pioneer vsx-922 av reciever transformer fuse keeps blowing

[adrian12]

Hi everyone
My AV receiver just died, I've opened the case and cannot see any visible damage to the boards or components , but found the transformer fuse blown , I replaced the fuse twice with the unit standing alone with no connections, but soon as I switch the wall socket on the fuse blows, I'm a complete novice at this but it don't sound good. I don't know if the power supply board or the transformer is at fault or something else is the cause . I have ordered a new av unit , but it seems a shame not to try and fix this one , sending it for repair I recon will be too expensive to justify . Is there anything I could try to find the problem?. 

Thanks for your help  Adrian.

[macboy]

One of the most common failures is one or both output transistors on an amp channel (s) failing short circuit. If both do this, then the power rails for the amp section will be shorted together and you will instantly pop fuses as you've seen. This is what is suspect first. You can measure the transistors in-circuit with your multimeter on ~1 kOhm scale. Measure between collector and emitter, usually the two outside leads. They should measure over-range resistance in both directions.

Before poking around in there, carefully discharge the two large capacitors using a 1 kohm resistor until no more than a couple volts. The DC voltage on those is enough to potentially kill, and the energy stored is enough to vaporize a probe tip if it slips and shorts out to ground (eg to heatsink), sending molten metal flying at your face.

[Trooper_Dan]

Basically you have a short as macboy said start with the outputs first if there are transistors desolder the  base and collectors and leave the emitter. Also check the number to determine pnp npn. If the fuse keeps blowing desolder the emitters then troubleshoot solder them one but one and test the power

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[Seekonk]

Do your diagnostics with a 40-60W incandescent lamp in series with the input power.  That will allow you to diagnose issues without burning up fuses.  Diagnostics is all about identifying things that are known and those which are unknown.  Make a list and figure out the possibilities.  Repair isn't that hard unless you are prone to just giving up.

[james_s]

Fuses almost never blow without a reason. I've seen mechanical faults a few times but if you replace the fuse once and it blows, don't bother replacing the fuse again until you know what the problem is.

Experience suggests that the transformer is very likely just fine. The problem may be in the power supply, but as someone else mentioned one of the most common faults in audio amplifiers is shorted output transistors. When that happens there is normally some collateral damage so if you aren't experienced working on these things you may want to find someone who is to help you. The annoying thing about audio amps is that if you replace a bunch of parts and miss one bad part, often all your expensive new parts will go bang the instant you power it up and then it's back to square one.

[Zero999]

What sort of power supply/transformer does it have? Switched mode, linear?

In a linear power supply, the transformer will be the first thing in the circuit, after the fuse and mains switch. The transformer will be a big lump of iron, which may or may not be directly soldered to the PCB.

In a switch mode power supply, there will  be a bridge rectifier, big smoothing capacitor and some transistors before the transformer. The transformer will be fairly small and will directly mounted to the PCB.

If it's a linear power supply, then you could try powering the transformer on its own, with nothing on the secondary. It's possible the transformer has developed a shorted turn, which is responsible for the fuse blowing. If it works, then you know it's something on the secondary side, otherwise you need a new transformer.

In a switched mode power supply, it's more complicated. Check the rectifier and main filter capacitor first. If they're fine, then it's most likely one of the switching transistors has failed short circuit.

[mzacharias]

Removing the amp module and disconnecting the power transistors is PREMATURE. Besides, there are 14 output transistors and seven bias transistors that would have to be un-soldered. Big pain in the ass.

I have extensive experience on this series of Pioneer receivers. In my opinion, although a  blown channel is fairly common, it does not cause the main fuse to blow because the small value emitter resistors go first.

I believe you most likely have either a shorted bridge rectifier or a shorted main filter capacitor.

To start with, you can unplug the larger three-lead power transformer secondary plug from the main board. If the fuse still blows with this unplugged you have a bad power transformer or some other short (probably) on the primary side. If the fuse does not blow, you can proceed to nail down the rectifier or filter cap. There is a smaller chance of a failed mica insulator on the seven-channel amp assembly.

"That's my story and I'm stickin' to it."

Until we know more, anyway.

[Hideki]

What sort of power supply/transformer does it have? Switched mode, linear?

It looks like it has both types: http://www.areadvd.de/hardware/2012/pioneer_vsx922.shtml

Most likely a large transformer for the power amps and a switch mode supply for all the rest. Considering the low price of this amp - unless you can easily find and fix the fault, it will quickly go, as Dave likes to say,  BER (Beyond Economical Repair).

[adrian12]

Thanks everyone for the info,

I have unplugged everything from the primary board so the mains runs just into the one board and the fuse blows when power is applied, does this mean the problem could exist in the primary board ?.

Thanks Adrian.

[mzacharias]

If you have indeed unplugged the transformer secondary windings from the main board, then yes, there is obviously a short somewhere on the primary side. Personally I wish I were there to do my own troubleshooting, but it is what it is...

Thanks everyone for the info,

I have unplugged everything from the primary board so the mains runs just into the one board and the fuse blows when power is applied, does this mean the problem could exist in the primary board ?.

Thanks Adrian.

[james_s]

So the transformer is connected to the mains but nothing is connected to the other side of the transformer? Or is even the transformer not connected? Either way assuming this is true then you know that the problem is somewhere in the input section or transformer. Can you post a picture of what is connected?

[mzacharias]

The transformer should not be connected to the mains until the power relay is engaged. If it blows a fuse prior to this, the standby power circuit is suspect.

[adrian12]

Hi everyone

I've powered up the board on its own and the fuse blows straight away , not sure what to check first ?.

Does anyone know and good link or site to get schematics or values for this board.

Thanks Adrian.

[james_s]

I would check the rectifier first. Looks like there are two of them, and that transformer looks like a switchmode power supply. Is there a chopper transistor near that? Use a multimeter with diode test function to test all the semiconductors.

[mzacharias]

I would say check virtually every component for shorts. When you find one, remove from the circuit and check it again. Do this methodically and you'll probably get lucky.

[adrian12]

Just like to say thanks to everyone for your replies , I went down the rectifier route and found one blown , and now for the Princely sum of 9 pence my av receiver is up and running again.

Thanks  Adrian.

[james_s]

Awesome! Good to hear.