Troubleshooting a switcher.

[itsmoked]

I have been handed a PS out of an old but valuable machine tool. "Can you fix it?"

Argh!


It's about a 100~200 watt unit. It outputs 5V, +/-15V, and 24VDC.

I have NO schematics and only a vague field wiring diagram.
Extensive web searches for a schematic have only resulted in wasted time.

I did a very careful visual and see nothing other than the usual slight board discoloration under a couple of the square ceramic power resistors.

The fuses have never blown.

I hooked up the required 220V to it, and while cringing appropriately behind my eye protection, threw the switch.

A relay clicks and one of the LEDs lit declaring "source". A voltage check shows 5V is present on a bevy of logic chips.

But! I see no other voltages and if you listen very carefully you hear a faint chirp-chirp-chirp with about a 1.5 second period.

So my question is: Any suggestions as to what should I focus on when the oscillation doesn't seem to get off the starting blocks?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[raithrovers1]

It probably is no comfort to you - but you are keeping us very entertained!!!!
Have been there, done that and did not last 2 hours while trying to draw schematic (aaaarrrggghhhhh!)
My sympathies are with you and I am sure you will get to the bottom of this.

UPS engineer

http://www.powerups.co.uk

 

[itsmoked]

Update:

While trying to draw that schematic there were the usual fuses, common mode chokes, and EMI filter caps leading up to a bridge rectifier.

From the rectifier we head to the two 200V large caps for storing the raw material for all the various outputs.

I was very puzzled because they are in series and I expected equal voltages on them. Instead one was like 170V and the other was 35V. This seemed wrong to me. Shining bright lights thru the board made it look like the + rectifier output was supposed to be going to one of the caps but it didn't have continuity. But what do I know? This is a fikken nightmare switcher with five outputs. Clearly some Mengele of power supplies designed this Frankestien and maybe it's supposed to have unbalanced caps.

Well, comparing the original now non-working unit with one of the new partially working ones, showed that indeed those caps where suppose to be symmetrically charged to about 150V each for about 290V total. This showed that there was no continuity where there should've been. Somewhere under the filter caps the trace was open. I soldered in a jumper and BINGO!

One down. Two to go.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Dear readers. We are now entering the test-for-life phase. Will it live through the night? Will it explode when the grandfather clock chimes midnight? The tension is immense. Stay tuned!

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MacGyverS2000]

Keith,

I would expect (hope?) that enough current to open up a trace would show as (at least) minimal discoloration around the area... could the board have gone through some flexure in that area, breaking the trace through fatigue?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

Hi Dan,


The failed trace is under the body of a large cap, in no way visible.

More likely the trace was injured when I removed the caps. I am painfully aware of top pad injuries when removing components. I have a fabulous solder-sucker that is very good at de-soldering and use it with skill and cunning but these are caps with lock-in pins. I did examine each removed cap for a lost pad and there were none. I also looked at the component side for lost or damaged pads. I did not see any, but, if you've ever de-soldered a lot of components you will see that sometimes pads are gone and you can't actually tell without high magnification. These pads are also down in component valleys of the board making them hard to examine well. I suspect the pad was still there but perhaps had a break going to the trace.


I'm guessing that the suggestions to change out ALL the E-Caps was the repair needed. The replacement of all caps repaired the board. However the trace failure was caused during the replacements rendering the supply completely dead. Subsequent repair of the trace then allowed the replaced caps to do their magic.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

That's a good explanation, Keith. I think you are on your way to success! But I will hold my breath for another couple of days.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...