Like Hugo said those readings are very suspicious, It looks like your bridge rectifier is shorted, You should not be getting any reading between the 2 middle AC ~~ pins, Your meter should read OL both ways when measuring the 2 middle AC pins ~~ A shorted bridge rectifier will very likely not be the only problem with the PSU.
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My C128d (plastic) just died and I need help
- Thread starter Overmann
- Start date
Well, that's a shame! I was hoping that this would most likely be the only problem with it
Well, at least it's something! At least these two components need changing then.
Last edited:
rittwage
Veteran Member
Well it IS a problem, so replace that first if it tests bad and we'll see what else might be bad.
Hugo Holden
Veteran Member
Normally to kill the bridge rectifier, something else fails, for example the main filter cap fails or the chopper transistor shorts out, taking the bridge out with it, if any fusing doesn't save it that is. It would pay to replace the three parts probably; the bridge, the filter cap and the chopper transistor. Also check the other components on the primary side of the circuitry associated with the chopper transistor.
Well, that's a shame! I was hoping that this would most likely be the only problem with it
I'm following this thread, it all makes little sense. You said the power supply was checked by an "expert" and he didn't notice a shorted bridge?
However, shorted bridge doesn't usually happen as a main failure. If it's a current-mode regulator, it has probably failed to regulate (or drive correctly the switching transistor) for some reasons and killed the primary driver transistor first, this made the bridge fail shorted and finally the fuse blew.
Failing to identify the original regulation problem will cause all the same parts to fail again and again.
Frank
Gary C
Veteran Member
Certainly, when my Apple II PSU failed, it took out the large smoothing capacitors and the bridge rectifier in one go. As rectifiers are so cheap, its worth having a go.
Hugo Holden
Veteran Member
Looking at the schematic, it is fairly straightforward.
A separate secondary feedback winding phased for positive feedback generates the supply to turn on the chopper transistor. This supply from the feedback winding is via a base-bias resistor R13 to provide the base current for the chopper transistor. The chopper transistor has a current sensing resistor in its emitter that drives T4's base-emitter, and T4's collector robs the base current of the chopper transistor. So even without any feedback, there is a limit set on the chopper transistor's max collector current.
The auxiliary circuit composed of T2 and T3 is wired as an SCR. If the base drive of the chopper transistor get too high, this switches on T2 & T3, which reduce the voltage feeding R13 and hence the base drive current.
The frequency of operation depends on when the primary current rises high enough to start to pull the chopper transistor out of saturation, when this happens its a positive feedback effect as it weakens the chopper transistor's base drive. That timing partly depends on how heavy the base current of the chopper transistor is. If the base current reduces, the time that the chopper transistor is in conduction will decrease. The current via C8 at turn on starts the circuit into oscillation by providing initial base current to the chopper transistor, before the drive from the feedback winding takes over.
Any increase in the collector-emitter current of T4 will lower the duty cycle and the supply's output voltage. So even with no feedback from the secondary circuit and lost feedback control, the chopper transistor's collector-emitter current reaches a limited value.But of course if the feedback failed its likely the secondary SCR based crowbar circuit would deploy.
The output voltage is monitored by the OP amp on the secondary side , when the output voltage reaches the reference level, the OP amp drives a transistor that sends pulses via the feedback isolation transformer TR2 (in lieu of using an opto-coupler) to drive the base-emitter of T4 and it reaches an equilibrium as this feedback loop lowers the supply output voltage. When T4 conducts, this resets the SCR function of T2 and T3, by shorting out the base drive to T2.
There is plenty of scope for more than one component failure. So all the transistors, the diodes and the resistors need to be checked on the meter , to detect any remaining faulty parts, otherwise one bad part can result in a cascade of failures with an SMPS design.
A separate secondary feedback winding phased for positive feedback generates the supply to turn on the chopper transistor. This supply from the feedback winding is via a base-bias resistor R13 to provide the base current for the chopper transistor. The chopper transistor has a current sensing resistor in its emitter that drives T4's base-emitter, and T4's collector robs the base current of the chopper transistor. So even without any feedback, there is a limit set on the chopper transistor's max collector current.
The auxiliary circuit composed of T2 and T3 is wired as an SCR. If the base drive of the chopper transistor get too high, this switches on T2 & T3, which reduce the voltage feeding R13 and hence the base drive current.
The frequency of operation depends on when the primary current rises high enough to start to pull the chopper transistor out of saturation, when this happens its a positive feedback effect as it weakens the chopper transistor's base drive. That timing partly depends on how heavy the base current of the chopper transistor is. If the base current reduces, the time that the chopper transistor is in conduction will decrease. The current via C8 at turn on starts the circuit into oscillation by providing initial base current to the chopper transistor, before the drive from the feedback winding takes over.
