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Old 03-03-2022, 08:32 AM   #1
jayjr1105
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Default 9v super basic wall wart issue

This is one of those worthless and time wasting repairs but it's bugging me and I want to figure it out. Simple 9v 1a SMPS wall wart and it's bouncing between 8.2 and 9.4v without load. I've pulled out every single cap and all tested fine. I've replaced the opto and same exact issue. All diodes are testing fine. I'm not sure if the IC can cause this or if the IC had issues you wouldn't get anything at all on output.
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Old 03-03-2022, 01:38 PM   #2
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Default Re: 9v super basic wall wart issue

power supplies tend to have minimum load, does it surge with some load?
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Old 03-03-2022, 01:49 PM   #3
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Default Re: 9v super basic wall wart issue

I'll check later tonight
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Old 03-03-2022, 03:56 PM   #4
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Default Re: 9v super basic wall wart issue

Replace that small electrolytic C3
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Old 03-03-2022, 07:44 PM   #5
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Default Re: 9v super basic wall wart issue

Look if C3 has something to with the feedback and change it.
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Old 03-03-2022, 08:56 PM   #6
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Default Re: 9v super basic wall wart issue

I wonder if C3 might be the issue
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Old 03-03-2022, 09:04 PM   #7
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Default Re: 9v super basic wall wart issue

The 431 may be hunting.

Try a 150-1k resistor across the IR LED in the opto.
Points are marked in attached image.

Change the small cap on pri side to 22u/50v or 47u/50v.
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Old 03-03-2022, 09:04 PM   #8
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Default Re: 9v super basic wall wart issue

I pulled all caps one by one and they all checked out. I can replace it anyway but don't know what good it will do
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Old 03-03-2022, 09:37 PM   #9
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Default Re: 9v super basic wall wart issue

Quote:
Originally Posted by kaboom View Post
The 431 may be hunting.

Try a 150-1k resistor across the IR LED in the opto.
Points are marked in attached image.

Change the small cap on pri side to 22u/50v or 47u/50v.
9.21v stable. How do I know which one fixed it? Replacing C3 or adding a 330ohm resistor to the opto?

Ok, pulled the resistor off and it's still good. Pulled C3 which was a 10uf 50v cap and replaced with a Panasonic FC 47uf 50v.

Last edited by jayjr1105; 03-03-2022 at 09:45 PM..
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Old 03-03-2022, 10:53 PM   #10
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Default Re: 9v super basic wall wart issue

Soldering heat from R&R'ing C3 may have temporarily "woken" it up.
The 431 seems to need a minimum load; when an unbypassed opto LED is the sole source of current to the 431, the latter can hunt.
Leading to voltage "bounce" at no load.

Perhaps it was C3.
With the original C3 in place, leave it out in the cold a while- it'll probably act up afterwards= cap(s) is/were suspect.

In similar SMPS wall warts, I've used 22-47u/50v caps, PW/HE/FC, for C3.
If a new C3 keeps it stable, even cold, you can probably leave the opto LED unbypassed.
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Old 03-04-2022, 08:50 AM   #11
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Default Re: 9v super basic wall wart issue

Quote:
Originally Posted by kaboom View Post
Soldering heat from R&R'ing C3 may have temporarily "woken" it up.
The 431 seems to need a minimum load; when an unbypassed opto LED is the sole source of current to the 431, the latter can hunt.
Leading to voltage "bounce" at no load.

Perhaps it was C3.
With the original C3 in place, leave it out in the cold a while- it'll probably act up afterwards= cap(s) is/were suspect.

In similar SMPS wall warts, I've used 22-47u/50v caps, PW/HE/FC, for C3.
If a new C3 keeps it stable, even cold, you can probably leave the opto LED unbypassed.
The 431 is the small voltage reference SOT23 part?
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Old 03-04-2022, 05:15 PM   #12
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Default Re: 9v super basic wall wart issue

BTW, got home and still tested very stable around 9.2v.
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Old 03-04-2022, 08:01 PM   #13
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Default Re: 9v super basic wall wart issue

glad you got it fixed! I ran into a similar issue before on a different power supply where it only supplied half of the voltage.

Last edited by CapLeaker; 03-04-2022 at 08:04 PM..
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