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zombienerd
June 7th, 2017, 07:28 PM
I've got a Packard Bell 300SX with a blown keyboard fuse. I've bypassed it for now, but I'd like to replace the fuse. The fuse shows BUSS 4A, but has no voltage rating on it. I know that keyboards run at 5V, but I'm not 100% sure if they would have used a 5V fuse. Any wisdom is appreciated. I don't want to put in a fuse that is over rated for the circuit.

KC9UDX
June 7th, 2017, 07:46 PM
Any voltage rating equal to or greater than the supply voltage is good. The higher the better, actually. 250VAC wouldn't hurt. What's really critical is current rating and physical size.

4A for a keyboard? Could it be .4A?

Chuck(G)
June 7th, 2017, 07:57 PM
I'd be surprised if the keyboard drew more than 1A--probably around 100 ma. But make sure that it's a fast-blow fuse. You didn't specify a physical size or if if was attached with clips or is wired in (pigtail leads) So a 1A fuse should do nicely.

If it's wired with leads, search eBay with the keywords "pigtail fuse"

KC9UDX
June 7th, 2017, 08:19 PM
.4A is a standard size. If that's what it is, (normally it will say 4/10A, but it could also say 0.4A or .4A) that's what I would use.

Maybe I'm paranoid, but I wouldn't use even a 1A fuse there unless the caps lock light is a Mazda!

Chuck(G)
June 7th, 2017, 08:49 PM
I doubt that a 1A fuse on 5V will do much harm.

KC9UDX
June 8th, 2017, 01:01 AM
It depends how big the traces are. Yeah, it should open quick enough, but if 1A was always good enough we wouldn't have .4A (or other).

glitch
June 8th, 2017, 05:16 AM
I've got an assortment of inline fuses about the size of a 1/4 Watt resistor, sometimes called (Grain Of) Wheat fuses. I think the assortment I bought years ago spec'ed them at 50V. Bought them for keyboard/USB fuse replacement.

Side note, apparently many Far East 386/486 boards just use a wire jumper, even though the component is called out as a fuse. Often there's a position for a fuse but it has a solder jumper in parallel, which is invariably closed.

Druid6900
June 8th, 2017, 07:16 AM
I would think that a piece of wire wrap wire, of sufficient gauge (AWG30?), should do it.

KC9UDX
June 8th, 2017, 07:49 AM
Fuses might be cheaper. They're certainly more reliable. Fuses on disconnectable I/O rarely need to work; but that one time it does makes it all worthwhile.

I used to work for a guy who would rebuild rebuildable fuses with strands of wire from phone cords. >shudder<

Chuck(G)
June 8th, 2017, 08:36 AM
I've got an assortment of inline fuses about the size of a 1/4 Watt resistor, sometimes called (Grain Of) Wheat fuses. I think the assortment I bought years ago spec'ed them at 50V. Bought them for keyboard/USB fuse replacement.

The trade name for those, I believe, is "Picofuse". Still being sold. There was also a variety used in some Amstrad equipment that are in 2-lead TO-92 packages. I forget the trade name for those.

lutiana
June 8th, 2017, 09:14 AM
http://www.littelfuse.com/products/fuses/axial-radial-thru-hole-fuses/pico-fuses.aspx

Chuck(G)
June 8th, 2017, 09:24 AM
What's the TO-92 variant called? Not used in US manufacture, as far as I know--mostly European.

Found them Rohm ICP series (https://media.digikey.com/pdf/Data%20Sheets/Rohm%20PDFs/ICP%20Series.pdf)

zombienerd
June 8th, 2017, 12:22 PM
The fuse on this board is a through-hole soldered axial lead component. It's about the size of a 1/4 watt resistor. It clearly states "BUSS 4A" although there could be a decimal point that has worn off or wasn't properly screened on. Currently, I'm using some super thin solder as my bypass jumper, just wrapped around the legs of the original fuse, I figure it would melt quickly if something actually shorted.

