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my xt clone stopped recognizing the keyboard

Ole Juul

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An excellent meter. It's been my only multimeter since the early 80s. I've only just recently upgraded to a Fluke 115, mainly to get 'true RMS' functionality (because of non-sinusoidal AC I'm encountering).
I don't know what a Fluke costs, but if I had to measure non-sinusoidal AC that was a single frequency, then I think I'd go cheap and just use a notch filter. Audio distortion is often measured that way. (If you're poor) :)

Regarding the "0" and "1" resistance, or whatever, what the OP needs to know is that a good fuse measures zero resistance, just like a dead short, whereas a blown fuse measures infinite resistance.
 

modem7

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I don't know what a Fluke costs, but if I had to measure non-sinusoidal AC that was a single frequency, then I think I'd go cheap and just use a notch filter. Audio distortion is often measured that way. (If you're poor) :)
A filter would remove components of a non-sinusoidal waveform that an RMS measurement needs to include.
So imagine measuring the RMS voltage of a non-sinusoidal waveform such as a near square wave.
The measurement needs to include the fundamental frequency, the harmonics, etc.
 

Lutiana

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ill test it as soon as i can, but if it is blown should I jump it

I did that once to a PSU. The damn thing blew up when I applied power to it, so I learned the hard way not to just jump fuses unless you know exactly why they blew.

With regards to the meter and working out if a fuse is blown, some of the low end meters have an audible continuity checker. Mine from Sears does. I'd highly recommend getting one that does, it really takes the guess work out of it and you don't need to look at the meter to see if it reads something, the chime indicates continuity. It so happens that on my meter the same setting can be used to test Diodes, very usefull. And I doubt the meter cost more than $20 (it was a gift). Craftsman 82141 FWIW.

On another note, did you try and use the machine on a modern KVM with an adapter? I find that his blows out keyboard fuses every time for me.
 

Chuck(G)

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My shorthand for an open-circuit reading of "1" is based on the property of almost every DMM that I've ever seen to display a single left-justified single "1" digit for an out-of-range value.

(One of the reasons that I dislike auto-ranging DMMs; when you get a reading, you have to look carefully to see what range has been auto-selected. If, on the other hand, I use a non-auto-ranging meter set on the 20 volt range, I know that "1" indicates that I've got a voltage that is much higher than I expected. Sometimes too much information is a hindrance).

I like DMMs with backlights and audible continuity testers. In the case of backlights, just be sure to get one where the backlight stays on long enough for you to see the number--some turn the light on for ridiculously short times.

And finally, it's worthwhile right off the bat to understand how the current measuring mechanism in your DMM works. Too often I've seen novices plunk the probes right across a power supply and *poof", the internal fuse is popped. Some DMMs don't use a fuse on the high-current (e.g. 20A) range and that can lead to meter melt-down.
 

bettablue

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I would love to find a good old fashioned needle movement multi meter. I used to have one of Radio Shacks best and miss it dearly. Is there by chance anyone who has either a Simpson or GOOD Radio Shack model I could possiby buy? If so, how much?


I'm not sure if autoranging (I use a Fluke 77, FWIW) is all that great, particularly in the hands of a novice. I find myself locking the range when doing serious work--I know what the reading should be and am looking for something unexpected. Autoranging just slows the process down. For almost all vintage work, a cheapie will probably be just fine. Save the money for a logic probe and some other useful tools.

But then, I cut my teeth on a Simpson 260. Still one of the finest analog meters around. Heck, I still own a working VTVM.
 

mikey99

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Simpson and Triplett are both good analog meters ......there are many for sale on the auction site
at reasonable prices.

I have a Triplett 630 and a Simpson 260 that I use regularly. These meters use two batteries
.....a 1.5 V 'D' cell, and a 30 v battery. The 30 v battery is rather expensive and only needed
when using the higher resistance ranges. So it's possible to use the meter with just one 'D' cell
installed... provided you don't need the higher resistance ranges.
 

