My C128d (plastic) just died and I need help

[Overmann]

Hi all,
My beloved C128 just died. It's the first computer me and my brother got and it's been with me all these years. I was playing around with it the other day and it suddenly just died. I heard a loud SPARK and the screen went white. At first I thought the monitor had failed (since the sound sounded like it came from the monitor speakers) but then I saw that the C128 had turned off, and would not turn on again.

I opened it up and all looks normal. No smells, no noticable burns or exploded components. I then opened the PSU and I saw that the first fuse had blown. I replaced the fuse and tried turning the machine on but It wouldn't, that fuse had blown as well.
I uploaded pictures and schematics to the PSU that I have here:
imgur link

I hope someone here has any thoughts or ideas. I'm not hugely competent with electronics and I do understand that I might need to get someone local to help me with this one, but I desperately need to get this computer working and any diagnostics work I can do would be great. This machine is part of the family, and I'd be damned if I let it die before I do!:huh:

 

[Caleb Hansberry]

Do you have a multimeter you can measure the PSU output with so you can see if the issue is the PSU or the computer?

 

[Overmann]

Yes, but when I unplug the PSU from the motherboard it still blows a fuse. I've been poking around with the multimeter to test for continuity and to my surprise found that there is continuiti between the positiv and the negative pole on the power socket (into the PSU). I don't know a lot about electronics, but that cant be right?

The PSU seem to contain the following caps:
2x 50v 10uf
1x 50v 1.5uf
1x 358v 100uf
1x 16v 1000uf
1x 16v 470uf
3x 16v 2200uf

 

[Overmann]

I've been poking around and desoldering components. First the big 385v 100uF capacitor, still continuity, then the other smaller capacitor on the highV side (50v 10uF), still continuity, and finally the choke filter. Now things seem to be beeping in the right places. No continuity between plus and minus all over the place.

The choke is an RN232-1.0/2 and it says 2x 18 mH on it. Does that mean that the newer RN232-1-02-18M which says just "Inductance 18mH" will not be a replacement? Do I need a 36 mH choke to replace this one?

 

[giobbi]

I've been poking around and desoldering components. First the big 385v 100uF capacitor, still continuity, then the other smaller capacitor on the highV side (50v 10uF), still continuity, and finally the choke filter. Now things seem to be beeping in the right places. No continuity between plus and minus all over the place.

The choke is an RN232-1.0/2 and it says 2x 18 mH on it. Does that mean that the newer RN232-1-02-18M which says just "Inductance 18mH" will not be a replacement? Do I need a 36 mH choke to replace this one?

I don't know anything about the C128 PSU, so I'm not sure what this filter serves for and where it was put in the electrical schematic; usually they put some are filter to avoid interference on the AC supply; are you sure that simply removing the filter it will not work the same?

 

[Overmann]

I don't know anything about the C128 PSU, so I'm not sure what this filter serves for and where it was put in the electrical schematic; usually they put some are filter to avoid interference on the AC supply; are you sure that simply removing the filter it will not work the same?

Yes, the PSU should work without the filter, and it might stricly not be needed, but I don't feel comfortable just leaving it out and wirejumping those points on the PCB. The filter must have been put there for a reason, even if that reason was dirly 80's power mains.

 

[giobbi]

Yes, the PSU should work without the filter, and it might stricly not be needed, but I don't feel comfortable just leaving it out and wirejumping those points on the PCB. The filter must have been put there for a reason, even if that reason was dirly 80's power mains.

sometimes the reason is because afaik in the USA every eletrical and electronic device must to be FCC compliant. Those aluminum papers inside VIC-20 and C=64, that make your ICs overheating and burning, and should be removed asap, served exactly for the same reason.

But of course I'm just guessing about that!

 

[iz8dwf]

I've been poking around and desoldering components. First the big 385v 100uF capacitor, still continuity, then the other smaller capacitor on the highV side (50v 10uF), still continuity, and finally the choke filter. Now things seem to be beeping in the right places. No continuity between plus and minus all over the place.

