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[SOLVED] Need help diagnosing or replacing a 5v transformer?

Thread starter #21
As an update to where things are

Transformer
A sine @ 15khz- 5v on pins 1-5 outputs 8.2v on pins 6-8
A sine @ 15khz- 5v on pins 2-4 outputs 8.2v on pins 6-8

Other components
Q10 was faulty on B-E
Q9 is ok
D6 (schottky) forward bias 0.3 is ok
ZD1 was ok, but replaced anyway.
All capacitors and inductors have been replaced. Some were leaking.
Traces and board are fine

Got to put it together and test.
 
#22
I do not have experience and trying to learn from who really knows.
[METAPHOR]How can you learn calculus when you don't have arithmetic or trig under your belt?[/METAPHOR]

You need to have a basic understanding of how passive components work, as well as the basics of diodes and transistors.

Otherwise, you won't be able to understand the whys of the answer.

A comment like Nigels and Audioguru are really what makes you want to give up on electronics.
Is that all it takes for you to give up?

Not only is Electronics a career, it's a labor of love for us.

And from all the forums I am on, this is the only one where you get people being agressive with beginners. That is on my other posts as well.
I quoted the two posts that you complained about, and took them apart. :)

As you've been trying to test a high frequency switch-mode transformer using a DC input it's pretty obvious you have little or no experience, or electronics knowledge.

The circuit is a DC/DC converter, to provide a 7.5V supply from a 5V input, it's VERY unlikely that the transformer would be faulty - unless you've damaged it by feeding it with DC.
That seems okay to me. You should have studied up on transformers first. If you did, you'd already know that testing a transformer requires an AC signal.

Other electronics forums have a "Beginners" section where some teachers answer questions correctly or wrongly. I have been a teacher a few times.
Most of us here went to school to learn electronics from real teachers and from good books. I taught my teachers a few tricks I found about electronics.

Gkmaia, it looks like you are trying to repair a defective circuit you have, and you are a beginner in electronics, so maybe your post should be in the Repairing Electronics or Homework section of this website.
I see nothing wrong with this post. He's even telling you that your post may be in the wrong section.

If you want to lead with emotions and run off, well that's up to you. Myself I'd just grow thick skin and study harder.

But that's just me.
 
Thread starter #23
MichaelaJoy not giving up, but I guess the point has been made.

I would rather focus on the electronics as it may start getting more personal than it should.
 

unclejed613

Well-Known Member
Most Helpful Member
#25
When T1 was on the circuit. I was getting:
- 0v on the left side of D6
- DMMV +5v (as it comes from the main board)
- DMMP low = 0v high = +5v
- IC4 (REG LINEAR 3V 250MA SOT89-3) output was 0v
- IC5 (REG LINEAR 1.5V 200MA SOT89-3) output was 0v
- Q9 & Q10 have been tested and both read 0.6v on the right direction

What you suggest? Is there a way to test T1 off circuit?
what is the voltage on Q9 base (that would also be the cathode of ZD1)? make sure DMMP is "off" first. if you're not sure that it's off, you can desolder and lift one end of R39. the goal here is to see if there's a problem with the oscillator part of the circuit. ZD1 being shorted or C26 being dried out might cause problems. also, i'm assuming all the resistors with the F suffix on their resistance values are "flameproof". there's often a good reason for them being flameproof, so check those and make sure they are 10K and 47. the reason for them being specified as flameproof is usually that they're expected to burn open with certain circuit failures (and those might be common failure modes if they went to the trouble of putting flameproof resistors in there).
 
Thread starter #26
what is the voltage on Q9 base (that would also be the cathode of ZD1)? make sure DMMP is "off" first. if you're not sure that it's off, you can desolder and lift one end of R39. the goal here is to see if there's a problem with the oscillator part of the circuit. ZD1 being shorted or C26 being dried out might cause problems. also, i'm assuming all the resistors with the F suffix on their resistance values are "flameproof". there's often a good reason for them being flameproof, so check those and make sure they are 10K and 47. the reason for them being specified as flameproof is usually that they're expected to burn open with certain circuit failures (and those might be common failure modes if they went to the trouble of putting flameproof resistors in there).
ok, thanks for that. I will follow your protocol and let you know.
 
Thread starter #27
I got a question. I just want to understand better the flyback topology. I've been reading quite a few articles and what has been said here. And that is what I understood.

Current flows down R41 and stops at ZD1 and C25 but flows to Q9 base. That allows the primary (2-4) and induce a current on the feedback (1-5) and on the secondary (7-9).

The feedback current will trigger the transistor to stop conducting and as the magnetic field collapses it flows into the secondary?

If that is right what phenomenon (or components and its characteristics) makes the current stops flowing to Q9’s base?

Also the wave generated by this oscillation wont look like a traditional AC sine wave. It will resemble a square wave more likely?
 

gary350

Well-Known Member
#28
Leave transformer in the original circuit then use meter to test voltage on transformer. If transformer is bad wires are usually cooked black and durnt. Transformer does not work on DC battery it only works on AC. Ohm meter will tell you if wires are good.
 
Last edited:
Thread starter #29
It is fixed now. Q10 was faulty and the transformer was fine.

The diode after the secondary reads 6vdc.

Thanks to all of you who toke the time and patience to provide guidance and ideas.
 

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