Theater Subwoofer amp troubleshoot. Blowing fuses.

[fastline]

I grabbed a sub from a friend. Don't really need it but thought I would see if it was repairable. Almost new. He moved it to a new location and then it started blowing fuses as soon as turned on. I have completely removed and isolated just the amp and it indeed has a hard short and blows the fuse instantly. Simple resistance testing across the AC inputs shows .9Mohms and nothing seems to be leaking to ground but obviously something is leaking bad when energized.

Has to be something in the front power stage to be blowing fuses like this. I am curious anyone has amp repair experience? Is is possible that the bridge rectifier is leaking within itself? Never seen it personally but I have desoldered a few things and inspected it thoroughly and all seems well. Confusing. Basically power comes in, goes through some filter caps and buck transformer, then through the bridge, then another transformer, and out to the circuits.

I suppose it could be a transistor too but certainly no visual signs of issues.

 

[crutschow]

You could try applying a low AC voltage with a Variac or a low voltage transformer (such as a 6.3V filament transformer) and see if you can trace the current path that way without blowing the fuse.

 

[fastline]

I thought about trying our function generator but doubt there is enough current there. I will admit my experience in PCB in circuit troubleshooting is limited. I usually resort to guessing components and desoldering for bench test. Any other ideas? I have heard of meters capable of testing components in circuit but not sure how that would be possible.

 

[wkrug]

Do you have some schematics of this amp?
Without that, it'll be a hard job to answer.
What kind of amp is it?
A traditional A/B Amp or a newer class D?

Very often the Power Transistors will be shorted.
You will measure between collector an emitter of them.
If there are 0Ohms the transitor is obvius defect.
Solder out and check the transistor again.
A good help are datasheets of the according part.
When Power transistors get defect, very ofter the driver transistors will be killed too.
To avoid further trouble you'll change them, when a defect power transistor ist detected.

Sometimes one diode of the bridge rectifier will be shorted. Check all 4 diodes from these part.
At least it is possible an Eletrolytic Capacitator is shorted.
All this reasons will blow fuses very fast ;-)

 

[fastline]

amp is A/B. no schematic.

I testing the transistors in circuit for obvious shorts and nothing found. Are there other tests I can try?

Still kind of suspecting the bridge for some reason but have never seen one fail like this.

 

[wkrug]

In most case the supply of an A/B amp is symetrical.

The middle connector of the secondary side of transformer is connected to GND.
The bridge rectifier is connected to both ends of this coil and built the positive an negative power supply of the amplifier.
Check out, is your amp designed like this?
Most times in positive and negative supply is included a fuse.
Sometimes this fuses are in front of the bridge rectifier.
Remove both and measure the Voltage to GND at input of both fuses.
AC or DC in depencence of built in.
If the primary Fuse will blown again there is obvious a problem with the transformer, the bridge rectifier or the capacitator.

In primary side of the transformer often an VDR and/or a capacitator is included.
It's rare, bot sometimes one of this parts is defect.
For testing it is possible to solder them out temporary and test again.

The AC Voltage is OK for the Amp?
If you'll take an 110V amp and put them to an 240V supply it'll make some trouble.
It is often a problem when some equipment would be imported from USA to Europe.

Do you have an external double power supply with current limitation for test?
Preset the current limitation to 1A that should be enough.
So you can connect it at the output of the bridge rectifier and the middle connector to GND then regulate both voltages up slowly.
The Error shoud be occur and the current will increase very fast until limit.

A schematic will be very useful, but you'll take the hard way ;-) .

 

[fastline]

If I understand correctly, you're thinking the mid point of the transformer secondary is ground terminated and each side is the outputs? Not sure if I fully understand the layout you are describing.

Power certainly is good at 120V. I think we can supply a current limited voltage to the input for testing but I am just not sure how that would ID my leak in circuit? Is is possible this thing needs energized to show the short?

 

[wkrug]

Is is possible this thing needs energized to show the short?

Thats right.
My suggestion is to power the secondary side of the Amp with an external power supply behind the Bridge rectifier. So you can take DC.
The failure will occur, but the current limiter of the external power source will avoid further damage.
The main voltage of the amp is not active while this test!
The power for the amp cames only from the external power source.

Check the circuit intensive before make this test, to avoid further damage oft the amp!

LF input must be open for this test - of course

Lock at this Link:
https://www.elektronik-kompendium.de/public/schaerer/endstu_p.htm
At "BILD6" ist shown a typical power supply for A/B amps.
The +12V -12V supplies are the supply for the Power Transistors.
The middle of the transformer secondary coil is connected to GND.
The Value of this voltages are dependent of the power of the amp.
My Idea is to power your amp by an external power supply at the + voltage, the - voltage and GND.
If there is a problem with the power transistors, the current limitation of external power supply will be started.
So you can measure in the Amp without risk of further damage of parts.

It sounds like your amp has an switching power supply instead a normal transformer?
Could it be possible the first transformer is an Power Filter to avoid network distortion?
After the Bridge must be than an big Capacitator.
After this capacitator is an switching power transformer, a power transistor and most time a switching power regulator IC.
At the secondary coil of this transformer is only one diode per Voltage mounted?
What type of power switching IC is used in your amp?
What type from witch manufactor is the amp?
Possibly there could be found a schematic in the internet.
Look at this, that's an typical switching power supply:
**broken link removed**