Corsair CX 430 with problems - Help diagnosing modified ATX PSU - Page 1
Forum section: electronics / repair

author: GreatField
author_rank: Newbie
timestamp: September 29, 2025, 03:27:12 pm
content: I need some help to understand and (hopefully) repair an ATX power supply. My electronics knowledge is quite basic, but I’ve been investigating and found some strange things.This PSU uses the CM6800 supervisor IC.The CM6800 gets its power through Q10.Normally, there should be a zener diode (ZD4) in this circuit, but in this unit someone replaced it with a “562” resistor (5.6kΩ).From what I’ve seen in pictures of other boards, ZD4 is always a zener, not a resistor. So it looks like a modification or a repair attempt.When I bought this PSU, it worked intermittently (sometimes turned on, sometimes not), and later it stopped working completely. I bought it from a guy who had tried to repair it.Right now, the PSU can enable the initial voltages, but it never asserts PGO/PGI (Power Good signals).My main questions are:What is the actual role of this resistor instead of the zener?Why can the PSU still bring up the voltages, but never output valid PGO/PGI?Could this modification be the reason for the unstable behavior?Any advice or pointers on how to properly restore this circuit would be greatly appreciated.See the ZD4 component on a fully working Corsair CX430 and compare it to the ZD4 area on the faulty unit.On the working board ZD4 is a Zener diode; on the faulty board it was replaced by a resistor marked “562” (5.6 kΩ).see the schematic CI PS229 and the voltages measured on board CX 430 with problem:

author: GreatField
author_rank: Newbie
timestamp: September 29, 2025, 04:57:12 pm
content: I have another Corsair CX430 that works, and its PGI measures 1.24 V.On the faulty unit, PGI measures 1.15 V.I believe the problem is on the PGI line.

author: MathWizard
author_rank: Super Contributor
timestamp: September 30, 2025, 02:23:23 pm
content: I'm looking at the schematics I made of a TX750, and I even have a zener labelled ZD4A, and I used a 20V one. I seem to remember a lot of 16 and 18V ones on these Corsair PSU, IDK about 20V. If that's right tho (if I measured it), it would be an over-voltage protection.These LV Vcc rails are usually around 10-15V, and the voltage is mainly set by the switching transformer winding and inductance ratio's. Here in NorthAmerica they use 170VDC as the input, and about 12-15V out. And then if it goes too high, this zener will start conducting, and cap the voltage.When working on these in the past, I remember expecting these zeners on these LV rails to be active regulators, but they weren't, they were just OVP.On these PSU's, there is a mini SMPS built in to the main PSU, that makes the LowVoltage Vcc on the primary side, and also 5V StandBy on the secondary side.The LV Vcc comes up out of a winding on a small switching-transformer, goes through a rectifying diode, and can then go through another diode right to the Vcc of the 8-pin LV/STBY switching IC, or through another diode and a 2R2 resistor and into a 100uF cap, that is the Vcc rail for the CM6800 and other stuff on the primary side.The ZD4A zener is in parallel with the 100uF capDo you have an oscilloscope ? If you could monitor the output rails on startup, and the PGI/PGO and opto signals from side to side, that should help narrow down when/where the issue is.What's the controller on the output side doing ? It can sense problems, and shut everthing down too, and those signals pass across the opto's. There's usually 3-4 of them, one for turning full PWR-ON, 1 for voltage feedback from the 12V rail, and 1 for feedback from the 5V STBY rail. And the controller on the secondary side can also be tied in to any off them or have it's own separate one, to reset or shutdown if there's a problem.Do you use the circuit simulator LTSpice ? I can upload a similar schematic if you want.