Repair of Voltage Regulated Transformer (nb later:switched mode regulated power)

[SteveGB]

Hi Guys, newby here so please be gentle! Just starting in electronics. If info stated isn't enough pls confirm what additional info is needed.

Have a 240v transformer with regulated voltage output of 5v, 7.5v, 12v etc dc. (little 'arrow' component turned via screwdriver).

It went kaput a while ago and took along to the Maker shed and changed an input resistor and hey-presto all good, seemed to work.

When I got home thought I'd check the output voltages and all seemed to correspond to stated DC voltage. May be odd but I also tested on with multimeter set to AC voltage and it was reading around double the DC voltage. Then tested for Hz and at initial switch on it it was reading 50Hz for about 2 seconds, then going to zero. This seemsed wrong.

Checked a pc regulated voltage supply and no such Hz reading, which is what I would expect.

Any idea where I should start with fault-finding???

[SteveGB]

'seemed'

[Kim Christensen]

When I got home thought I'd check the output voltages and all seemed to correspond to stated DC voltage. May be odd but I also tested on with multimeter set to AC voltage and it was reading around double the DC voltage. Then tested for Hz and at initial switch on it it was reading 50Hz for about 2 seconds, then going to zero. This seemsed wrong.

Many multimeters set to AC will give the wrong reading when measuring a DC source. I'll bet your multimeter will show around 18VAC when measuring a good 9V battery.

And the Hz reading on a multimeter is just a frequency counter. It'll trigger on fairly low ripple so doesn't really tell you a lot.

The best way to test is with a resistive load and then checking the DC voltage with your meter. Use an oscilloscope to see the ripple. If you had a high quality multimeter capable of reading AC voltages up in the 100Khz range then you may glean some useful info from that, but if the switching speed is higher than 100Khz even a fancy fluke meter is going to underreport the RMS ripple.
(The above is assuming your "Voltage Regulated Transformer" is of the switching type and not the old school 50Hz transformer type)

[SteveGB]

OK, thanks for the advice Kim.

I've now attached some pics - is this 'old school' rather than switched?

Uses an LT7233 chip.

Does this change the plan of action or carry on as described?

[SteveGB]

<<Many multimeters set to AC will give the wrong reading when measuring a DC source. I'll bet your multimeter will show around 18VAC when measuring a good 9V battery.>>

Thanks. Just tested my existing meters using a 9v DC battery, two of the oldest and most simple meters measured double the DC current if set to AC voltage measurement, the newest two correctly measured 0V. I think I measured the regulator with the old ones so may be spurious.

AC spec for best meter is 40Hz to 1 KHz

Will try the load/oscilloscope test as suggested.

[SteveGB]

Kim, markings indiate a diode, so I dont think switching - see pics

[SteveGB]

My mistake, when in doubt 'read the label'!

[Kim Christensen]

Kim, markings indiate a diode, so I dont think switching - see pics
My mistake, when in doubt 'read the label'!

If you find that the ripple is unacceptable, then the circled capacitors are your most likely suspects. If they're bad, make sure you replace them with low ESR capacitors designed for switch mode supplies. (Usually have ESR spec'd at 100Khz or higher)
I think the cooked brown goo on top of the caps is just glue from elsewhere as they don't appear to be bulging. But if this has been leaking from the caps, then they are definitely bad.

[wasedadoc]

If the replaced "input resistor" was the component in black sleeving near the mains input wires in the bottom left of the photo, then I suspect it was a fuse.  Those usually do not fail without a reason.

[james_s]

That's not a transformer, it's a switching power supply. By far the most frequent cause of problems in them is worn out electrolytic capacitors.

[SteveGB]

Thanks for the advice Kim, I think they are fairly beefy (450V) so will exercise a bit of caution when trying to diagnose tonight at the maker lab.

[SteveGB]

No, the resistors we changed are a 3 part 'daisy chain' (to get the needed value) in the upper left of the picture above the red tape. Obscured by other bits in the picture

[IanJ]

High voltage...... Please be careful!......have somebody experienced at hand.

[Kim Christensen]

Thanks for the advice Kim, I think they are fairly beefy (450V) so will exercise a bit of caution when trying to diagnose tonight at the maker lab.

The ones I circled should be fairly low voltage ones. One of the 3 appears to have 35V written on it.
Usually the high voltage (450V) input filter capacitor is not the problem. Not saying for sure that yours is good: Just that generally this one is less stressed than the output filter caps on the low voltage side.

Caution: Do not put your scope probe across the 450V capacitor or any part of the mains side of the circuit!

[SteveGB]

Quite right Kin, the 3 caps are lower voltage.

Hooked up to the scope Tuesday evening and everything seemed OK both with an isolated ground transformer and direct to mains using the scope.

The feeling is that the small AC 50 Hz ripple for a couple of seconds was just a a quirk of the meter.

Gunk on/across caps looks like a stabilising glue.

[james_s]

There isn't really any reason to scope it at this point, the best tool would be an ESR meter but if you don't have one of those just replace all of the electrolytic capacitors with new ones and go from there. Most of the time that will fix it.