trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

I have just realized that the mosfet might still be broken.

I was following a theory that I had learned, that if I tested the mosfet in diode mode for junctions BE and BC and they tested fine, then if I reversed the polarity of the probes and found no junction (no voltage drop) that the transistor would be good. Also that if BC was higher than BE then it's likely to be ok.
I did not use any theory about Gate, Drain and source but I assume the test is the same for a transistor switch and a mosfet.
But for clarity, I would be testing the equivalent of BCE:
Base is the Gate,
Collector is the Drain,
Emitter is the Source.
So I would be looking for GS ~ 0.5V, CS ~0.51 V or more, and nothing in reverse.

But, I just checked a pre-amp transistor that from a radio that I made, and I know it was broken. I know because I put 5V in a 3V radio and it stopped working. Then there was the smell of burning and that transistor was hot. When I changed it and lowered the voltage back to 3V the radio worked.

I kept the broken transistor for reference. It's a BC550C (PNP).
I just tested it using the theory above and it tested BE ~0.59 V and BC ~0.58V. I checked the data sheet and BE is supposed to be higher than BC in this instance.
When I reversed the polarity I did not find a junction and it measured fine.
Note that my multimeter might have more of an effect on readings than more expensive ones.

This means there is an error in this theory since I know the transistor was broken and yet it has the same readings as a BC550C that I know is not broken.

So, then the mosfet could still be broken even though no reverse junctions were found in circuit tests. And even though it was at room temperature.

However, a mosfet works differently from a pre-amp transistor. Perhaps someone with more knowledge than me could shed some light on this. Perhaps the test works for mosfets and transistor switches but not transistor amplifiers?

Anyone know any good tests to verify if this mosfet is broken or not, perhaps with power on?
How do I find out if it's the mosfet, IC or both that are broken?
Any advice is appreciated.

[petehall347]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

It is 1% resistor coded with EIA-96 label, value 4990 ohm




.

are you sure ? i make it 680 ohms . the B meaning add a zero after 68 .

[harp]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

On the component on the top left, it says 688 or 68B.
What is that and what is the value?

It is 1% resistor coded with EIA-96 label, value 4990 ohm

The bottom left component.
What is that value and what does it mean?

Common resistor marked 393, mean 39k...


When check resisror in circuit, there shall be always less or equal ohm then marked, else is broken. Indeed, they can be broken and show not suspicious reading if is paralel connected with other stuff. Say 393 marked resistor, if you measure 42k across it, it is broken, but if you measure say 30k it may be ok or broken. Small signal resistor are rarely broken, and often visualy corupted (burned marking, hole, edges melted) in broken state.

[petehall347]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Thank you.
Can I do this test before removing D8 or not?

if you like then yes . just be sure its unplugged and capacitors discharged .

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Thank you.
Can I do this test before removing D8 or not?

[petehall347]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

putting schematic here so easier to find .
if you do remove d8 check ohms on the ic pin 5 to pin 1 . it may take a short while waiting on c3 to charge from your meter . if it appears low ohms then remove c3 and try again .

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

I measured C2 with the power on using crocodile clips on it's wires.
It measures 337 Volts DC and 0 Volts AC.

Also, I have found that there IS an alternate path across the two wires of D8 the Schottky diode, so the split second reverse bias reading means nothing and I would have to remove it to check it properly. Also I think it is a fast recovering diode and not a Schottky diode since I think a Schottky diode should read less than 0.5V forward bias that this reads.

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

New microscope setup (on a kitchen roll stand).
Note: the metal should be insulated but I haven't got around to that yet.

On the component on the top left, it says 688 or 68B.
What is that and what is the value?

The bottom left component.
What is that value and what does it mean?

Could someone give me some ideas where I could look to find a replacement IC please?
You don't have to give me an exact place to buy it, just where I need to look. Data sheet attached.

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

No I haven't measured C2. I didn't want to solder that many wires in one go.

Yes, I figured that F1 possibly only operates during a short circuit, and that with only 10R the voltage across it would be low.

R1 measures around 1.2MR rising to 2.2MR (and probably more) in circuit. Obviously there is a capacitor somewhere effecting the reading.
R8 still seems to measure in reverse bias for a split second. I suppose I should take them out and measure them again.
And to measure C2 also.
If I connect a multimeter to the pulse output of the IC I should see a flicker on the meter, but can't measure the frequency with that.

