than the proper frequency you will draw more current than the rating of the radio fuse and it will blow, the power supply fuse will not and does need to. If you use a fast blow fuse in both places you will blow them both at the same time since there will be no delay in the power supply fuse to allow the radio fuse to blow first.
73
Frank
KL7IPV
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