SMPS ticking, outputs oscillating - Page 1
Forum section: electronics / repair

author: redshot
author_rank: Newbie
timestamp: December 06, 2016, 02:27:19 pm
content: Hi, I'm having a strange issue with this SMPS from a mixer that suffered an overvoltage. I've searched these forums and encountered useful threads, but I still can't figure out what's wrong with this thing.The SMPS in question is from a Peavey PV 20 mixer, which is about 2 years old. Apparently it worked fine until it suffered an overvoltage a few months ago, started working erratically and then stopped completely. This is what I did:- Initial check, 1.5 A fuse was blown. Replaced fuse and plugged the mixer again with no load, and fuse blew again;- Traced a short circuit in the TOP244YN switch, so I ordered replacements. Replaced it with a new one, turned the SMPS on with no load... And it popped the switch almost immediately, without even blowing the 1 A fuse! Fortunately I had ordered more...!- Replaced the 68 uF filter capacitor on the primary side, and 4x 470 uF capacitors on the secondary which were slightly bulged and with better voltage tolerances (25 V to 35 V)- All the other electrolytic capacitors, resistors, diodes, all seem to check out. De-soldered most of those to test evertyhing on an open circuit.At this point, the switch doesn't blow anymore, it just fails to start, ticking away every second or so. With every tick, the filter capacitor voltage drops from 340 VDC to 320 V, and then charges back up again. The outputs, with no load, oscillate between 22 and 25 V on the +-25V outputs. With the mixer connected, the LEDs blink for a fraction of a second every time the SMPS ticks.I can't identify the source of the problem. I replaced the TOP 244 yet again to make sure it worked, and it behaves the same. I'm inclined to look at the feedback circuit but I have no idea how to measure it, I only know that there is feedback, when the switch ticks, there's a signal going through the optocoupler. The secondary doesn't seem to be shorting, because all the outputs are measuring "something" (+25V, GND, GND, -25V, 48V).Pictures of the board tonight.I'm running out of things to de-solder.

author: tautech
author_rank: Super Contributor
timestamp: December 08, 2016, 01:37:29 am
content: Welcome to the forum.Like with any of these SMPS controllers you're best to grab its datasheet to see if a "typical application" shown is much the same as the circuit you're working on, they often are.http://www.mouser.com/ds/2/328/top242-250-9213.pdfP20-24 show typical circuit designs.From what you describe the SMPS is trying to start which suggests the dropper resistor (or divider) from rectified mains might be the problem.Read :Line Under-Voltage Detection (UV) on P8.Other components that I might/could/would suspect are the opto, electrolytic caps associated directly with the IC and diodes.

author: wraper
author_rank: Unknown
timestamp: December 08, 2016, 02:23:21 am
content: QuoteUnless you know for sure that overvoltage event was there, most likely there was not.QuoteWhat do you mean by "open circuit"? Just measuring out of circuit or what?Quote.Those ticks probably is overvoltage or some other kind protection kicking in. Did you check if parts in the flyback snubber is not faulty? (Like on the picture) Particularly diode(s) not shorted, capacitor(s) have proper capacitance.

author: wraper
author_rank: Unknown
timestamp: December 08, 2016, 02:33:45 am
content: Also I would certainly check if the circuit around the "C" pin has no faults, and check the whole feedback circuit too.

author: Armadillo
author_rank: Super Contributor
timestamp: December 08, 2016, 02:55:24 am
content: Quote from: redshot on December 06, 2016, 02:27:19 pmTicking - sounds like partial winding shorted or arcing.Take T1 out of the circuit and ring-it or best replace or swop one.

author: wraper
author_rank: Unknown
timestamp: December 08, 2016, 09:12:36 am
content: Quote from: Armadillo on December 08, 2016, 02:55:24 amThis is among the last things I would expect. If there was a short, then unlikely to have around 25V on the output at all. If it was arching, no way it would happen as ticking with constant pace.QuoteAs if it is possible to reasonably easy to get a replacement transformer.

author: xavier60
author_rank: Super Contributor
timestamp: December 08, 2016, 09:19:30 am
content: If the recovery after the drop in HVDC is slow enough to see it on a DMM, it could mean there is a high resistance like a damaged anti-surge resistor.

author: BradC
author_rank: Super Contributor
timestamp: December 08, 2016, 10:01:20 am
content: I had these symptoms on a F&P dishwasher recently. It was the bootstrap start cap on the switcher, so it was charging up off line but had lost enough of its mojo that it couldn't sustain the switcher until it got going. Probably not your failure, but the symptoms were regular ticking sound from the transformer and the device LEDs (and all other output rails) pulsing in sync with that.