The choke is an RN232-1.0/2 and it says 2x 18 mH on it. Does that mean that the newer RN232-1-02-18M which says just "Inductance 18mH" will not be a replacement? Do I need a 36 mH choke to replace this one?

If you remove the common mode choke, there won't be any continuity on the primary side anymore, it's like you cut all the power input to the rest of the PSU. The short is likely somewhere else, likely on the bridge rectifier. If the bridge is shorted, it's usually not the only fault also.
You're not going to fix anything by blindly removing parts I'm afraid.
Find someone near you that can repair it or find a matching replacement.

Frank IZ8DWF

 

[Overmann]

If you remove the common mode choke, there won't be any continuity on the primary side anymore, it's like you cut all the power input to the rest of the PSU. The short is likely somewhere else, likely on the bridge rectifier. If the bridge is shorted, it's usually not the only fault also.
You're not going to fix anything by blindly removing parts I'm afraid.
Find someone near you that can repair it or find a matching replacement.

Frank IZ8DWF

Well, I've wirejumped the connections left open when i removed the choke of course. If I didn't I wouldn't know if removing the choke fixed the problem. The choke had continuity between all pins. I'm not getting the same plus-and-minus-represented-everywhere anymore, but there might of course still be issues. I'll have to get help if this fix doesn't work i suppose.

 

[iz8dwf]

Well, I've wirejumped the connections left open when i removed the choke of course. If I didn't I wouldn't know if removing the choke fixed the problem. The choke had continuity between all pins. I'm not getting the same plus-and-minus-represented-everywhere anymore, but there might of course still be issues. I'll have to get help if this fix doesn't work i suppose.

A shorted choke would be a rather unusual fault. However the PSU should work even without it, if we are talking about the one at the AC input.
These are usually wound on separate bobbins, so it's difficult they can short. Please post a photo of it, if you can. It must have made a good
burning marks where it shorted.
Frank

PS: sorry, I should have read better all you wrote.

 

[Overmann]

I've picked up a replacement choke today and yeah, that didn't solve the issue.. So I guess I WILL have to get some help with this.
@iz8dwf there was no burn marks actually. I would have taken a picture but I threw the old one out yesterday. I didn't think any of it. Damn, perhaps it wasn't bad after all. Well, there was continuity on the pins that were on the same side, so thats not right..
Either way the choke clearly wasn't the only problem, and I'm back to square one. There should be someone a town over or so who can deal with stuff like this. I'm out of a job at the moment so I'm hoping to NOT have to buy a replacement. These C128D PSU's are hard to come by.

 

[Hugo Holden]

You remarked:

I hope someone here has any thoughts or ideas. I'm not hugely competent with electronics and I do understand that I might need to get someone local to help me with this one

Also you said that the capacitors you tested had "continuity".

Capacitors do not have continuity, they are charge storage devices and do not pass direct current. They have an impedance to alternating currents.

It is nearly impossible (unless through freak luck) that a person can repair a typical computer power supply without understanding the basic Physics of devices like capacitors, resistors & inductors etc, then there is the required understanding of the properties of semiconductors, and the issues of poking around in high voltage circuitry (not without safety risks), test gear and the interpretation of the data from that gear.

After all, the best instrument in the laboratory is the thing between the ears. It does always amaze me how confident people are to have a go, without the required knowledge or experience, on a complex task. Still, I actually admire that bravado, as we all once had that when we were young and learning.

Smps power supplies can be tricky to fault find, even or the experienced and it sounds from what you have said you need help fixing it. I would avoid any more interventions at this point and seek a person local in your area who knows what to do.

 

[Overmann]

You remarked:

I hope someone here has any thoughts or ideas. I'm not hugely competent with electronics and I do understand that I might need to get someone local to help me with this one

Also you said that the capacitors you tested had "continuity".

Capacitors do not have continuity, they are charge storage devices and do not pass direct current. They have an impedance to alternating currents.