Chuck(G)

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Indeed, the Simpson 260 is still manufactured. If it's an analog meter you need, I can't recommend one more highly. Get one with a mirrored scale, however--it improves the accuracy of your readings.

VOMs aren't as useful as they once were, however--it's that nasty "ohms per volt" resistance that gets in the way of measuring anything on today's low-voltage MOSFET-input solid-state circuits. If that's a concern, a VTVM or an old "FET VTVM", such as a B&K 290 might be useful.
 

redhawk579

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I did that once to a PSU. The damn thing blew up when I applied power to it, so I learned the hard way not to just jump fuses unless you know exactly why they blew.

With regards to the meter and working out if a fuse is blown, some of the low end meters have an audible continuity checker. Mine from Sears does. I'd highly recommend getting one that does, it really takes the guess work out of it and you don't need to look at the meter to see if it reads something, the chime indicates continuity. It so happens that on my meter the same setting can be used to test Diodes, very usefull. And I doubt the meter cost more than $20 (it was a gift). Craftsman 82141 FWIW.

On another note, did you try and use the machine on a modern KVM with an adapter? I find that his blows out keyboard fuses every time for me.

i used an original ibm 5150 (pc) keyboard
 

Ole Juul

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A filter would remove components of a non-sinusoidal waveform that an RMS measurement needs to include.
So imagine measuring the RMS voltage of a non-sinusoidal waveform such as a near square wave.
The measurement needs to include the fundamental frequency, the harmonics, etc.

Gotcha, I see my mistake now. :) (And I thought I was being so clever- lol)
 

modem7

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the fuse is blown what do you think could have caused it
One possible cause: Your keyboard is shorting out the +5 volts fed to it.

To test that:

1. Refer to http://www.beyondlogic.org/keyboard/keybrd.htm
2. About half way down the page is shown the pin configuration for the '5 Pin DIN' keyboard plug.
3. On your keyboard's plug, measure the resistance between pins 4 and 5.

On my good 5150 keyboard, I measure about 1K ohms.
What does yours measure?
 

modem7

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I would expect that the 5 volts goes through the fuse and from there goes directly to pin 5 of the DIN connector. But there could be components on the motherboard after the fuse (such as a capacitor), one of which is faulty.

With power turned off, and the keyboard disconnected from the motherboard, what resistance do you measure between ground (e.g. power supply case) and the keyboard end of the blown fuse?
 

Chuck(G)

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Personally, I'd just get a few 1A fuses and try them. If the first one blows, you know you still have a problem. If not, don't worry about it.

Tantalum and some film capacitors tend to be "self-clearing" faults--they blow up and leave an open circuit, but before they do, they draw a lot of current. I'm sure you've got some of those in your keyboards.
 

redhawk579

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Personally, I'd just get a few 1A fuses and try them. If the first one blows, you know you still have a problem. If not, don't worry about it.

Tantalum and some film capacitors tend to be "self-clearing" faults--they blow up and leave an open circuit, but before they do, they draw a lot of current. I'm sure you've got some of those in your keyboards.

ill try the 1 amp fuse thing
 

Chuck(G)

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all my tantulums are good on this one

FWIW, I said "tantalum and film". But not all tantalums fail explosively. Sometimes, they just clear the short and keep on going.

If you can always tell that a capacitor is good just by looking at it, then you should get a job doing just that. You'll clean up.
 

bartman2589

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For most of my basic testing I use a very simple cheap autoranging meter I got from Radio Shack several years ago, it's only got a couple of functions and the leads coil up into the flip open case, it supports AC Voltage, DC Voltage, Diode Test, Ohms Resistance, and an audible continuity test (very well suited to testing fuses as in this case). When I need a little more functionality I dig out the WaveTek Meterman I purchased back when I worked as a mechanic, it's essentially a clone of a Fluke (not sure which one though) and has features like a logic probe and ammeter functionality in addition to various fixed ranges I can choose from. Anyway for basic testing I think something similar to the cheap Radio Shack one I have is more than sufficient for all but the most serious PC enthusiasts and would probably serve the OP quite well for now.
 
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