I wanted to err on the side of caution with the replacement, I'd rather the fuse be under rated than over rated. I am thinking that 0.4A at 5V will probably be sufficient. If it pops regularly, I'll go bigger.

Off to mouser and digikey to see what I can find :)

lutiana
June 8th, 2017, 03:26 PM
The fuse on this board is a through-hole soldered axial lead component. It's about the size of a 1/4 watt resistor. It clearly states "BUSS 4A" although there could be a decimal point that has worn off or wasn't properly screened on. Currently, I'm using some super thin solder as my bypass jumper, just wrapped around the legs of the original fuse, I figure it would melt quickly if something actually shorted.

I wanted to err on the side of caution with the replacement, I'd rather the fuse be under rated than over rated. I am thinking that 0.4A at 5V will probably be sufficient. If it pops regularly, I'll go bigger.

Off to mouser and digikey to see what I can find :)

I am not sure you're going to find one rated at 5V. You may need to go 24V or 125V. But as long as it's 0.4A then you'd be good.

gekaufman
June 8th, 2017, 07:33 PM
Why not measure the current and go from there. I'd bet the keyboard draws .1 - .2A - if so it was likely a .4A fuse.

Chuck(G)
June 8th, 2017, 08:41 PM
I can only imagine what my keyboard-port powered parallel-to-SCSI adapter or my Zip drive powered from the keyboard port would do to a 100 ma fuse.

KC9UDX
June 8th, 2017, 09:11 PM
Sounds like a job for a cassette port...

Chuck(G)
June 9th, 2017, 07:31 AM
The keyboard port is a great way to steal power for an external device. There were 5-pin DIN adapters to do that--I've got a couple. Much more convenient than a separate "wall wart".

If you'll tell me where to find the cassette port on my 486 box, I'd be grateful. :)

MikeS
June 9th, 2017, 08:27 AM
Sounds like a job for a cassette port...

I doubt that his P-B has a cassette port; do they even have power?

Chuck(G)
June 9th, 2017, 10:57 AM
You have to ask yourself what the purpose of the fuse really is--and that's where I have a problem.

It can't be electrical circuit protection--the keyboard is supplied with +5 only, which, if dead-shorted, will cause the PSU overcurrent to kick out. The signal lines are straight TTL, so shorting them to +5 won't hurt anything.

About the only thing that I can think of is that there was a fear of a resistive short somewhere in the keyboard that might cause a fire. I can't ever remember a keyboard starting a fire.

Most systems don't have a keyboard fuse, so you have to wonder why this one does--and why.

KC9UDX
June 9th, 2017, 11:07 AM
Most of mine do. You'd never know they were there unless you were looking for them though. They just look like green 1/4W resistors.

I've never had to replace one. I've blown mouse port fuses though by unplugging a powered-on mouse.

Whatever the reason the system on question has one, it apparently did its job.

KC9UDX
June 9th, 2017, 11:09 AM
I doubt that his P-B has a cassette port; do they even have power?

But isn't this the reason PC/XT/AT/Clones are superior? They have those slots where you can install anything. And anything must include cassette ports.

Cassette ports at one time were the most popular place in the world to get external 5V power from on a personal computer.

MikeS
June 9th, 2017, 11:19 AM
But isn't this the reason PC/XT/AT/Clones are superior? They have those slots where you can install anything. And anything must include cassette ports.

Cassette ports at one time were the most popular place in the world to get external 5V power from on a personal computer.
Certainly in the Commodore world and elsewhere, but I didn't think that 5V was available in the PC (and compatible R-S) cassette ports?

In my experience, in pre-USB days if you needed 5V you either tapped into one of the disk drive lines or the keyboard connector unless current draw was so low that you could steal it from the parallel or serial ports.

krebizfan
June 9th, 2017, 11:31 AM
Certainly in the Commodore world and elsewhere, but I didn't think that 5V was available in the PC (and compatible R-S) cassette ports?