The traces have come off on one of the mains input wires and one end of F1 and the trace on the other end of F1 is moving up and down. I had to bridge the mains to F1.

[R_J]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Have you checked the voltage across C2?

[harp]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

if f1 was broke then c1 voltage would be zero .f1 is mains fuse

F1 is 10R
I think he is measure voltage across F1...?

[redwire]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

I took some readings with a multimeter
[...] C3 = 1.3 V DC. 0 V AC.
Does the measurement on C3 show U1 is broken?

Yes, or C3 or the diode D8 has a problem. You said R1 is good. The IC could be hiccuping too, but it wont start unless over 15V is present.

When working on the board, always ensure the main filter cap C1 is discharged before going in. You might have to discharge it or wait for the 335V to dissipate.

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

D2. That is the light shining off it. I check and there is no hole in D2.
Also, the hole in the board near D4 never had a component in it. It was covered with solder and appeared when I soldered near it.

[petehall347]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

if f1 was broke then c1 voltage would be zero .f1 is mains fuse
oh and why has d2 a hole in it ?

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

I took some readings with a multimeter with the adaptor plugged in.
Voltage and temperature tests. Powered on. Insulated jacket, rubber gloves, protective goggles.

Voltage Tests (Powered On)

F1 = 1.5mV AC. ~ 0V DC.
C1 = 335 V DC. 0 V AC.
C3 = 1.3 V DC. 0 V AC.

Temperature Tests (Powered On)

Equipment = a digital room thermometer with insulated probe on a plastic stick.

Everything I tested was at room temperature. I think that was weird. I expected if U1 and Q1 were working they would be above room temperature.
I also tested the largish smd diodes (M7...Schottky) on the reverse side and the small smd capacitors and the resistors near U1. They were all room temperature.
I tested the transformer, both inductors and the fusable resistor and they were all at room temperature.

Any advice is appreciated.
Should F1 have more voltage?
Does the measurement on C3 show U1 is broken?

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Thanks. I thought if I test it first using a multi-meter and only connect the scope to ground or remove the ground it would be ok. Or only test things that have a capacitor between them and negative or postive. 400 V does sound too high. I will take your advice and not use the scope.

I am starting to think that it might be the IC that is faulty. Since you don't advise using a scope to test it,
does anyone have any ideas on how to test the IC to verify if it is shorted?

Less than 50% of the wires showed continuity and there are other paths to that, so that test was inconclusive.
I know other measurements on capacitors have been suggested, but if they go ok, I think I am likely to end up testing the IC anyway.

There seems to be a problem ordering them, since though I have the data sheet, the only company that I have found that sells them is in China and I doubt they accept small orders and they don't even have an ordering system.

Since I have started working on this adaptor, I noticed a smell that I think is similar to the smell when an transistor or a diode is burnt. I don't know what a burnt capacitor smells like. Since I don't seem to have found any other shorted semi-conductors by testing in circuit I suspect the smell came from the IC.
Though the fast acting diode did seem to conduct for a split second in the reverse direction.

If the IC is faulty and I can't find a replacement then I have to end this project as it can't be fixed.

[R_J]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

If this is a mains powered (not battery powered) oscilloscope DO NOT connect it to the primary side of this adaptor without an isolation transformer, you will blow up the bridge diodes at a minimum.

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Thank you.
I am focussing on assembling the current limiter first. So, any more measurements will have to wait until I that is finished.

Also, I have another 6V adaptor but the wrong size jack and I might just swap the jack lead and rewire the faulty adaptor mains input lead with longer wires so that it is easier to work on. Atm those wires are falling off due to strain from the mains plug hanging onto it. I had to re-solder them twice already.

I was also thinking of wiring up my oscilloscope to the ouput of U1 and to the it's ground to see if there is a signal coming out.

No-one said anything when I mentioned that the fast acting resistor temporarily measures a voltage in the reverse bias in circuit. Should I assume that's coming from the circuit?

An advice is appreciated.

[stj]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

if you cant see anything wrong, check R1 and replace c3 with something decent

[petehall347]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

i would start with q1 voltages and see what's not right there . then go backwards towards the beginning .