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Old 10-14-2021, 07:12 AM   #21
Crystaleyes
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

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Originally Posted by reformatt View Post
The cap being open cct caused the output to rise to 100KV and blew the yoke of the TV through the top of the cabinet!
Holy crap!



The only through-hole components which still haven't been tested are these white ceramics, so that'll be done today.


I understand the differences between resistance, impedance, reactance and the effects of back emf's caused by collapsing magetic fields through coils and hence the need for diode protection, but no doubt I could brush up in this area.
As for the opto-isolators, again, I understand exactly how they work, however I am not yet on top of how, or from where, the feedback signal which passes through them, comes from...


Anyway, unless these ceramics are open, then it will time to dive into the smd section on the printed side.

If so, I'll list all the different chip, FET and transistor types and we'll see if we can't find something open.
I am assuming something has opened, as the fuse hasn't blown, eh?

Last edited by Crystaleyes; 10-14-2021 at 07:15 AM..
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Old 10-14-2021, 07:53 AM   #22
Crystaleyes
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

These white ceramics are just a TO-220 housing, containing a mystery (to me) component.

There are no markings, and this is the first time that I have encountered this set-up.

This particular one was on the same heat-sink as the two FETs which are directly connected to the transformer secondary terminals, so is on the output side of the PSU.
Even though it is adjacent it is not immediately obviously connected to these two FETs, however both sides of this component ultimately end up at pin 4 of the output connector.


Photos enclosed...

DMM readings in both directions were almost 10k in the ohms scale and around 2.6v under diode test.

The component tester reckons it is a resistor.


??

Any ideas, anyone?


..
Attached Images
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File Type: jpg P1010045.JPG (2.79 MB, 1 views)

Last edited by Crystaleyes; 10-14-2021 at 08:24 AM..
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Old 10-14-2021, 10:19 AM   #23
stj
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

thermister - for reading the heatsink temp
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Old 10-14-2021, 10:59 AM   #24
Crystaleyes
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

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thermister - for reading the heatsink temp
Nice one.

Obvious really...
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Old 10-14-2021, 04:50 PM   #25
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

I'd personally just be hitting all the solder joints with an iron first. I've had that on an iMac power supply once that wouldn't start and a blanket resolder job fixed it. I'm old school and despise lead free solder. I really don't trust it.

If you don't have shorted components everywhere on the primary side, then it's likely either a short on the output or a cap low in uf. Just ensure that anything you put in is known to be good and installed correctly. You can quickly go down a rabbit hole if you install a faulty component and then assume it's good and ignore it in your troubleshooting. Just one of the lessons you learn.....
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Old 10-17-2021, 06:22 PM   #26
Crystaleyes
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Default Re: iMac A1224 PSU, 614-0421 - no standby voltages

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I'd personally just be hitting all the solder joints with an iron first. I've had that on an iMac power supply once that wouldn't start and a blanket resolder job fixed it. I'm old school and despise lead free solder. I really don't trust it.

If you don't have shorted components everywhere on the primary side, then it's likely either a short on the output or a cap low in uf. Just ensure that anything you put in is known to be good and installed correctly. You can quickly go down a rabbit hole if you install a faulty component and then assume it's good and ignore it in your troubleshooting. Just one of the lessons you learn.....
Went away for the weekend but shall be back on it tomorrow.

Thanks for the support.
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