Another SSI Power supply repair - Page 1
Forum section: electronics / repair

author: Photoman
author_rank: Regular Contributor
timestamp: June 09, 2020, 05:45:58 pm
content: HI All,Same friend dropped off a different model of SSI power supply.This one has a strange fault. No power at all to the NE5560N switch mode chip.All of the transistors, fuse and caps check out. Though I cant seem to find the fault anywhere.As far as I can tell, there are no shorts at all on the board. Also fuse does not blow....well it does if I try to measure the MosFet with my DMM.I have injected a sine and square wave from my function gen through the AC terminals and I also tried DC.I still cant find where its stopping or why.I tried another tact. Disconnect the AC line and just power the switch mode NE5560N from my DP832.That worked. The chip started up and was outputting its sawtooth. That made it all the way to the transistors.Even stranger, the DC made it all the way to the bulk filter caps. Something that doesn’t work in the opposite direction.I figured that maybe a diode was faulty but I checked every one in the path.Though I have no way to test the 800v fast recovery rectifier diodes other the DMM diode mode and one of those Arduino tester things.For further testing, I was wondering if I could power the NE5560N externally and apply the AC to the input?I want to see if the rest of the supply is working but I don't want to blow up my DP832.On that trend, if I cant figure out why the NE5560N isn't getting power, could I rig up a separate supply permanently?Photos of the supply in question attached. I haven't cleaned the flux off and lost a few pads (not electrically connected, at least on the bottom).Thanks!

author: bob91343
author_rank: Super Contributor
timestamp: June 09, 2020, 06:43:24 pm
content: A schematic diagram would be helpful.

author: Grandchuck
author_rank: Super Contributor
timestamp: June 10, 2020, 03:59:08 pm
content: Found this in old post:The NE5560 is an old chip now, the datasheet says 1994 and some of the terminology is not the same as used today.They refer to UVLO as: Low supply voltage protection and it means the same thing. On P3 it's mentioned as: low supply voltage protection thresholds and the value to use is 10.5 V on pin 1 (VCC) relative to pin 12 (Gnd).10.5V must be present for the chip to start and continue running and SMPS will sometimes start then fail as there are generally 2 sources of current to supply the SMPS IC and the second (run) supply might not take over. For the start supply there is generally some high value resistor or chain from rectified mains, in this case I believe via R422 and then another supply from a feedback winding on the SMPS transformer. This supply (run) I believe is via R421 and fed from winding 2.The components in each supply chain need be checked as each must be OK for the IC to start and continue running.Monitor pin 1 and if it falls below the UVLO after start, there is your problem.They (the IC) can be coaxed to run with a bench supply and output then checked with a scope but this should be done without mains connection.

author: wraper
author_rank: Unknown
timestamp: June 10, 2020, 04:07:39 pm
content: QuoteIf it gets no power, you should check where it initially gets it power. Not checking random transistors or caps.Say like this:

author: Photoman
author_rank: Regular Contributor
timestamp: June 10, 2020, 04:29:32 pm
content: Thanks for all of the tips! I will check them out today.My hunting and pecking approach was based on another SSI SQM supply I fixed.It had a couple of bad caps, MosFets and suspect joints.I should have known better (Thou shalt check voltages) but I just went with my previous experience with themI also bought another SQM 250 supply on ebay to use as a comparison.It’s a different form factor so I still need to repair this one.Thanks again!

author: Vicus
author_rank: Regular Contributor
timestamp: June 10, 2020, 08:06:47 pm
content: Quote from: wraper on June 10, 2020, 04:07:39 pmQuote.

author: Photoman
author_rank: Regular Contributor
timestamp: June 10, 2020, 09:33:47 pm
content: Quote from: Vicus on June 10, 2020, 08:06:47 pmMy bad, I forgot to quote when I replied.So the saga continues. I was so excited when I found a film cap that was an order of magnitude low. Replaced, no change...Red herring.I took off the control transformer T1 and I think it may be the culprit.For reference I tried probing the pin (pinon the transformer that is connected to the emitter of the BUF420A, blew it and the fuse (I didn’t short anything).When I say probe I didn’t even have the other probe connected to anything yet. It was in my hand!I was just using my DMM (Agilent U1272A), DC volts.After taking the transformer off, I measured the resistance of the windings.Pin 1-2 are 0.7, 1-8 is 0.352-8 is 0.4, 3-6 1.08and 4-5 is 0.58.Pin 7 is NC

author: wraper
author_rank: Unknown
timestamp: June 10, 2020, 09:58:20 pm
content: Quote from: Photoman on June 10, 2020, 09:33:47 pmWhat were you trying to find there if PWM controller has no power? You can probe or replace all the parts you want but until you fix initial power delivery to PWM controller to ensure it's starting, nothing will work. Do you understand that until PWM controller starts, nothing will happen at the transformer? PWM controller may or may not have dual supply. But there should be initial supply from high voltage to ensure PWM controller starting, and looking for it at transformer is just stupid. + there may be additional rectified voltage from auxiliary transformer winding (only after PSU started). The issue with latter (considering there is no power on PWM controller at all) may be only if diode failed short and is thus shorting PWM controller supply to GND through the winding. My judgement is that you blew some parts by doing things with completely zero reason/ thinking behind your actions.