Power Transformer Troubleshooting Tips

[Richard Crowley]

Transformers do not operate on DC (as from a car battery, etc.) They operate only on AC.
In order to make them work, there are associated circuits with switching transistors that create "chopped DC" to simulate the AC necessary to travel through a transformer.

Just because you can trace the supply voltage to the transformer doesn't mean you are home free. What's up with that transistor(?) over to the left? Fixing switch-mode power supplies is not an easy thing even for experienced technicians, even with a schematic diagram for reference. Without any documentation, you have a huge job ahead of you.

Frankly, with decades of experience I wouldn't even tackle this unless I had the schematic diagram and it was really worth the difficult task of diagnosis and repair.

[Audity]

Thanks for the info and yeah, switch-mode power supplies definitely aren't my specialty lol. I was hoping to open it up and just see a simple little short or easily-replaced component, but oh well. It's just a cheap old thing that I thought would be a fun time consumer to fix. And DUH! I should smack myself for trying to read VDC across an AC transformer lol, but at any rate it's got full supply voltage on the primary and nothing, AC or DC at the secondary.
To the upper left of the transformer is the thermistor and part of the smaller MOSFETs that are normally screwed down to the chassis, and toward the bottom left are 2 solid core coils with one end apiece soldered to the +V trace near as I can tell, it's got about 2 pounds of sloppy wave solder under the PCB lol.
I spose I'll tinker about with it for awhile longer and if it's not fixable it'll make a nice parts cadaver lol

[Lotus 7]

Originally Posted by Audity

... but at any rate it's got full supply voltage on the primary and nothing, AC or DC at the secondary...

If there is no AC across the primary, only DC, clearly the switching circuitry is not "switching". Since the most common failure mode for transistors (and especially power transistors) is a short, that's a high probability.

Get a schematic, or at least check the power transistors with an ohmmeter, and figure out why it's not switching. Also, putting the full DC supply voltage on the transformer for any length of time probably means you're pumping way to much (DC) current through that winding and risking damage to the transformer.

[Minion]

Thats not a Transformer , it a torroidal inductor .....

[Richard Crowley]

A torroid is a "doghnut shape" An inductor is a coil of wire. A transformer is an inductor with more than two leads (either a full iso transformer or an "auto-transformer" without separate primary and secondary windings.

Indeed the component in the photo is a torroid as anyone can see. Furthermore it is an inductor which is also obvious upon inspection. However it also clearly has several different windings which fully qualify it as a transformer.

So Minion's component identification score:

1) Torroid TRUE correct
2) Inductor TRUE correct
3) Transformer FALSE incorrect

67% correct

[Lotus 7]

And of course, there are "Transformers" which are not toroidal, and have no inductance.

[tINY]

More than meets the eye



-tINY

[Minion]

Originally Posted by rcrowley

A transformer is an inductor with more than two leads (either a full iso transformer or an "auto-transformer" without separate primary and secondary windings).

You are right on that as long as they have a coupled magnetic flux and they are used as transformers but it seems from the pic and having worked on a few car amps that used simular inductors that it is used as a inductor and not as a transformer ....
There are several models of dual or more inductors on the market that use the same core for two or more inductors ......
Then again I have used a primary of a transformer as a choke in a tube amp so I guess it depends on the application of said inductor/transformer .....

Cheers

[LeeYoo]

Hi Audity

Car amps like this one have two sections.

1: The part with the multi-winding transformer toroid dougnut inductor
This DC/DC supply makes someting like +and- 30volt for the amps for more output power, because straight 12volt on the amps would only be 20watt RMS (4ohm load, bridged amps) per channel.

2: The actual amp(s).

Find two big caps with a voltage rating over 25volt.
That's the output voltage of the DC/DC converter.
Measure that voltage.

It will be there if :
The DC/DC converter is in working order.
And the amps are not shorted.
And there is 12volt input.
And the power switch wire is connected to 12volt.

The main power wire of a car booster is permanently connected to the battery.
A thin switch wire (yellow? or red?) turns the amp on or off.
This is normally connected to output of the power switch in the car radio, the motorized aerial output (yellow).
Turn the radio on, aerial goes up, and amp powers up.

Intermittend working could just be a dryjoint.
Leo..

[Richard Crowley]

Originally Posted by Minion

... it seems from the pic and having worked on a few car amps that used simular inductors that it is used as a inductor and not as a transformer ....

One can clearly see at least one primary and one secondary coil. One is thicker wire with reddish lacquer insulation, while the other is thinner wire with orange color lacquer insulation. There may actually be more than one primary and or secondary coil there, but without seeing the connections all around the base it is impossible to tell for sure.

There are several models of dual or more inductors on the market that use the same core for two or more inductors

That is actually impossible. if two coils share the same core, then they are the SAME INDUCTOR as they share the same magnetic waveform. The coils simply convert electrical current to magnetic force and back again. This is how transformers work, actually, so two coils sharing the same core is what we call a "transformer".

[tINY]

Originally Posted by rcrowley

That is actually impossible. if two coils share the same core, then they are the SAME INDUCTOR as they share the same magnetic waveform. The coils simply convert electrical current to magnetic force and back again. This is how transformers work, actually, so two coils sharing the same core is what we call a "transformer".

...unless it's a 1:1 transformer used as a common mode choke...

But I don't know what one of those whould be doing in a car amp. It could be an inductor with a "sense" winding. Those old SMPS designs were often pretty creative.



-tINY

[Richard Crowley]

Originally Posted by tINY

...unless it's a 1:1 transformer used as a common mode choke...
But I don't know what one of those whould be doing in a car amp. It could be an inductor with a "sense" winding. Those old SMPS designs were often pretty creative.
-tINY

Even the common-mode choke is a type of "transformer" It is wired up "sideways" to allow the DC to pass through, but is actually "shorting" any AC component back upon itself because of the transformer action.