trouble with high power amp

[bombardon73]

Hi there, I hope someone can help me identifying the problem I have with my Bell PCX 9024 amp. It's a professional ampflifier that can (normally) deliver 2x600 watts (4ohm) and 2x400 watts at 8ohm.

Lately the amp seems not to be able to give that power anymore.
It stil works, but you can't push the amp as far as before.
You can also see it at the LED's, the amp has several green LEDs and two red LEDs. At 1st you could push the amp so that the 2e red LED was blinking, no distortion or whatsoever. But now when I turn the amp upto the 1st red LED that amp allready starts to distort and when I push it a bit further the amp shuts into portection mode. You also hear clicks and pops when it distorts, especially when the limiter mode of the amp is turned on.

I know it's NOT the loudspeakers, I tried several.

I allready opened up the case to check out the tranistors, none of them is getting even warm when I push the amp.

Someone allready told that it could be the large capacitors, because when they don't work correctly the amp gets a powerfailure. Is that right?

There are four of these big guys in this amp (33000uF 63v). When I try to discharge them I noticed that one of them is still charged, one other a bit and the other two not at all. Could this be sign of bad capacitors?

I'm not a electronic wizz so I ask you guys overhere (I'm most of the time in the loudspeaker forum) to shed some light on this problem.

 

[OMNIFEX]

Lately the amp seems not to be able to give that power anymore.
It stil works, but you can't push the amp as far as before.
You can also see it at the LED's, the amp has several green LEDs and two red LEDs. At 1st you could push the amp so that the 2e red LED was blinking, no distortion or whatsoever. But now when I turn the amp upto the 1st red LED that amp allready starts to distort and when I push it a bit further the amp shuts into portection mode. You also hear clicks and pops when it distorts, especially when the limiter mode of the amp is turned on.

Transistors Are Shot.

I allready opened up the case to check out the tranistors, none of them is getting even warm when I push the amp.

You Transistors Are Gone.

Time to put it the shop, if you don't know how to fix amplifiers.

 

[bombardon73]

So, if your transistors are gone your amp can stil be functional?

Because thats what it does, it's still functional but with less power.
Also, both channnels have the same problem. It occurs at the same power level.

 

[OMNIFEX]

Yeah.

You'll just lose around 50 - 80% of the output.

You tried different speaker wires when you tried other
speakers right?

 

[bombardon73]

Yeah, I tried several things, including cables.

I tried to test the transistors with my digitalmultimeter, but I guess I can't trusts the results since I measure the transistor in the circuit. E and C should not conduct either way, right? but they do.

 

[anatech]

Hi bombardon73,

1. You can not test for transistor leakage in-circuit.

2. If your transistors were shot, they become expensive wire. You blow fuses and that is not happening.

To test filter caps, run the amp no signal and look at the waveform on the capacitor terminals. When they start to go bad you will set "pips" on the leading edge of the waveform. You need an oscilloscope for this. Low AC voltage could do this also. It is also possible you have circuit damage, but who can tell with this much information? When transistors go, it's due to excessive die temperature normally, and they tend to go short. Rarely do they go open. Other stuff tends to change colour rapidly and the magic smoke escapes.

-Chris

 

[bigmike216]

are you running the amp at a lower load than you were when you could push it right to the top with no distortion? If you were running 8 ohm speakers before, and now are running 4 ohm, this could easily happen, and be perfectly normal.

 

[bombardon73]

bigmike216 wrote: are you running the amp at a lower load than you were when you could push it right to the top with no distortion? If you were running 8 ohm speakers before, and now are running 4 ohm, this could easily happen, and be perfectly normal.

I run the amp with an 8 ohm load right now (also tested it with 16 ohms). The days that I could run it without distorting the speakers were 4 ohms.
The amp actually should handle a 2ohm load (you can read that on the back of the amp).
But when I run a 2 ohms load on it now, it starts to distort faster.

 

[ilimzn]

Check the power supply, namely the rectifier diode or bridges, you may have one dead and the power supply running half-bridge.

 

[mbates14]

yea, it definately sounds like the rail voltages are a tad low.

Or, the bias could be a bit off, but it would sound "tinny"

 

[OMNIFEX]

Originally posted by anatech


2. If your transistors were shot, they become expensive wire. You blow fuses and that is not happening.

Not all the time.

Pending on the amplifier you can indeed lose the output transistors, not blow fuses, and have the amplifier not go into protect mode.

 

[anatech]

In that case you would have both emitter resistors and base resistors open. I have seen that very occasionally. But if the rest was working you would still get full power until the rest blew.

It has to be supply.

-Chris

 

[bombardon73]

Guys, there's something else. The amp also goes into protection mode when there are NO speakers attached to it.
When the 1e red LED starts to blink and you turn it up a bit the amsp goes into protection mode. So there's no way I can led the 2e LED burn.

I don't know if this is common with these amps, but they use some kind of technologie that checks the in and out(speaker) signal. Maybe that has something to do with it.

here's a link that tell something about that.
http://www.bell-audio.de/PRODUCTS/PSX_Power_Amps/psx_power_amps.html

 

[OMNIFEX]

Dude, to be honest, you may need to contact bell-audio.

I remember an amplifier that would go into protect mode, and, stay there (You had to literally turn off and on to reset it) if it clipped too
hard.

With your amp offering half output, its quite possible the amp
is going into protect mode because its seeing an overload once
it clips.

Amp protection circuitry can be anything the manufacter wants.

I can't say I've ever seen two different brands of amplifiers react
the same when it comes to the protection circuitry.

And I'm talking Pro Amps here............

Home amps are a little more predictable.