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idflyfish

Working on C 128 PSU

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My C 128 PSU seems to be dead. I got 0 on my multimeter when I checked for 5 VDC. According to what I have read I should check to see if a fuse inside of the power supply is blown. Is it safe to pull apart the PSU while it isn't plugged in? Is there anything I need to be aware of before I do this?

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The PSU is not like the CRT, as long as it's unplugged it should be safe to poke around inside.

 

ftp://zimmers.net/pub/cbm/schematics/computers/c128/power/ps-128.gifhas the schematic for external 128 power supply. Nothing sticks out dangerous, just be sure it's really unplugged.

 

There are some caps inside, so it's not 100% safe.

Tho for poking around fuses it's safe enough, and the power that can last in a PSU shouldn't do more than giving you a shock.

 

Just don't poke your fingers everywhere, and if the fuses are soldered, be sure to use a grounded iron tip.

Edited by CatPix

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I went down to a electronics store and the owner told me that a 2a 250v fuse is a suitable replacement for a 4a 125v fuse. I really don't know much about electronics but this didn't sound right so I questioned it. I was then told by the owner that he had "gone to college for this" and that it would be fine. A quick google search proved this is horse crap. Anyway, I ordered a 5 pack of 4a 125v fuses from ebay. I will let everyone know how it goes.

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Technically he's right - intensity varies depending on your voltage. What doesn't change is the power (watts).

2 amps X 250V is 500watts

4 amps X 125V is 500 watts

(and now you can see why most electrical hardware is given for 125V and 250V instead of 120V and 230V)

 

So technically, a 250V 2A fuse and a 125V4A fuse are both meant to support 500 watts.

However, as you can see, the intensity is diveided by two for 250V, and intensity is what will make the fuse... well, fusing, incase of problem. and in your case, if you are on a 120V grid, this mean that the intensity will be off, and your fuse might won't work as intented? I'm not sure about it..

It feels like it should go far before it should, which is probably a good thing in fact :D as I doubt your PSU will even use 500 watts :D

Anyway, it's best to stick with the original specs.

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If it blew when it wasn't plugged into a 128, there is a problem in the PSU for sure. If it blew while plugged into 128, there may be a problem in 128 that is drawing too much.

 

Sometime ago I had a flaky 64c that kept killing the fuse. After some trial and error, one of the sram was shorted out and drawing too much power.

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It blew while not plugged into the 128. Couldn't that be attributed to using a 2a 250v fuse instead of the proper 4a 125v fuse?

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I don't think so. Either way, this fuse should not fuse before the load reach 500watts. So there is a short circuit in your power supply; probably a "small" one that cause too much intensity to go between the neutral and live, and make the fuse to melt after a time.

I'd bet on either the transformer itself, or a capacitor, since it seems to be an issue on the 5V line.

 

Caps like C9, C10 (on the schematics) might have gone bad and short the circuit, makign your fuse playing his role and breaking it open.

But it could be any chemical cap, or a resistor, or a transistor. Tho the transformer itself and chemical caps (the large cylindrical ones) are the most likely to be guilty.

Edited by CatPix

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It blew while not plugged into the 128. Couldn't that be attributed to using a 2a 250v fuse instead of the proper 4a 125v fuse?

I pretty much agree with CatPix. It isn't a problem with the fuse. You have a problem in the PSU. You don't say where you're from. The PSU for the C-128 are quite different depending on if they are 120 volt North American or 240 volt European, or ????. It might be the transformer, or a regulator. It is rarely a Capacitor in these power supplies.

Are you good with a multimeter? that will tell you where the problem is.

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I pretty much agree with CatPix. It isn't a problem with the fuse. You have a problem in the PSU. You don't say where you're from. The PSU for the C-128 are quite different depending on if they are 120 volt North American or 240 volt European, or ????. It might be the transformer, or a regulator. It is rarely a Capacitor in these power supplies.

Are you good with a multimeter? that will tell you where the problem is.

I am in North America. I have virtually no experience with electronics but I do have a digital multimeter and I am more than willing to learn, though it might take me a while.

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I am in North America. I have virtually no experience with electronics but I do have a digital multimeter and I am more than willing to learn, though it might take me a while.

There are tutorials on youtube that might help you:

I hope this help you out.

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On the second and third pics, it definitively looks like that a resistor fried and cooked.

That might be what cause your fuse issue... but resistors doesn't burn like that, so it's probably more of a symptom than a cause.

I'm more and more thinking that the transformer is dead.

We'd need to know where this resistor is located in the datasheet to be sure.

Edited by CatPix

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I have no idea. Might be some resin that is put on the PCB to glue/protect some parts, or something that melted.

I don't know what the C128 US PSUs looks inside, I only have a C128D PSU and from what I heard, they are totally different things.

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I replaced the fuse again. I am going to leave the PSU plugged in for a while to see what happens. This is because the last time I replaced the fuse I did plug it into the C128 which caused a load hum through the monitor speakers. However, the fuse blew after I had unplugged it from the C128 and while I was testing it with my MM.

 

Anyway, I get 5.21v DC when testing pins 1 and 4. However, I only get 2.54v AC when testing pins 3 and 5.

 

The troubleshooting continues.

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I replaced the fuse again. I am going to leave the PSU plugged in for a while to see what happens. This is because the last time I replaced the fuse I did plug it into the C128 which caused a load hum through the monitor speakers. However, the fuse blew after I had unplugged it from the C128 and while I was testing it with my MM.

 

Anyway, I get 5.21v DC when testing pins 1 and 4. However, I only get 2.54v AC when testing pins 3 and 5.

 

The troubleshooting continues.

Your PSU board looks very different from my two 128 PSU here. obviously something else fried the resistor (and fuse). Could be the transformer, or it could also be the diode bridge (used as a rectifier)

 

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Your PSU board looks very different from my two 128 PSU here. obviously something else fried the resistor (and fuse). Could be the transformer, or it could also be the diode bridge (used as a rectifier)

 

 

 

Yes. I notice mine is very different from any images of 128 PSUs I have seen on the internet.

 

I have ordered a new one which I will promptly inspect before plugging it in.

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I received my new power supply and tested it. I was able to get 5.21v DC on pins 1 and 4 but got 2.6v AC on pins 3 and 5. This totally puzzled me until I removed the 1.6a fuse and found that it was blown. I guess I expected a blown fuse to not show any reading because that was what I experienced when the 4a fuse was blown. Anyway, I replaced this fuse and now everything works.

 

Thanks to everyone for their help. I'm going to continue learning about the innards of the C 128.

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