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Author Topic: HELP! Which fuse should I use? (fuse blew on po  (Read 8977 times)

Posts: 35

« on: November 30, 2001, 09:36:31 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 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

Posts: 35

« Reply #1 on: November 30, 2001, 10:33:40 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?


Posts: 787

« Reply #2 on: November 30, 2001, 01:07:31 PM »

The description of the "spring" inside the fuse makes it most likely that the fuse was a slow blow fuse.  

The voltage rating on a fuse is the MAXIMUM allowable voltage it can be used on. Most glass fuses are rated at 250 Volts, but some that are intended for automotive use are rated at 32 Volts. You can use a fuse rated at 250 Volts on your 120 Volt circuit.

As far as the amperage rating, I'd go with a standard (not slow blow) 5 Amp fuse. If the new fuse blows right after you turn the radio on then you've got a fault somewhere in the power supply or the radio, and that's why the old fuse blew.

Don't replace the 5 Amp fuse with increasingly higher amperage fuses until you find a size that doesn't blow, because you'll likely have a smoking/flaming rig shortly thereafter.

Posts: 35

« Reply #3 on: November 30, 2001, 01:12:57 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.

But after reading your post, it sounds like I should give the 5amp one a go.  I may  well do that instead.

Thanks for your advice!


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