repairing a switch mode plug pack

Peter: Dust off your DMM or VOM and ESR meter... and test all the discrete components like diodes, transistors which include bipolar & FET, resistors, electrolytics, etc. If all of that checks out then it is probably the chip. electricitym . . .

On Sat, 20 May 2006 13:06:11 +1000, "Peter" put finger to keyboard and composed:

The figure of 8V seems low. The datasheet for the UC3842 stipulates an UVLO of 16V during startup and 10V during run mode. I'd check whether the resistor that feeds Vcc is open or high.

- Franc Zabkar

Thanks Franc resistor feeding it is 300K - and is on value. There is a zener from that pin to ground - it's not an 8 volt zener though - I've checked it with a current limited psu - at 8 volts it doesn't trigger. it's not open circuit On it is just has the number 48

Peter

I was going to make exactly the same suggestion. 8v is definitely low for a startup on a 3842. It's unusual to have a zener at that point. Normally, it just relies on the current drawn by the chip and the value of the startup resistor, to provide some kind of arbitrary voltage of about 12-14v. Once the supply is running, a self regulated run voltage is provided from the transformer. It is normal, however, to have a series diode at the pin, so that when the run voltage comes up, this diode is blocked, so current ceases to be drawn via the startup resistor. It may be that you've not followed the circuit quite right at this point, and it is the series diode you're looking at, and the " 48 " on it refers to it being a good old 1N4148.

Anyway, if the starup resistor's value is correct, then the next immediate ( and very common ) suspicion, is the decoupling cap for the supply pin. It's usually quite a small value. 1 - 10uF would be typical. Check it with an ESR meter. Capacitance value is unlikely to reveal anything amiss with it. Failing that, just replace it.

Arfa

I've looked at a know good plugpack and compatred voltages - on the working one there is 8.9 volts on pin 7. I've swapped chips and the known good unit works with both pwm ICs In the working unit there is 2.4 volts on pin 2 and 2 volts on pin 1 In the faulty unit there is no voltage on either pin.

I found a faulty 0.01 ceramic cap between pin 2 and 4 of the CM3842 on the faulty unit. Replacement fixed the problem. The unit uses a PC 817 IC to feedback some of the output to the pwm chip

Thanks all

Peter

Bad solder joints or high ESR electrolytic caps, chopper. Everything else is much lower probability.

See the SMPS repair guides at the site below.

--- sam | Sci.Electronics.Repair FAQ:

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I have just spent a couple of days trying to fix an almost identical SMPS 6V plugpack but so far without success. It also had no physical signs of component failure and the standard components each measured ok. The PWM chip in mine was the 3843 but same pinout as the 3842. I even stripped down the board and individually measured/tested all caps resistors diodes opto-coupler and semiconductors. The 3843 worked fine when substituted into a known good SMPS. The only possible faulty component was a 7.2V zener connected directly across the DC output which had gone leaky or changed specs. I then drew out the schematic and rebuilt the lot with a new zener and got it to work a bit. It produced 6V unloaded but this sagged when any load at all was applied.

Am almost at the stage where I have to admit this has me beat but I don't like letting this happen.

BTW, you can get an identical SMPS from Jaycar (MP3140) for AUD24.95. You don't need to pay $55.

The unit I was playing with also used a 300K from Vbulk - pin 7 and the startup voltage was 8.6V which is ok for the 3843 but not the

"Peter" hath wroth:

(...)

Such 5V 2A or 3A switching power supplies can be found for much less than $55. See: |

I should have read your last post first. It seems you have fixed the problem.

Hi Ross - did you see my pic of the board? Is yours similar?

Many thanks Jeff

here in OZ we have to pay a lot more for things - also the A$55 was for a genuine d-link replacement - rip off!

Peter

Forget about testing the capacitors, replace them. This sounds like a classic SPMS electrolytic capacitor problem to me. There is no convenient

Try warming up each cap in turn and see if it restores normal operation temporarily, that should pinpoint which cap is weak.

Dave

Gotta disagree with you on that one Dave ! I don't think I can recall ever having had an electro that was causing trouble on a switcher, that didn't show bad on my trusty ESR meter, which is definitely convenient to use, and in my opinion totally conclusive when you've learnt to correctly interpret what it's telling you. In that respect, I suppose, it is a bit of a black art, so I might concede 100% conclusive for those people not using an ESR meter all the time ;~}

Arfa

I disagree. A shorted or leaky electrolytic cap will pass an ESR test ;-) An ESR meter is not an absolute indication of the serviceabilty of an electrolytic cap. In many cases, the only 100% effective proof of a faulty or marginal cap is to replace it. An ESR meter does not simulate the working conditions of an SMPS capacitor, and an intermittant or marginal problem may not arise when under test.

I sort of agree with that, I'd say that combined with other techniques, an ESR meter is a very useful tool indeed. However, the OP has reached a dead end and IMO the most likely culprit is capacitors, regardless of what his ESR meter is saying.

Dave

Dave

Sorry Peter, I didn't see it before now. And yes, mine is exactly the same in layout and components.

All academic now Dave, as the OP has posted his final finding, which was a defective 0.01uF ceramic cap across two pins of the 3842 ( posted on 20th oddly, the same as the original post ) so neither your technique of blitzing all the electros, or mine of applying an ESR meter and a bit of black magic, would have found it anyway !!

Arfa

Arfa, I was replying to Ross Herbert, not the OP! Check out his post on

Dave

The thread breakdown in my newsreader indicates otherwise...

Tom