Repair of SMPS for Cambridge Audio x300 subwoofer - Page 1
Forum section: electronics / repair

author: hooverphonique
author_rank: Contributor
timestamp: May 21, 2024, 09:18:43 pm
content: 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?

author: perieanuo
author_rank: Frequent Contributor
timestamp: May 22, 2024, 07:27:23 am
content: 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

author: xavier60
author_rank: Super Contributor
timestamp: May 22, 2024, 08:20:05 am
content: 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.

author: hooverphonique
author_rank: Contributor
timestamp: May 22, 2024, 12:28:04 pm
content: Quote from: perieanuo on May 22, 2024, 07:27:23 amDid 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

author: hooverphonique
author_rank: Contributor
timestamp: May 22, 2024, 12:37:40 pm
content: Quote from: xavier60 on May 22, 2024, 08:20:05 amI 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 amIt 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 amR54 is already in series with D22 (10R), or did you mean between D22 and C28?

author: xavier60
author_rank: Super Contributor
timestamp: May 22, 2024, 12:41:35 pm
content: Quote from: hooverphonique on May 22, 2024, 12:37:40 pmYes, between D22 and C28. Just to see what happens.

author: xavier60
author_rank: Super Contributor
timestamp: May 22, 2024, 12:59:58 pm
content: I think the 15V isn't meant to supply much continuous current.Is there something else that's supposed to take over powering of the control circuit when the main supply starts?Edit: but then Q2 would not be able to turn off the main supply.

author: hooverphonique
author_rank: Contributor
timestamp: May 22, 2024, 02:38:23 pm
content: Quote from: xavier60 on May 22, 2024, 12:59:58 pmIf I power the 15V circuit with an external power source, it pulls around 35mA, so that's probably ok.No, this flyback psu powers everything low power all the time, including the pwm controller for the high power psu (when STB is pulled to 5V).

author: MathWizard
author_rank: Super Contributor
timestamp: May 23, 2024, 05:57:03 am
content: The D22 looks more like a low voltage supply tap for the output side. Q2 doesn't look like it's used for regulation, as a series pass element to draw more/less current and load down the windings. But the output side chip could monitor it's voltage and then send info to the primary side chip, I guess over some opto off schematic, to change switching speed or duty cycle, etc.So where's the rest of the schematic ?

author: hooverphonique
author_rank: Contributor
timestamp: May 23, 2024, 07:56:01 am
content: Quote from: MathWizard on May 23, 2024, 05:57:03 amQ1 and Q2 are only there for the standby function. There's no feedback from secondary to primary side - the flyback uses its primary side controller supply winding also for voltage sensing on pin 4.I've attached the full schematic for your pleasure.. Please note that it says x200, but the psu is the same for the x300.