[SOLVED] 5VDC power supply repair

Thread Starter

cdebackere

Joined Nov 2, 2022
10
I have a dead PSU.
The obvious first issue was a slightly swollen main capa (C301), which confirmed leakage when unsoldered. Replaced it, but PSU still not delivering.

I see no obvious damage to any component.

Find attached the circuit diagram.

Since I am not at all into analog PS design, I can't find what's going wrong.


My main suspects are the OP301 and Q301 ... but any advise on how to identify/confirm would be helpfull.

I measured some voltages (with 230VAC connected, no load connected) and can measure more if needed.
Note: I cannot accurately measure below 5mV, so 0 could be anuthing between 0 and 5mVDC

- output 5VD: 0VDC
- output DET: 0.1VDC
- across C301 : 335-345 VDC (as net fluctuates)
- IC301 (TOP 266EG):
- V 4.9VDC
- D ~340VDC
- C 0VDC
- X 0VDC
- F & S 0VDC (obviusly, since reference)
- directly across R307, R308, R309 I measure each time 90-92VDC, BUT strangely (checked 5 times), when measuring from 0V(IC301-S/F) to intermediate connections I get:
- 0 to IC301.V : 4.9VDC
- 0 to R309-R308 : 96VDC (consistent with above)
- 0 to R308-R307 : 189VDC (consistent with above)
- 0 to top of R307 : 340VDC. So, minus the 189: a voltage of 151 VDC! I don't get it

- OP301, left side: 1.1VDC across terminals
- R315, R320, R321, R318 : 0VDC across
- Q301 : BE ~1.0VDC , BC ~0.9VDC
- D301, D304 : 0VDC across

Who can help me diagnose?

Cheers,
Christof.
 

Attachments

Dodgydave

Joined Jun 22, 2012
10,591
This is a switchmode psu, the 3 resistors in series( R307/8/9) are for the startup voltage for the chip, the optocoupler only comes on when the TL 431 zener fires due to the voltage set by it's reference pin, which pulls the npn on to slow the chip down.
Probably the Top266 chip has blown.
 

Thread Starter

cdebackere

Joined Nov 2, 2022
10
Another typing error .. so sorry .. the replaced capa is the electrolitic one, the C303, there is no C301
And thanks for the much more ergonomic inline schematic upload
 

Thread Starter

cdebackere

Joined Nov 2, 2022
10
This is a switchmode psu, the 3 resistors in series( R307/8/9) are for the startup voltage for the chip, the optocoupler only comes on when the TL 431 zener fires due to the voltage set by it's reference pin, which pulls the npn on to slow the chip down.
Probably the Top266 chip has blown.
Hi Dave,


I followed your reasoning, ordered the IC and after some messing around to get the old one out .. the new one got is flawlesly .. and woke the PSU back to life!

Thanks a lot for your assistance!!

BR
Christof
 
Top