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Repair of SMPS for Cambridge Audio x300 subwoofer

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hooverphonique:
I'm trying to fix the psu above..

The low power part of the psu (fairly simply flyback circuit) is constantly restarting due to what I believe is the psu thinking there is an overcurrent problem..

I've attached the schematic for the low power part of the psu (psu_lo.png)..

If I put a resistive load on the + or -7V outputs (150ohm), there's no problem, but as soon as the output for driving the hi power part of the psu (through D23) is enabled and loaded (e.g. with a 1kohm resistor - the unloaded voltage of this output is ~15V), it tries to restart approx once per second..

All the diodes seem to check out, and the electrolytic caps also don't appear to be much off in capacitance or esr.
I've changed C28 and C22 (and also D22 in case it should be leaky) but that didn't make a difference.

I can't find what model the switching controller is.. it's marked what looks like 32A18.. the 2 could be a 7, the 8 a B. I've tried to reverse engineer the pinout (see IC3.png). The switching frequency seems to be ~750Hz when the circuit is unloaded (i.e. D23 removed, and nothing connected to CN3). The frequency rises with load on +-7V outputs, but the gate pulse width stays the same (~1.4us), indicating this isn't a pwm controller?

What confuses me is that the +-7V outputs seem to work fine, but the third one going to D23 immediately triggers (what I think is) the overcurrent protection set by R19 (2.0 ohm) when enabled by connecting STB to +7V. This should indicate that the problem is on the secondary side of the transformer, but but only with the output going through D23?

Loading directly on C28 +, bypassing Q1/Q2, makes no difference.

Has anyone got any ideas to why only the output going through D23 seems to cause trouble, and/or what the switching controller chip model could be?

perieanuo:
restart can also be some electrolyte capacitor like C2 (the one filtering IC supply). overcurrent should be noticed by resistive measurement on output.
smps is diagnosed with scope or with experience, choose your pick

xavier60:
It's unusual that D22 is rectifying the forward half cycle. I'd expect the rail to have poor line regulation.
Carefully check R19. A DMM with at least 0.01 ohms resolution is needed.
Try putting some resistance in series with D22.

hooverphonique:

--- Quote from: perieanuo on May 22, 2024, 07:27:23 am ---restart can also be some electrolyte capacitor like C2 (the one filtering IC supply). overcurrent should be noticed by resistive measurement on output.
smps is diagnosed with scope or with experience, choose your pick

--- End quote ---
Did you mean C22? I already exachanged that.
I do have a scope, and used it to figure out some of the things I wrote above. The experience part is probably subjective, so no need to discuss that here ;)

hooverphonique:

--- Quote from: xavier60 on May 22, 2024, 08:20:05 am ---It's unusual that D22 is rectifying the forward half cycle. I'd expect the rail to have poor line regulation.

--- End quote ---
I guess it has to be that way for the voltage to be above the shared gnd potential. But yes, this circuit probably has poor load and line regulation, but the supplied circuits seem to not care too much about that.

--- Quote from: xavier60 on May 22, 2024, 08:20:05 am ---Carefully check R19. A DMM with at least 0.01 ohms resolution is needed.

--- End quote ---
It measures 2R0 spot on (as is stamped on it) IIRC, but I'll double check.

--- Quote from: xavier60 on May 22, 2024, 08:20:05 am ---Try putting some resistance in series with D22.

--- End quote ---
R54 is already in series with D22 (10R), or did you mean between D22 and C28?

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