Any increase in the collector-emitter current of T4 will lower the duty cycle and the supply's output voltage. So even with no feedback from the secondary circuit and lost feedback control, the chopper transistor's collector-emitter current reaches a limited value.But of course if the feedback failed its likely the secondary SCR based crowbar circuit would deploy.
The output voltage is monitored by the OP amp on the secondary side , when the output voltage reaches the reference level, the OP amp drives a transistor that sends pulses via the feedback isolation transformer TR2 (in lieu of using an opto-coupler) to drive the base-emitter of T4 and it reaches an equilibrium as this feedback loop lowers the supply output voltage. When T4 conducts, this resets the SCR function of T2 and T3, by shorting out the base drive to T2.
There is plenty of scope for more than one component failure. So all the transistors, the diodes and the resistors need to be checked on the meter , to detect any remaining faulty parts, otherwise one bad part can result in a cascade of failures with an SMPS design.
daver2
10k Member
Simple test first...
Remove the bridge rectifier completely and power up.
Does the fuse blow or not this time?
You should be able to measure approximately 9V AC (with your multimeter) at the appropriate pins from the non SMPS (via the second fuse).
Don't forget, there is mains voltage on the AC pins of the (now removed) bridge rectifier - so keep your fingers well away. Make sure there are no solder splashes anywhere and put the power supply back in the case before testing.
Dave
Remove the bridge rectifier completely and power up.
Does the fuse blow or not this time?
You should be able to measure approximately 9V AC (with your multimeter) at the appropriate pins from the non SMPS (via the second fuse).
Don't forget, there is mains voltage on the AC pins of the (now removed) bridge rectifier - so keep your fingers well away. Make sure there are no solder splashes anywhere and put the power supply back in the case before testing.
Dave
I understand the confusion. What I wrote was "someone much more competent then me". Not an expert, but someone who regularly brings old commodore-machines back to life. PSU's might not be within his comfort zone though. I don't know any experts, sadly.
I'm trying to learn as much as possible from all this, and I'm going to be as careful and attentative as possible. I clearly won't be able to understand how this works, but you guys are being so helpful, I'm feeling fairly confident that I'll be able to make this PSU work again. I'm sure you're all wondering why I'm not just sending this somewhere for repair, but the sad truth is that I won't be able to afford to pay anyone for repairs for a while.
"Hardship teaches a naked woman to weave" as we say in Norway.
daver2
10k Member
Don't worry - it's amazing how many people come here with an old machine that they had in their youth that doesn't work any more and absolutely zero knowledge of electronics. Usually, with joint working, miracles do happen and their machine comes back to life once more !
Some even catch the 'vintage computer bug'...
So studying up on electronics is always a good thing to see.
Incidentally, we had a very enjoyable holiday in Norway (many years ago now), but we must go back someday...
Dave
!Some even catch the 'vintage computer bug'...
So studying up on electronics is always a good thing to see.
Incidentally, we had a very enjoyable holiday in Norway (many years ago now), but we must go back someday...
Dave
Don't worry - it's amazing how many people come here with an old machine that they had in their youth that doesn't work any more and absolutely zero knowledge of electronics. Usually, with joint working, miracles do happen and their machine comes back to life once more
:D
Oh, I've been suffering from that since I was a teenager, but I've been remarkably lucky in that I've only ever had to recap motherboards, change ram-sockets/chips and program the odd bios/rom chip. Nothing difficult, and no ACTUAL electrical troubleshooting. I've been trying to learn a bit more about electrical circuits because I've been building audio devices (compressors, preamps etc.), but this PSU is a completely different. :/ It's fun though, and I'm very greatful for the help!
Hugo Holden
Veteran Member
yes that would work.
I should probably leave the chopper transistor out as well, as it's faulty, or should I leave it on for the test?
daver2
10k Member
It shouldn't make any difference at all - as removing the bridge rectifier should result in none of the switch mode part of the power supply from even being energised.
If it is out - leave it out...
Dave
If it is out - leave it out...
Dave
9V AC measured at 11.7 V (no load, obviously
Both fuses are OK.
Nah, that's about right for no load. I regularly see 9V transformers putting out 12V with no load or 12V transformers putting out 15V, for example.
daver2
10k Member
That's fine.
So we know that the problem is from the bridge rectifier onwards.
Ok, that's what we thought - but this test confirms it...
Dave
So we know that the problem is from the bridge rectifier onwards.
Ok, that's what we thought - but this test confirms it...
Dave