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

[redwire]

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

So far only the fusible resistor was bad, everything else you have checked is OK. Seems a bit odd. To me that happens when I've glossed over and missed testing a part, or I tested it wrong etc. so I will back up and recheck the top suspects.

Can you measure (carefully without slipping with the probes and shorting things) the voltage across C3 (same as empty "C1" on the board's backside). That is power for the IC. It might be pulsing or hiccuping. RJ also asked for this.
The SF5920S IC needs over ~15.5VDC to startup, ~9.5V it goes to UVLO shutdown, and ~33V is OVP shutdown.

[Tarot Superstars]

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

Thanks.
R0 measures 3 ohms in circuit.
The mosfet is not small. It's the size of a normal through hole transistor.

[redwire]

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

The fusible resistor blew due to a short or overload on the primary side.
That would be a bad rectifier diode, or shorted mosfet and that fault current ends up going through the sense resistor who usually loses.
So far I don't see the cause of the overload, although OP slipped with probes and shorted something out.

Rectifiers test OK, mosfet tests OK- so something isn't making sense here.

[R_J]

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

Based on post #26 I suspect the R0 resistor is ok. The mosfet is a 1N65T

C1 and C2 must be the two 4.7µf/400v capacitors, What is the voltage across each of them? (it should be around 300vdc). What is the voltage across C3 (it should be around 20vdc) and the voltage across C3 is important as it will tell us the state of the ic.
Best I can do with Paint

[redwire]

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

Can you check the resistor R0 2.7Ω current-sense is not open-circuit. It looks a bit dodgy.
I don't think I've ever encountered a small TO-92 HV mosfet before. What is it's part number?

Also, note this PSU does not have an opto-coupler so it must be using the aux winding for voltage feedback.

[Tarot Superstars]

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

I changed F1 today and there was 0V coming out of the jack so that didn't fix it.
However, capacitors C1 and C2 are charging now, and they weren't before.
F1 was not broken after the tests and measures ~ 10 R now.

I tested the fast recovering diode in circuit, marking RS1M.
It tested ~ 0.5 V one way.
The other way, was 1.9V for a split second then 0V.
I'm not sure if this behaviour is from a nearby capacitor, another component or the diode.

[Tarot Superstars]

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

I checked the resistance on the continuity/200 ohm setting and yes that is the value.
Thanks for the data sheet. I see there are other pathways from the wires and that's where the readings are from.

[lotas]

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

0S429 - 0Sxxx - SF5920S

[R_J]

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

You are not measuring the ic, you are measuring the external resistors, for example Pin1 is hot ground, pin3 is the current sense pin, and R0 2.7Ω is across those pins, R0 is the source resistor for the mosfet.
Are you sure pin 4 to 1 is 42.8Ω and not 42.8KΩ

[Tarot Superstars]

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

If you mean AC input is the primary winding then.
Transformer Winding A:
AC: With one probe about 5V
but both probes: 0.23 V negligible.
DC: 0V.

Other wise I haven't measured the AC input or AC input stage.

IC Resistances

4 to 1 ~ 42.8 R
4 to 3 ~ 42.2 R
1 to 3 ~ 3.0 R

[R_J]

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

How much resistance is in a BEEP?
Pin 1= ground
Pin2 = gate drive
Pin3 = Current Sense
Pin4 = Voltage sense
Pin 5 = Vcc

[Tarot Superstars]

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

In circuit shorts read with multi-meter set on continuity are:

4 to 1
4 to 3
1 to 3

These all read continuity and beep.

Oh, yes, the title is wrong. It's 6V 0.5A.

There has been a burning smell since I opened the adaptor too. I was hoping it was just one of the capacitors that I changed.

[stj]

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

are any of the pins shorted together on the chip?

[Tarot Superstars]

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

I just checked D1, D2, D3, D4 in circuit on the diode setting.
None are shorted and they all read ~0.5V one way and 0V the other way.
Anyway, I have ordered F1. Maybe replacing that would have fixed it.

There is a fast recovering diode D8 too. Marking RS1M.

[R_J]

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

You are just picking up stray ac, by using only one probe. What voltage do you measure on the AC input? You won't measure any DC voltage with F1 10Ω resistor open, which it is.

F1 is supposed to be 1W 10R and it measures open circuit

[Tarot Superstars]

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

PS: I have all of the components ready to make a current limiter, but I won't be putting it together right now.

[Tarot Superstars]

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

Thanks everyone.

I previously performed a test on the transformer with the power on, and I got a result I wonder if you could explain.
It involves getting a 5 V reading when placing only one probe onto one of the transformer wires and leaving the 2nd probe unattached. The multi-meter is a RS Beckman T100B and it is battery powered with no connection to anything else, including earth.

The transformer and meter were on a rubber mat and insulated from each other.
Also, the image was not the one I tested. I tested the adaptor with all of the components connected.

I have attached a Jpeg with windings A, B and C labelled. I also included the resistance of each winding.

Results

Transformer Winding A:
AC: With one probe about 5V
but both probes: 0.23 V negligible.
DC: 0V.

Transformer Winding B:
AC multi-meter setting: With one probe about 5V
but both probes: 0.23 V negligible.
DC multi-meter setting: 0V.

Transformer winding C:
AC: With one probe about 5V
but both probes: 0.23 V negligible.

I checked voltage across the two 4.7uF 400V capacitors on the input and there was no voltage on them. 0V. So they had not charged.

Note: I wrote negligible voltage meaning 0V but there was some fluctuation in the voltage reading even without connecting the meter.

My question is how am I reading 5V AC when only one probe is connected?

I was guessing that electrons went up the probe and registered on the meter, but I don't really have a proper explanation for that. I thought there had to be a circuit with a potential difference between the two probes to get a reading, but this gave a reading similar to the way a grounded oscilloscope does with only one probe and no earth connected.

How is there 5V AC on only one probe?

Any advice is appreciated.

[stj]

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

mov1 is the empty outline
d1,2,3,4 form a bridge rectifier circuit - check none are shorted.

[Tarot Superstars]

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

I just hope it isn't the IC, since I don't know where to get that.
Do you mean the diodes in the bridge rectifier?

The legend says it's MOV1 and the component has colour code brown, black, violet, silver. There are Jpeg's I attached. I checked and there is no other legend near that component.

I have changed things since then. The large cap on the left was one I replaced and the one next to it has now been replaced too. 4.7 uF 400V.
In the 3rd photo, the bridge rectifier is on the underside, left hand side.
In the 1st photo, there are two diodes on the top rhs and they are in series cathode to anode.

I measured the mosfet on the top side in circuit and could see the junctions of BCE and they measured ok in circuit, with BC ~ 0.5V and BE ~0.7V, but from watching videos, it could fail at higher temperatures.
The mosfet is Q1 3 wires 1N65T 1424 1.2A, 650V n-channel power mosfet.

Are you sure that I mistook an inductor for an MOV?
It looks like there is legend for a MOV but it's not fitted. I don't suppose it would be on the underside.
Ok, I checked again, the L1 legend is under the inductor wire. So, yes, it's an inductor. It measured 14R when set on 200 R and I was expecting 100 mega ohms.

[stj]

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

the fuseable resistor is open circuit - so you have a short on the primary side.
usually the rectifier or mosfet circuits.
you dont have a mov fitted - if you did it wouldnt have coloured stripes anyway.
your mistaking an inductor for a mov.

and you didnt upload pictures - people dont want an ebook or a youtube video - just jpegs.