In my experience, in pre-USB days if you needed 5V you either tapped into one of the disk drive lines or the keyboard connector unless current draw was so low that you could steal it from the parallel or serial ports.

No power to the PC cassette port. There were a number of expansion cards intended to be used for external devices that had ADB sockets for power so the manufacturer could reuse their for the Mac design.

Chuck(G)
June 9th, 2017, 12:16 PM
Most of mine do. You'd never know they were there unless you were looking for them though. They just look like green 1/4W resistors.

I've never had to replace one. I've blown mouse port fuses though by unplugging a powered-on mouse.

Whatever the reason the system on question has one, it apparently did its job.

Oh, I know what a picofuse looks like--got a bunch of them here.

But are you talking about a fuse in the keyboard or the motherboard? A quick check of three motherboards on my pile doesn't reveal any.

lutiana
June 9th, 2017, 12:29 PM
But are you talking about a fuse in the keyboard or the motherboard? A quick check of three motherboards on my pile doesn't reveal any.


The only ones I have ever encountered were on the motherboard, usually near the keyboard plug. I've had to replace at least one before (it's the reason I a few new pico fuses somewhere). And when that keyboard port did not work on the PB, the first thing I did was look for a fuse on the motherboard and test it.

So I'd not say that keyboard fuses were super common, but I would not describe them as super uncommon either.

Chuck(G)
June 9th, 2017, 01:12 PM
I'll have to look through the rest of my pile to see if I can see any fuses.

I can see a cable short blowing one, though you'd think that a short would also just trigger the PSU overcurrent cutout.

You can see them in other places, though. For example, I note that a TV converter box that I have has the OTA RF tuner with one.

And then there's DEC--a CE that I once knew referred to their practice as "transistor-protected fuses'. :)

KC9UDX
June 9th, 2017, 02:33 PM
Oh, I know what a picofuse looks like--got a bunch of them here.

But are you talking about a fuse in the keyboard or the motherboard? A quick check of three motherboards on my pile doesn't reveal any.

Yes, I apologise by saying "you", I didn't mean you specifically.

I don't recall ever seeing one in a keyboard, all mine are on motherboards.

Stone
June 10th, 2017, 02:29 AM
I doubt that his P-B has a cassette port; do they even have power?Doesn't appear to:

http://www.wiki30.com/wa?s=IBM_Cassette_tape

zombienerd
June 10th, 2017, 12:04 PM
From my reading, the logic behind them is in case of a short in the keyboard, the fuse will blow before a trace on the motherboard melts. I'd rather it be there than not, as someone else mentioned, it being blown means that something bad happened and it did its job :)

Chuck(G)
June 10th, 2017, 01:12 PM
So, if you're protecting against that, the rating of the fuse is pretty broad. A 4A fuse will certainly blow if shorted across the 5V line. I'd probably use 2A, but that's only because I've got a lot of those.

lutiana
June 10th, 2017, 02:00 PM
So, if you're protecting against that, the rating of the fuse is pretty broad. A 4A fuse will certainly blow if shorted across the 5V line. I'd probably use 2A, but that's only because I've got a lot of those.

I strongly suspect that it's a 0.4A fuse, 4A seems VERY high, expecting 20W of power out of a keyboard port is silly.

Chuck(G)
June 10th, 2017, 02:40 PM
Agreed--my point was that a 4A fuse will do the job if the protection is against a short-circuit.

zombienerd
June 13th, 2017, 07:18 PM
Fuse has been replaced with a 125V 0.375A littelfuse.

Works perfectly, and gives me the peace of mind I was looking for ;)

I ordered 4 total, so I have 3 spares I'll probably never need.

http://www.mouser.com/Search/ProductDetail.aspx?R=0251.375MXLvirtualkey57610000 virtualkey576-0251.375MXL

KC9UDX
June 13th, 2017, 07:34 PM
You'll never know if it works perfectly unless you test it. I've heard a rumour that the military does this.