EP1500 blowing caps

[ATech]

Someone help please

One of the 100v 3300uf caps have blown on both modules of the amp. They keep blowing even after being replaced. Does anyone have the schematics for this amp as I cannot find them anywhere or can anyone tell me which component is regulating the voltage as I don't want to make operating conditions any worse. The current line voltage supplying the individual caps is between 130 - 135volts. The other caps are operating as they should at 100v. The amp is fully operational and still intact. The output transistors are fine etc...

Regards AT

[djk]

Pretend this says EP1500 on it, they're the same.

http://www.qscaudio.com/support/libr...es/rmx1450.pdf

[Mooly]

Strange circuit, there does not appear to be a ground reference to the mains transformer. Is the output AC coupled using the smoothing caps as coupling caps ? Where does J104/11 go.
Is the center point of the 3300mfd caps taken to another part of the circuit.

[megajocke]

Looks like that. If the output gets a big DC offset the caps could blow. (check the voltage over C-E of the different banks of output devices)

[ATech]

Many Thanks Djk.

[nigelwright7557]

Originally posted by ATech
Someone help please

One of the 100v 3300uf caps have blown on both modules of the amp. They keep blowing even after being replaced. Does anyone have the schematics for this amp as I cannot find them anywhere or can anyone tell me which component is regulating the voltage as I don't want to make operating conditions any worse. The current line voltage supplying the individual caps is between 130 - 135volts. The other caps are operating as they should at 100v. The amp is fully operational and still intact. The output transistors are fine etc...

Regards AT

If your line voltage is too high then it could be a transformer problem or a dropper in the cct has gone short.

If the caps keeps blowing then its supplied with too high a voltage and you need to find out why.

Or could be someone else has tried a repair and fitted the wrong caps ?

[ATech]

I checked the line voltage comming from the bridge rectifiers just to ensure that my transformer was ok. The voltage checked out at 365v and 364v stable accross the two channel inputs. This is ok since they are 400v rectifiers. I am still within my voltage limit comming from my transformer. I went on further to check each individual capacitor just to see that everything was the way it should be. The capacitors seemed to check out ok with an ohmmeter so I moved the remaining capacitors from channel two and installed them into channel one. I went on to powering up the unit and the capacitors seemed fine until I connected a speaker to the speakon terminals. I experienced a strange low frequency sub harmonic rumble for about 2 minutes then... One of the capacitors exploded sending thick white smoke into the air. I quickly turned on a fume extractor. ops:

[megajocke]

Those voltages sound totally wrong, but I can't think of anything other that broken multimeter, bad battery in multimeter, wrong connections or trying to measure DC with the AC setting of a cheap DMM causing that.

I'm pretty sure it's the last one - measuring rail-to-rail voltage on the AC-setting. The correction factor in these multimeters converts the half-wave-rectified measured average to the corresponding RMS value. Shown value = (measured halfwave average) * pi / sqrt(2)
This would suggest a voltage of 164V which is about correct for the amp. (164*pi/sqrt(2) ~= 355V)

That the cap blows when you connect the speaker while the speaker hums sounds like there is a DC offset on the output. The speaker won't see the offset because of the caps, but if it's more than 20V the caps will blow up on one side. Measure DC voltage between C-E on the output transistors - it should be almost the same for NPN and PNP side.

Is there any distortion? Asymmetric clipping?

[DRC]

I hope you have it wired for 240v

There doesn't seem to be a lot of mF's for 4 pairs of outputs & 70+V rails. The ripple current will be high - what was the ripple current spec of the caps you replaced.

I assume the CT of the transformer secondaries is connected to ground - but it is not shown on the schematic

[megajocke]

"I assume the CT of the transformer secondaries is connected to ground - but it is not shown on the schematic"

It isn't. The power supply is floating with the output. Some models use the center tap though connected to the output node but this model doesn't.

I'd suspect R118, R119, R218 or R219 (maybe they have other numbers in the behringer?) being bad or a bad connection somewhere. You could check the connectors.