It is nearly impossible (unless through freak luck) that a person can repair a typical computer power supply without understanding the basic Physics of devices like capacitors, resistors & inductors etc, then there is the required understanding of the properties of semiconductors, and the issues of poking around in high voltage circuitry (not without safety risks), test gear and the interpretation of the data from that gear.

After all, the best instrument in the laboratory is the thing between the ears. It does always amaze me how confident people are to have a go, without the required knowledge or experience, on a complex task. Still, I actually admire that bravado, as we all once had that when we were young and learning.

Smps power supplies can be tricky to fault find, even or the experienced and it sounds from what you have said you need help fixing it. I would avoid any more interventions at this point and seek a person local in your area who knows what to do.

I have a hard time knowing where to start with this comment. But I'll give it a try:
I never intended to imply that the caps had continuity, but that there was continuity between positive and negative (on the pwr input). I might have mistyped or you might have misunderstood, it makes little difference.

I've been trying to learn electronics and basic repair tasks for a while, and always find it's good to be upfront about my lack of knowledge, but also take it for granted that the explenation of my issue will inform people of that fact. It's not BRAVADO to try to learn something, and it's not BRAVADO to seek out the advice of people more competent and knowledgable. As someone who has needed to learn several completely different skillsets throughout my life it's absolutely necessary and absolutely not something that seeks your admiration.

Advice, like what I got from giobbi and IZ8DWF, is not really expected to solve all my problems, but rather to help me understand where I need to focus my attention, what I need to look into and understand, and which topics I should try to learn more about. And because of that threads like this, while specific in nature, might help other people learn more about related or unrelated topics in the future.

It's all about wanting to learn, and a few being so generous with their time that they are willing to teach. If anything, the last part is the aspect to admire. Not the "bravado".

 

[Hugo Holden]

It's all about wanting to learn, and a few being so generous with their time that they are willing to teach. If anything, the last part is the aspect to admire. Not the "bravado".

Don't read too much into the word bravado, it was just used as a term to convey that a person was confident, nothing more, and that is an admirable quality in my view. I had certainly tackled a lot of jobs in the past where my knowledge was not up to scratch for the task at the time, but I was somehow confident.

You had also said: "There should be someone a town over or so who can deal with stuff like this"... which led me to think you were considering having an experienced person look at it, which seemed to be a sensible option.

The other option that will work too is to learn more about power supplies, there is only so much another person can teach you, its more about self teaching & self learning from the books. If you learn the basics of the electronic components involved, everything else will start to make more sense, the schematics, the results of tests you perform etc, including any advice from people on the forum trying to make helpful suggestions. Then you may succeed repairing it yourself, which would be a rewarding experience.

From the remark you had made about the capacitor's continuity, it led me to think (rightly or wrongly and sorry if I got it wrong) that you might not understand the basics of the components, therefore perhaps making it risky, in more ways than one, for you to proceed with the repair, regardless of any advice.

 

[Hugo Holden]

.....Generally in these sorts of supplies, as shown on the schematic, the mains input, after passing via the common mode choke is full wave rectified and charges the main storage capacitor on the primary side of the transformer. This DC is chopped up into the primary by a power switching transistor. Isolated feedback from the voltages on the secondary of the supply is feed back to control the switching duty cycle of the primary which is how the supply regulates.

The main suspects if there are shorts at the line input area are the bridge rectifier and the chopper power transistor, if the main filter capacitor has already been checked/eliminated as the cause.

 

[Overmann]

I had someone much more competent have a look at it today, and it seemed to him that the power transformer was shorted. He had a pretty thorough look at it, but said that he had no such transformer and didn't know how to get hold of one.

 

[Hugo Holden]

I had someone much more competent have a look at it today, and it seemed to him that the power transformer was shorted. He had a pretty thorough look at it, but said that he had no such transformer and didn't know how to get hold of one.

Its possible but its not very likely. Generally these sorts of transformers have a low Ohmic resistance of their windings (especially compared to a usual line transformer in an analog supply), large diameter wire too and seldom does the wire insulation fail.

So on a meter for example the measurement of the resistance on the primary terminals (and the other windings on the secondary too) could look very low and perhaps trick somebody into believing they were shorted out.

