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AT PSU repair

retrogear

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Yea, without a schematic I'm clueless at this point. Just googling that IC brought up some diagrams that are quite similar but not the same.
Best to come up with a substitute supply.

Larry G
 

mR_Slug

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Disabling the protection circuit is not a good idea, Have you checked for shorted / Leaking diodes on the secondary side as Chuck suggested earlier, Also The small Caps with no vent and resistors.

Ahh yes diodes, i will desolder and check. Most of the small caps i have replaced. I have not checked the values of all the resistors, except in immediate areas to where i have been checking. I've been poking around that IR3M02 since my last post before Xmas, and everything seems to be ok-ish. I haven't been exhaustive, partly because its a bit beyond me and partly because it is only an AT PSU. I have been trying to determine if the transistors on the primary are firing, but since it shuts itself down, i guess the IR3MO2 stops this. Hence the idea to disable the shutdown/protection. I will have a look at it tomorrow.

thanks for all the help.

Andrew
 

retrogear

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A scope in DC mode would be required to measure voltages during the short on-time of the supply. Most of my troubleshooting back in the day was using my scope as a DC meter also.
DMM's are just not fast enough. My Fluke meter has a hold function but still not accurate enough. I'll still give free advice as long as you're willing to pay with your labor :p

Larry G
 

Malc

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Scopes are definitely handy, Years ago i picked up a scope and scope calibrator that was in a job lot i bought, I played with the scope a couple of times and it got put away and hasn't seen the light of day since, I must dig em out one day though i suspect after all this time they have died in their sleep, I've also got a Fluke that cost me an arm and a leg and a couple of other DMM's and analogue meter which i mainly use.
 

1980s_john

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Ahh yes diodes, i will desolder and check. Most of the small caps i have replaced. I have not checked the values of all the resistors, except in immediate areas to where i have been checking.

Hi, good luck with replacing components, have you the tools to test capacitors? Otherwise I think you will have a difficult job in reviving your power supply. I would suggest a capacitor meter, an ohmmeter and an ESR meter (for electrolytics).

The best guide I have found for SMPS repair is by Sencore, TT205 archived here :

https://web.archive.org/web/20061230062617/http://www.sencore.com/custsup/techtips.htm

If you suspect the protection circuit, I would suggest you check for poor solder joints. Then check how the comparators are performing, to do this you will need a couple of DC power supplies.

Set up (as per Sencore guide) would be as follows:

Disconnect SMPS from mains - for this test don't apply any voltage to the SMPS input.

Power up the SMPS control and comparator circuitry - this needs a DC supply of around 10 to 15V (check datasheets for ICs). Connect DC supply via a current limiting resistor (eg 680 ohms) to SMPS IC - check voltage on each IC to determine that supply is within range, increase DC voltage until all ICs are powered up, check voltage drop across current limiting resistor for current drain.

This would be a good point to use an oscilloscope (as you don't have any dangerous mains voltages / earth loops etc to worry about) to check the control IC is generating a suitable output at say 20kHz to 40kHz. Then check the comparator ICs, each probably contains two or more comparators, each has two inputs and one output. If the + input is higher voltage than the - input the output would be high (or low if inverting), and vice versa.

If all comparators are OK, then use the second DC supply to 'power up' the SMPS 5V output - ie feed a voltage back in to simulate a working PSU. One of the comparators will be for over voltage, eg increase output from 4.5V to 5.5V, one of the comparators should flip - tracing the circuit should show which!

HTH,
John
 

Johan

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Jun 14, 2019
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SMPS repair

SMPS repair

I know this is an old thread ,but I will give my experience here:

I have dealt with an old XT psu. I struggled to figure out why it wouldnt switch on after adding new caps and that the 12v rail was 9v when I took out some of them. thease old psu's regualtion works on the 5v rail.Put a old car or trailer ligtbulb as a load an the 5v or a 2ohm 5w resistor. Many of these supplies need a load to work.If it still doesn' t workThe IMOR2 is easily swapped with a Texas Instruments TL494. They are cheap. Replace all the electrolytics. Trust me ,they all have dried out a long time ago.They dry up without use in less than a decade
 
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