Aydin Ranger 21/Iiyama MF-5021

[Sanxion]

I need some clarification.
By transformer, I assume you mean the large one (T931)? If so, I can't see a pin labeled 4.
By rectifier, are you referring to D901?

[Welsh Anorak]

Yes, T931 - the transformer that straddles the thick white line. There isn't a label on the pin - it's the fourth one counting from the left top row.
However it might be easier to use chassis earth which links most of the negative pins of the large capacitors above the line (NOT the ones you have measured earlier).

I can't quite make out the rectifier numbers. However if you follow the print from the other pins of the upper row from the transformer you'll see several of them go to rectifiers - there are three big chunky ones.

Measure the voltage on each of these - the positive side. We're looking for a voltage here on one or all of them.

You may find this easier to measure on the component side of the board where you can clearly see the rectifiers. On your first post, first picture, they are directly below T931.

[Sanxion]

If I have done this correctly, I don't have anything other than mini volts on each of the positive side of the three rectifiers. I am measuring on the underside of the board placing the negative lead on pin 4 (although pin 5 is also connected to pin 4) and the positive on the positive of each rectifier. The rectifiers are the black components with a screw through the upper side?
I have attached a picture to show the points I put the leads on. Black negative and red positive.

[Welsh Anorak]

Absolutely right.

So this proves the power supply isn't working which is as we thought.

Now disconnect the mains and, using your meter's diode test or continuity function, check all the rectifiers coming off the transformer's pins. They should show a reading (depending on your meter) in one direction but not in the other. What they should not show is a short circuit.

[Sanxion]

D960: 0.75 mega ohms (both ways)
D976: 0.76 mega ohms (both ways)
D967: 20.8 k ohms (positive to negative) - negative to positive produces strange results. Sometimes an increasing value and sometimes decreasing.
None of the rectifiers produce a short.
Is that of any help?

[Welsh Anorak]

Yes - there isn't a short. We now need to look at the primary side of the circuit

[Sanxion]

Any particular components?

[Welsh Anorak]

Sorry for the delay.

Could you connect a meter across the big capacitors as you did before - where you measured 340v? Switch on for a moment, then switch off keeping the meter in place and see how slowly the voltage decays which will give us a clue.

Could you also email me close-up pictures of the print and component side of the primary side of the board? Specifically the area to the left of the two large capacitors with those three big resistors. I'll be more able to see components with a higher resolution image.

[Sanxion]

It takes over a minute to completely discharge but interestingly, the drop from 340V to 80V takes about 20 seconds...
I have attached some closeup pictures of that area.

[Welsh Anorak]

That sounds like the primary is actually working to an extent.

You will notice two resistors, R906 and R907, both 150k in series, fed from R913. Usng the primary earth (negative as you used just now) measure the voltage on the end of R907, beside the two small yellow capacitors with a grey blob of silicone holding them.

What's the number of IC902 (I think) - the 16-pin one?

[Sanxion]

I have measured the voltage at the end of R906 and I am sure the first time I did, it registered 250V. However, every attempt afterwards reads -3/4V...

IC902 is IR3M02A

[Welsh Anorak]

It's the end of R907 (by those yellow capacitors) you want to measure with respect to the primary earth.
I'd expect a voltage of aound 12v which may be varying rhythmically.

[Sanxion]

Sorry, in my previous message, I meant to say the end of R907 not R906. Red was on the end of the leg of R907 and black was on pin 4 of the transformer.

[Sanxion]

I think my negative point is wrong here? If I use the negative point of C926, I receive a stable 13V.

[Welsh Anorak]

Sorry for the delay in replying.

Stable 13v means the IC is receiving the correct working voltage. The fact it isn't vrying suggests it's not in trip mode.

Hmm.

The IC could be faulty - it's easily available as it's a Sharp part. Do you have a 'scope?

[Sanxion]

Yes, I have an oscilloscope.

[Welsh Anorak]

Great. However to work on this type of PSU an isolation transformer is also a good idea.

[Sanxion]

Unfortunately, I don't have one of those...

[Welsh Anorak]

As I have always used an isolation transformer I think I'll need to ask on here the best way to proceed with this type of power supply.
Disconnecting the earth on the 'scope works but isn't ideal....

[Sanxion]

I have replaced IC902 i(IR3M02A). Before I reassemble and test the monitor, is there anything I should check first?