Electronics > Repair
Help me understand this complex flyback PSU
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F1GWR:
Central structure of the primary stage of this Artesyn NFS177-7630 (used in an HP 35670A analyzer) is reverse-engineered here : see enclosed files

Please help me understand in the daughter (= fille) board around the UC3842A how work these weird structures:
- the oscillator network with Q3 Q4 and what should I expect as a working frequency
- the voltage reference with VR1 associated with Q1 and Q2

As this PSU is completely dead (but Hi DC voltage is OK when supplied by mains (= secteur) or 12-28VDC to DC converter), my aim is to inject low cyclic pulses in the gate to force it to waken somewhat and consequently be able to locate the faulty component(s). Therefore I need to know the approximate oscillator frequency and the reference values.

Complex secondary of T1 is not depicted. The links from there are represented by T3 and the two opto-couplers. The only inspection finding is in the main board R16 that is discolored, measured 140 Ohms in circuit both ways and PCB yellowish.

Please note: Attached schematics below have been edited with error corrections since this first message has been issued. Therefore some wise remarks in the following thread may not appear consistent anymore!
wasedadoc:
Where is Vcc generated?
fzabkar:

--- Quote from: wasedadoc on July 28, 2024, 06:38:33 pm ---Where is Vcc generated?

--- End quote ---

CR5 and Q1, AFAICT.
wasedadoc:

--- Quote from: fzabkar on July 28, 2024, 06:48:13 pm ---
--- Quote from: wasedadoc on July 28, 2024, 06:38:33 pm ---Where is Vcc generated?

--- End quote ---

CR5 and Q1, AFAICT.

--- End quote ---
OK. I'd start by checking Vcc.
shakalnokturn:
Sorry for not answering all those questions.
Having repaired plenty often without going into all the details of how they're designed, like others I'd start by checking on PWM Vcc.
A fair chance that your problem is R2 or R4/5 on "platine principale".

I wouldn't recommend simulating gate control, even at low duty cycle there's potential to cause unnecessary damage.
If you must, add a series light bulb between primary bulk capacitors and transformer.

Isn't T2's primary on platine principale meant to be in series with T1/MOSFET for primary current sensing?

R16 is in the snubber, they often run hot, not necessarily a defect although it may have drifted up in value.
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