Behringer BX4500H, hum (not DC), no signal

[allante666]

Hi, I have a Behringer Ultrabass BX4500H. Seems like almost brand new with hardly any use.
All i get is a hum on the output. There is no DC on the speaker, none of the controls alters it. Using the line out it works fine so i guess the preamp is ok. I havent come across one of these before so am a bit lost. Massive heatsink in the middle with 6 big power transistors on it. Ive checked all those and they all seem ok (desoldered them and checked them with a transistor tester). I also took out all the big pot resistors around them and checked those also, all ok. Nothing looks like its fried, the whole thing looks new and clean and shiny!
Its not a loud hum but there is absolutely no signal getting through.the speaker pops when you switch it on but it doesnt stay pushed in or out and there no dc on it.

This things too modern!! Any help appreciated!

[Enzo]

First check the filtration. There is a pair of 4700uf/80v main filters. See that BOTH power rails for the power amp are up to about the same voiltage and free of riple. No, I don;t know what the voltage is, 50v more or less? If one measures low, flip the meter to AC and measure across each cap. There whould be very little AC On those DC supplies. If ther is substantial AC on one of them, try a new filter cap, or at least try resoldering the old one.

Oh yes, unlike more common arrangements, the collectors on the power transistors are grounded, it is the emitters that get the power rails.

[allante666]

Thanks, Ill try that tomorrow, these are the two big smoothing caps in the middle of the heatsink yes? Is there one cap for each supply?

[Enzo]

Yes, one for V+ and one for V-. Cyeck for solder, then check their function electrically.

[allante666]

ok just had a quick look at this, its a bit hard to tell whats what with it being double sided pcb and no schem but i measured no AC ripple on either one, I had 130 VDC on one of them and No DC on the other but I cant tell where the supply to these is coming from. The main transformer looks ok, its got a 97 volt supply and a small 15-0-15 volt one.

[allante666]

Enzo do you mean that I should have an individual DC on both caps seperately or should I be measuring the voltage between collector and emitter of the transistors. Im guessing one supply is for the PNP's and the other supply is for the NPN's yes?

[Jazz P Bass]

Power Supply

I would think that a + & - power supply is what is called for.
Look at the main filter caps.
The positive supply will have the - sign to ground.
The negative supply will have the + sign to ground.
If the rails are not shorted I would check the bridge rectifier & or fuses.

[allante666]

Hi thanks, Im measuring 130dc across one cap and nothing across the other, what i'm trying to say is are these two caps both fed on totally independant supplies. i.e if i have +130DC on one should I have -130DC on the other with a common ground? Im guessing I should but its just so awkward to work out where it is supplied from without the schem. its double sided board so hard to follow (with my eyesight anyway!!)
Also is 130 too high?

[Jazz P Bass]

Behringer

Sorry, but you are on your own.
Behringer will not provide schematics.
Period.
May I suggest you check the capacitor connections as posted above.
If the amplifier is class ab & it can provide 450 watts into 4 ohms, then it most probably has a + & a - power rail.

[allante666]

Success!! well,kind of... I found the problem. One rail was indeed down due to it shorting out to the heatsink. If I had done the job in the right order and tried it with the board removed from the heatsink I would have got it straight away but hey ho, we live and learn. So, I found that I had a voltage of 60 V between the heatsink and the chassis, this killed the voltage as soon as i tried bolting it back in place. I found that the little thermistor which presumably shuts it down if it overheats (theres two, one on each side, one for each rail) had one of its legs catching the heatsink. I simply squeezed them together a bit annd it was fine. rails came back up to +60 and -60 as they should.

Only problem I have now (which I cant see being connected) is I have an awful hum when I plug a lead in. I can plug one in the FX return and it is fine but if i put it in the input (either of them) it hums like mad. If i put my hand anywhere near the wires from the back of th e input sockets it gets worse. Ive tried the usual like turning off the lights and anything else thats switched on nearby.
Im assuming an earth somewhere?
.

[allante666]

Sorted! I found the source of the hum...duh! I picked up a spare mains lead and it had no earth connection in it! Put a good one in and hey presto! One working amp!
thanks for all the pointers guys.