Power Supply Repair

[Cozzmo]

Howdy all.

I've got this 24V / 2A power supply which has an issue.

All its putting out is ~5v DC at the moment which can be varied ~0.5V by the trimmer pot.

I have replaced the main transistor TIP3055 and the smaller one next to it BD681 and still the same result.

The Main IC is an LM723CN.

Nothing has released the magic smoke as far as i can tell. The 240V AC input transformer is putting out the right 24V AC voltage and i'm seeing 35VDC after the rectifier diodes.

All caps tested OK with my ESR Meter.

I can easily get another main IC, they're not expensive. I thought seeing as my first guesses were wrong, i'd ask for a little more expertise before just throwing a new IC at it. I don't have a scope so just basic measurements with a DMM and an ESR meter is all i've got.

[Paul Price]

Since there are just three control elements to get the power supply to work, replace the 723 and it will probably fix the problem.  This assumes you are getting these unexpected voltages at the output with no load...yes???

[Cozzmo]

Yes that's correct, voltages were measured with no load.

I'll grab a replacement and see how i go tomorrow. Thanks for your response.

[peter.mitchell]

Looking at these supplies, they seem nothing special, yet, I myself have used them...
Why didn't I open it up at least to see what was in it!?

[digsys]

I buy heaps of TacTec P/Supplies for a particular customer. They're regarded as one of the more reliable and stable P/Supplies available,
especially in security systems etc. The designs are pretty straight forward, with some being even quite clever.
The circuit will be pretty close to what's in the application notes for the LM723, so if you've done the standard tests, change the IC.

[Cozzmo]

Changed the LM723, same deal.

*sigh*. I like it when things blow up. At least then you can see where the problem really is. Looking at the circuit it doesn't match the typical applications, although some things do, some things don't. So i'll probably have to do a bit of Dave CAD to try and figure it out and check and calculate the values properly to see what is going on i guess.

This is the first one that i'm aware of that has died on the site and it was put into service in late 2005 some time stuck in a hot roof.

[Cozzmo]

So i pulled a few things off to test them out of circuit & found the Frequency Compensation capacitor C3 (224/0.22uf) doesn't register on my LCR meter (says bad or unknown part). Its a cheapy crappy chinese thing (just a PCB with an LCD), but all the other caps tested OK and close to the spec that they should be.

I have one 0.33uf tantalum and i have some other ceramics and greencaps in the 0.22 range. Does it matter what type of cap it is? the one that's on the board "looks" like a tantalum (glossy yellow casing), but doesn't have a +/- marking on it. Its the one just above U1 in the picture.

I don't want to throw a random cap in there and have it damage something else in the circuit, but i think it should be ok to use the tantalum i have with the + side to the pin on the LC723?

I can't find any references in the datasheet to the specific value that the frequency compensation cap needs to be, just the recommended values which vary depending on the circuit configuration.

[SeanB]

Use a ceramic cap, that one can have a voltage across it which is positive or negative. You can also use a film capacitor there. The value is kind of a suck it and see value, but is thankfully not too critical. close enough or a little bigger will work.

[Cozzmo]

Fixed & working.

That was it. Always the way, the 2c part dies and cripples the whole thing.

Turns out the ceramics i had were 330p, not 330n so i checked the parts box and found a 330n film cap, worked perfectly on first power up, 23.2v, tweaked the trimmer to a little over 24v to account for a little load and looks like it should to the job.

Thanks SeanB. Glad i didn't just jump in and throw the tantalum in.

[peter.mitchell]

I buy heaps of TacTec P/Supplies for a particular customer. They're regarded as one of the more reliable and stable P/Supplies available,
especially in security systems etc. The designs are pretty straight forward, with some being even quite clever.
The circuit will be pretty close to what's in the application notes for the LM723, so if you've done the standard tests, change the IC.

While this may be true, they cost a small fortune!