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# Troubleshooting a switcher.

## Troubleshooting a switcher.

(OP)
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

### RE: Troubleshooting a switcher.

Hello Smoked! It hasn't smoked yet? Has it?

It might be that there's nothing wrong with that PS. Some won't work if not loaded at least five or ten percent of rated load. Try put som load on it and see if it makes up its mind. The problem could be in the external wiring in the machine. No connection - no output.

Or scrap it and buy a modern unit. Smaller, more reliable and definitely less costly.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

### RE: Troubleshooting a switcher.

That sounds almost like a standard PC supply.  Gunnar is right, it needs to have at least a light load to regulate properly, and this may be what the chirp is... the units will turn power on, look for a proper current draw, then turn off for a second or two if none exists.  The PSs for home walkways lights work the same way.

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Troubleshooting a switcher.

Non-working SMPS, but no burns.

The power resistors you see with a little brown around them are probably snubbers. They always heat.

Chirp-chirp-chirp. SMPS when unhappy will click, buzz lightly, or chirp. These sound come from the magnetics. Could be a overcurrent/softstart cycle re-occuring. This could also occur if the minimum load requirement is not being met.

Generally, as a SMPS ages, the electrolytics slowly dry-out and loose capacitance. They may look perfectly fine, or may be slightly bulged, have the bottom rubber seal slightly bulged out, or show a faint sign of leakage. This can happen to both power electrolyics, and small electrolitics used for signal conditioning

Optoisolators can degrade with a lot of age, CTR increases and the control loop may "open" resulting in the output going (usually) higher in voltage which may cause over-voltage protection circuits to activate, making the supply then shut-down in overcurrent. A properly designed SMPS may pull little input current when it's in overcurrent mode.

Also check the thermal pads under power devices. If it's the old zinc-oxide or silicon based thermal transfer, these can dry-out. Clean it completley and re-grease, or use a more modern type of thermal pad. But, your problem doesn't sound like this. A thermal problem usually shows up under load.

Also, some older SMPS use various inrush current protection schemes so the input electrolytics don't spike the line when power is applied. These inrush circuit sometimes eventually fail making it so there no or inadequate power  on the primary circuit side.

### RE: Troubleshooting a switcher.

(OP)
Greetings Skogs!
Smoked? Negative!  Nice and clean. All the smoke is still inside.

Not loaded enough?  Dang, since all the modern supplies don't care anymore I forget that this 20 year old might.  I shall try that.

Scrap it?  That was my first thought but subsequent inspection showed that it has an 80 pin IDC connector  and about (25) 74TTL chips on it.  It plugs directly onto a mother board.

I could also mark every device on the supply. Send it in to a repair place. Pay the $800 repair charge and see which parts get replaced. Then do the the other 5 supplies the same way. LOL Keith Cress kcress - http://www.flaminsystems.com ### RE: Troubleshooting a switcher. Definitely not entertaining. For anyone. Gunnar Englund www.gke.org -------------------------------------- 100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again... ### RE: Troubleshooting a switcher. Keith, If the caps were suddenly cooked at some point (rather than slowly going bad), they may have bulged due to gas expansion. Once the pressure is gone, the can is still puffy... I would be tempted to pull one out and see what a cap meter has to say. Dan - Owner http://www.Hi-TecDesigns.com ### RE: Troubleshooting a switcher. I'm just a novice on this stuff and am interested as much as anything else in learning how you guys toubleshoot these things. So please excuse my questions if they are too basic. Looks dark in an area around what look to be two fuses on the lower right hand side of the picture. Is that an optical illusion? Overheating? Also looks like some components in that area that would make up the input rectification and filtering for the supply. Have you checked the voltage of the two big caps? ### RE: Troubleshooting a switcher. Is the goopy stuff around some of the caps epoxy? ### RE: Troubleshooting a switcher. (OP) Hi Bob. Naw, just picture lighting. The entire unit looks out-of-the-box new. With the exception of the aforementioned slight darkening underneath the board under the power resistors. There is not even a trace of dust on this 15 year old. IR; ALLLLLLLLLLlllll the electrolytics have those blobs of what appears to be RTV somewhere on their bottoms. Often between them. It is obviously factory applied in the usual business like manner. Perhaps to mitigate vibration work hardening of the leads holding a big device. I told them this is just too hard not having documentation nor a working model. They said "I think I know where to get some documentation." They also said there was another machine that was dead NOT because of the supply. And that they could probably get me a working supply. So that's where we're at on the long path to.... Keith Cress kcress - http://www.flaminsystems.com ### RE: Troubleshooting a switcher. Sheesh, if you can get a hold of a working supply, it would be easy peasy to compare major voltage points. Dan - Owner http://www.Hi-TecDesigns.com ### RE: Troubleshooting a switcher. "There is not even a trace of dust on this 15 year old.". Suggest you pull some of the electrolytics (both the big and small) and measure them. They're old enough to have dried-out, which happens with time, heat, and use. If they have, you will find the capacitance is 10% or less of what you expect. ### RE: Troubleshooting a switcher. Before it became Kaypro, and long before it died, NonLinear Systems sold a magic box that could check the health of most capacitors in situ, among many other things. I think it cost$400, when that was a _lot_ of money.  Doesn't anybody make stuff like that anymore?

Mike Halloran
Pembroke Pines, FL, USA

### RE: Troubleshooting a switcher.

Halfway along the copper-colored sink, near edge:
Is that:
- a water mark?
- a witness mark from a plating electrode?
- bleached from heat?

