Electronics > Repair
Help for open frame dual output PSU - model FSP012-2P01
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jd213:
All of the resistors and diodes seem fine when tested out of circuit.  I believe the transistors (Q10 and IC30) are also fine since my TC1 component tester recognized them.

Would anyone have a guess as to what else could be the problem? Should I test the film/ceramic capacitors?

Not sure how to test U30 (UC3842B) or PC20 (marked L0421 817A, not sure what kind of component this is).  Are either of those responsible for the negative voltage?

Thanks in advance.
Chris56000:
Hi!

. . .PC20 is a Sharp PC817 Opto–Coupler!

. . .The best way to debug any power supply based on the UC384x series is simply to measure the d.c. voltage between pins 5 and 8 – pin 5 is power ground and pin 8 is the Reference Output, connected internally to the protection circuits within the chip!

. . .If the voltage between pins 5 and 8 is 0.5 V or less, either the chip is locked out due to an under–voltage lockout, a short within the chip itself or across the supply decoupling between pins 5 and 7!

. . .A pulsing voltage indicates the chip is tripping due to either excess current, most likely a defective output rectifier diode or capacitor, (open–circuit or high e.s.r. output capacitors also cause tripping!), or  (rarely!) short–circuit turns in the power switching transformer!

. . .Loss of, inadequate negative feedback from the output side can be due to PC20 (Sharp PC817) going low–emission, will also cause the chip to persistently trip, substitution with a new replacement will prove this!

. . .If the voltage between pins 5 and 8 is 5V approximately and the board is not producing power, then the components between pin 6 (output) and the gate (usually the left–hand leg) of the power switching MOSFET may be defective – a defective MOSFET can destroy these components , commonly a resistor from 22–100 Ω and a protective zener diode of about 15 V 400mW between gate and source of the power MOSFET.

. . .Dry–joints where the transformer lead out wires are soldered to the bobbin/PCB pins is another common cause of persistent tripping or failure to start, particularly if that lead–free muck has been used to make the original board, as is very likely – removing the transformer from the PCB and resoldering the leadout wires where they connect to the bobbin pins is the cure for this!

. . . Rectifier diodes do sometimes short out as soon as power is applied and then promptly test OK when power is removed – binning the lot and fitting suitable replacement types will eliminate this possibility – the "UF" series is suitable as replacements in most cases!

Chris Williams

Harry_22:
Hi!
The PaintNET diagram will help you.
jd213:

--- Quote from: Chris56000 on June 02, 2024, 06:17:18 pm ---Hi!

. . .PC20 is a Sharp PC817 Opto–Coupler!

. . .The best way to debug any power supply based on the UC384x series is simply to measure the d.c. voltage between pins 5 and 8 – pin 5 is power ground and pin 8 is the Reference Output, connected internally to the protection circuits within the chip!

. . .If the voltage between pins 5 and 8 is 0.5 V or less, either the chip is locked out due to an under–voltage lockout, a short within the chip itself or across the supply decoupling between pins 5 and 7!
--- End quote ---

Ah, thank you!!  Looks like it is under 0.5V, but not pulsing.  I'll try replacing UC3842B and PC817A then, doesn't look like they will cost much.


--- Quote ---. . .Dry–joints where the transformer lead out wires are soldered to the bobbin/PCB pins is another common cause of persistent tripping or failure to start, particularly if that lead–free muck has been used to make the original board, as is very likely – removing the transformer from the PCB and resoldering the leadout wires where they connect to the bobbin pins is the cure for this!
--- End quote ---

Oh yeah, I've dealt with lead-free solder before... thankfully this board is from 2005 and seems to use leaded solder.


--- Quote ---. . . Rectifier diodes do sometimes short out as soon as power is applied and then promptly test OK when power is removed – binning the lot and fitting suitable replacement types will eliminate this possibility – the "UF" series is suitable as replacements in most cases!

Chris Williams

--- End quote ---

Hmm, when read in-circuit, D21 read 150 both ways and D50 read at 43 both ways, but the readings were the same both with and without power to the board.  Should I assume they're bad?

Thanks again!
jd213:

--- Quote from: Harry_22 on June 02, 2024, 08:21:43 pm ---Hi!
The PaintNET diagram will help you.

--- End quote ---

Yes indeed, that came in quite handy when testing the board when powered on! Thanks a lot!
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