Use the meter on the "beep or diode position" to check the voltage drops of each of the diodes in the bridge rectifier (should be around 0.6 to 0.8V roughly each).

If the the bridge rectifier is ok and the main filter capacitor ok,the other capacitors on the input side checked for shorts and the input is still shorted out (or low resistance) and its drawing current, blowing the fuse, then most likely the main chopper transistor is shorted out. You could confirm that by removing it and testing it on the meter. It should read like a diode (conducts one way with a drop around 0.6V and not the other) from the base to collector connections , and from the base to emitter, and no conduction in either direction from the collector to emitter. If it was shorted out (its junction melted) the collector to emitter could be shorted. It is much more likely this transistor fails, than the transformer in an SMPS.

 

[Overmann]

Its possible but its not very likely. Generally these sorts of transformers have a low Ohmic resistance of their windings (especially compared to a usual line transformer in an analog supply), large diameter wire too and seldom does the wire insulation fail.

So on a meter for example the measurement of the resistance on the primary terminals (and the other windings on the secondary too) could look very low and perhaps trick somebody into believing they were shorted out.

Use the meter on the "beep or diode position" to check the voltage drops of each of the diodes in the bridge rectifier (should be around 0.6 to 0.8V roughly each).

If the the bridge rectifier is ok and the main filter capacitor ok,the other capacitors on the input side checked for shorts and the input is still shorted out (or low resistance) and its drawing current, blowing the fuse, then most likely the main chopper transistor is shorted out. You could confirm that by removing it and testing it on the meter. It should read like a diode (conducts one way with a drop around 0.6V and not the other) from the base to collector connections , and from the base to emitter, and no conduction in either direction from the collector to emitter. If it was shorted out (its junction melted) the collector to emitter could be shorted. It is much more likely this transistor fails, than the transformer in an SMPS.

Thank you for that post, Hugo! That was very interesting. I'll get on that as soon as I figure out where the bridge rectifier is. :D
I was in the process of testing tracking the signal when I read your post. I just desoldered the transformer to see if I can check on it a little when not on the PCB.
View attachment 61411

The purple and red lines show continuity to the transformer, and the green and black lines show continuity between the pins on the transformer itself.

EDIT:
Ok, I found what must be the bridge rectifier. I measure 0.06v between the diodes. Whch is a lot lower then 0.6v. Does that mean that, if nothing else, then at least my bridge rectifier is bust?

 

[Hugo Holden]

EDIT:
Ok, I found what must be the bridge rectifier. I measure 0.06v between the diodes. Whch is a lot lower then 0.6v. Does that mean that, if nothing else, then at least my bridge rectifier is bust?

That is very suspicious.

One thing to avoid any mix ups is to use the meter you have to test a known good silicon diode of any kind, like a 1N4001 etc, or a 1N4148, from spare parts or some defunct pcb, and ensure that the meter is correctly reading a good diode. If it shows a known good diode to read around 0.5 to 0.7V (very roughly) then if the diodes in the bridge give a drop of 0.06V on the same meter, the bridge rectifier definitely is shorted out. There are 4 diodes in the bridge of course that need testing. It might pay to remove the bridge rectifier for testing in case of interactions with other components if they are faulty too.

 

[Overmann]

Ok, I had another look at it and now I have it in a holder and I do indeed measure just over 0.5v. There are four pins, -~~+, measuring - ~ = 0.06v, -~=0.5v, ~~ = 0.5v, ~+ = 0.06v, +- = 0.5v and +~ = 0.5v
That was a difficult and confusing way of writing it.. All points measure 0.5v except the + and - to it's closest ~, where it measures 0.06v. I had to clean the pins a little, as I was having trouble getting readings before and that gave me the 0.5v where previously I had .0L on those pins.



I suppose that means it's actually fine.. So I should start looking for that main chopper transistor.
Would this be it?:


On that guy, assuming it's correct, I get .058v from base to emitter and collector, and 0.05v from collector to emitter, desoldered.