Mike Halloran
Pembroke Pines, FL, USA

### RE: Troubleshooting a switcher.

(OP)
Mike; Sharp there.

It looks to be an anodizing drip..  Nothing translates onto the board.

Rather than desoldering the existing caps I soldered on the back, more caps.  So if the capacitance was dropping to a point where the zeros and poles were too badly misplaced it would drag them back.  Made no difference.  I guess I'll wait for them to cough something else up.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Troubleshooting a switcher.

(OP)
I have been told they will pay several Bills for even an unsuccessful attempt.  Yes!  Release the hounds!  So I am about to commence with a full scale shotgun attack.  I will keep you posted.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Troubleshooting a switcher.

Man, wish I could find a company that would pay me money for non-guaranteed results!

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Troubleshooting a switcher.

Wow! Thrill and joy are back to Eng-Tips! We will be with you all the way - either way it goes!

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

### RE: Troubleshooting a switcher.

(OP)
Well I've been dragged back onto this baby.
I can now get it to attempt to turn on.

It's 24V supply ramps correctly as does it's 5V.

It's 12V only ramps to 8V and the supply's jungle logic trips the entire supply off in a fault condition that lights the 12V fault light.

This all transpires in about 20ms from live switch-on.

I now can hunt backwards thru the unit to see where the ~8V is coming from.

In the photo below the the red area is the lair of the 12V switcher.

The blue area is the area, including the transistor switch on the heatsink, that cuts the 12V off instantly when the fault is declared.

The Green Area is the actual 12V switcher. This includes the switches on the heatsink.  That transformer has around 10 pins on it.  (It's rumored to be + and -12V but there are no obvious -12V outputs anywhere.)

My immediate question is why does the 12V cap, (top of green area), have it's positive pin hooked DIRECTLY to one of the pins on the transformer?

If this makes sense to you maybe you can tell me what topology I'm looking at.  Then I can continue troubleshooting with one more arrow in my quiver.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Troubleshooting a switcher.

A real RS-232 port, common on machine tools, requires -12V at a few milliamperes.  They are used to drive attached printers or terminals for doing data dumps.  It doesn't have to be a particularly good supply, and may be weird.

<tangent>, not necessarily uh, unconnected.

There may also be a 120V socket in the machine tool, provided for the express purpose of powering said local printer or terminal. That socket's ground and neutral may not be connected where you think they are.

I.e., if you connect a data device to the machine's RS232 port, _and_ power the data device by plugging it into any line socket other than the one provided on that machine tool, ... you will instantly smoke the power supply of the data device, and some very expensive circuit board in the machine tool.

Twice.

I didn't believe anyone could possibly _design_ a machine tool that way.

That's when I started buying opto-isolated RS232 connectors for machine tools.  Always.

</tangent>

Mike Halloran
Pembroke Pines, FL, USA

### RE: Troubleshooting a switcher.

(OP)
Excellent point Mike.  This power supply actually has the old original RS-232 driver chip in it.

I don't believe there is any printer in the picture.  There is a paper tape reader.

Another interesting anomaly: I scope captured the signals seen on the switch hooked to the transformer.  There was total random AM hash on it, like the oscillation was uncontrolled and wild.

I doubled the filter cap capacitance - with no change.
Perhaps some small cap in the driving circuitry is bad?

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Troubleshooting a switcher.

See if that chip has a charge pump inside... I cannot recall if they made chips without them (requiring external voltage source), but if it has the pump, look elsewhere for an answer.

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Troubleshooting a switcher.

(OP)
No, its the old original "88" chip. Four drivers and nuthin else. So -12V is required somewhere. Plus since this supply runs a mother board with drive controllers and a dense CPU board something out there probably needs a small amount of -12V anyway. Makes sense.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Troubleshooting a switcher.

This is a cliff hanger! Sounds like your circuit for the presumed 12 V is somewhat unorthodox. An unorthodox circuit is what I would be looking for if several individuals fail after prolonged time. Of course, I would look for traces of high temperature also. But you did that already.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

### RE: Troubleshooting a switcher.

(OP)
Yes sir.  And since there is the zener/logic circuit for disabling the supply on any out-of-range issues there could be an overload that never gets a chance to heat up the board.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Troubleshooting a switcher.

Any news? No gnus are good gnus - or not?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

### RE: Troubleshooting a switcher.

(OP)
Ah.. I was pulled off onto a job I get no  for. (see shooting ones self in the foot)<sigh>.

I was able to get them a backup so the heat is off a bit.
I also ordered every single electrolytic capacitor ~ 30 - and they came in Monday.  Hopefully I will be back on it later this week.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Troubleshooting a switcher.

(OP)
Downdate:

All thirty two electrolytic capacitors were very carefully replaced.

The unit was powered up and now, not even the 5V logic supply functions anymore.

I was also handed 2 more identical <bad> supplies.  One has a smoking habit.  The other works just like the original except instead of a bad 24V supply it has a bad 12V supply.

If I can ever get the original running again I can at least compare the the alternate good sections between them.

I started drawing a schematic.  After about 2 hours I found myself searching for new ways to kill myself.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Troubleshooting a switcher.

KUTGW!

'smoking habit' - good one!

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

### RE: Troubleshooting a switcher.

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

### RE: Troubleshooting a switcher.

(OP)
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

### RE: Troubleshooting a switcher.

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

### RE: Troubleshooting a switcher.

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

### RE: Troubleshooting a switcher.

(OP)
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

### RE: Troubleshooting a switcher.

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

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