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My C128d (plastic) just died and I need help

Overmann

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I just put in a new chpper transistor and bridge rectifier. The PSU powered on, whining and buzzing for a couple of seconds before the fuse blew again. The new chopper has shorted.
 

Hugo Holden

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I just put in a new chpper transistor and bridge rectifier. The PSU powered on, whining and buzzing for a couple of seconds before the fuse blew again. The new chopper has shorted.

The most likely reason why this would happen would be either the current rose too high through the chopper transistor, melting it's junction, or that it's collector voltage got exceeded. Previously I posted how the circuit likely operates so it requires that the other transistors associated with the chopper are in good order, were they all checked ?

Also was the chopper transistor replaced with one of an identical part number ?
 

Malc

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Also Have you checked components in the secondary side of the PSU, Capacitors / diodes etc
 

Overmann

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No, I Charles Bronsoned it and just started the PSU without testing transistors or diodes. I ordered several of these choppers (etc), but it might still might have been a vulgar thing to do.
I will be desoldering and checking the transistors and diodes tonight after I put the toddler to bed.
I was unable to find a chpper with the exact same part numer but order both the one I linked to at the previous page (GBU10K) and the one @Malc suggested (KBP206G). I ended up using KBP206G today.

EDIT:
SORRY, that was wrong. BU508AF was the chopper. Ang I acutally ended up using the GBU10K as the bridge rectifier. The bridge rectifier still appears to be OK, but the BU508AF died. KBP206G was quite a lot smaller in physical size to the rectifier that was on the board before.. It's been a looong day :p
 
Last edited:

Hugo Holden

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The three associated transistors on the primary side are very important, two, acting as a latch limit the chopper's base current and the other the emitter current, so if they are not working the chopper transistor can be destroyed. Also, any substitute part for the chopper must have suitable collector voltage ratings or it will fail immediately, which is often why for smps supplies it pays to find the exact transistor or spend time selecting a replacement with better ratings, but sometimes that can be tricky due the the many other properties of the transistor.
One solution, is to replace all the primary side semiconductors in one go with exact parts (check all R's and C's there too. If it still fails after that, you could be confident the fault is on the secondary side, though that is less likely.

When you powered it was it loaded ? If not its possible that voltage overshoot might have caused the SCR crowbar circuit, on the secondary, to deploy.
 

Overmann

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Apr 21, 2017
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102
Location
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The three associated transistors on the primary side are very important, two, acting as a latch limit the chopper's base current and the other the emitter current, so if they are not working the chopper transistor can be destroyed. Also, any substitute part for the chopper must have suitable collector voltage ratings or it will fail immediately, which is often why for smps supplies it pays to find the exact transistor or spend time selecting a replacement with better ratings, but sometimes that can be tricky due the the many other properties of the transistor.
One solution, is to replace all the primary side semiconductors in one go with exact parts (check all R's and C's there too. If it still fails after that, you could be confident the fault is on the secondary side, though that is less likely.

When you powered it was it loaded ? If not its possible that voltage overshoot might have caused the SCR crowbar circuit, on the secondary, to deploy.

I'm checking the components, as best I can, on the primary side now. Regarding the chopper I tried to find one that match, and asked here if it would be a good replacement, however I was unable to find (or perhaps understand) the different values, and what eventual differences would mean for the my PSU.
I know it's possible to find the exact chopper on Ebay, but I wanted to get it quickly and with things the way they are it takes a long time to get packages even from the UK.

The PSU was not loaded. I'm scared to connect this to my C128D when testing. There is a risk that it could cause damage, right, if it hasn't already?
 

Overmann

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IMG-20200608-WA0008(1).jpeg

So, I'm assuming the one in red is busted. It measures nothing from base to emitter. It sohuld be .65v I suppose :p
 

Overmann

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IMG_20200626_112017.jpg

I did some more testing, and as far I can tell they are checking out good so far.

EDIT: And I was mistaken, it was not the chopper I had asked about. Sorry about that. There is a lot to keep straight for me here :p
 
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Hugo Holden

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I would suggest, order the exact three small transistors and the chopper transistor. Fit them at the same time (you have checked the diodes).And order a few extras.

Load the 5V supply output with something like a 6V 21W auto lamp. Or you could buy a power resistor to load it, but the lamp gives a visual indication and has a low initial (cold) resistance which will be helpful at start up.

And try the supply again. Make sure to fit the new small transistors with the correct orientation.

If it still fails then its onto the secondary side for fault finding.
 

Overmann

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Apr 21, 2017
Messages
102
Location
Norway
I would suggest, order the exact three small transistors and the chopper transistor. Fit them at the same time (you have checked the diodes).And order a few extras.

Load the 5V supply output with something like a 6V 21W auto lamp. Or you could buy a power resistor to load it, but the lamp gives a visual indication and has a low initial (cold) resistance which will be helpful at start up.

And try the supply again. Make sure to fit the new small transistors with the correct orientation.

If it still fails then its onto the secondary side for fault finding.

Thank you, I'll do that. The chopper transistor on my PSU was a BU508A, and the one I replaced it with is a BU508AF. They are indeed not the same.. Having looked more closely at the specs. They are close however. I can't find anything new that exactly matches the BU508A so I'll have to order NOS or used to find the exact part. I'll look into getting a 6v 21w lamp.
 
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