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
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
Pretend this says EP1500 on it, they're the same.
http://www.qscaudio.com/support/library/schems/Current/RMX Series/rmx1450.pdf
http://www.qscaudio.com/support/library/schems/Current/RMX Series/rmx1450.pdf
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.
Is the center point of the 3300mfd caps taken to another part of the circuit.
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 said:
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 ?
RE: Blowing Capacitors
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. 😳
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. 😳
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?
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?
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
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
"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.
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.
Hi,
Have you got anywhere with it. I have seen this kind of circuit before -- I think it was called a reversed output amplifier.
If you look at the speaker terminals on the PCB they are "reversed" polarity wise to what is normal. I can't help feeling that there is more "circuit" than is shown in the link. There was a design published like this years ago in Electronics World.
Did you find where J104/11 went.
Have you got anywhere with it. I have seen this kind of circuit before -- I think it was called a reversed output amplifier.
If you look at the speaker terminals on the PCB they are "reversed" polarity wise to what is normal. I can't help feeling that there is more "circuit" than is shown in the link. There was a design published like this years ago in Electronics World.
Did you find where J104/11 went.
I'd like to point out that the QSC RMX and the Behringer EP series are similar, but not identical. The schematic topology is alikea and the board layouts are resembling each other, but the differences are in the board details, the power supply and the input circuitry. Also, as already noted, the component designators are likely different.
As of hearsay around here the RMS series is the more powerful, more reliable construction. 😉
As of hearsay around here the RMS series is the more powerful, more reliable construction. 😉
If they DID use the centertap then the capacitor wouldn't do this - so it must be identical in this respect. Another possibility is that the cap is installed backwards.
Mooly: J104/11 goes to J204/11. They have a pretty strange way of drawing schematics.
Mooly: J104/11 goes to J204/11. They have a pretty strange way of drawing schematics.
Hi,
Thanks for that Megajocke. It's difficult without the thing in front of you 🙂
R157 and R158, not open circuit are they ?
I can't really get my head round this one without actually seeing it.
The way it looks to me is this.
The collectors of the outputs are linked as in any amp of this configuration. The output would be taken from here via the speaker to the "zero" line at the PSU.
This amp though calls that point ( the collectors) speaker negative.
I think what you need to do is this, see first what DC voltage is across each 3300mfd cap with no speakers attached. I would expect them to be equal pos and neg. If they are not then look around the circuitry at R157/8 etc. Are the rails to the Op Amp OK. They are floating-- look at the returns from the 15 volt zeners, they go to the "output", point P1 which is the collectors of the outputs. The supply to the OpAmp i.e. +/- 15 volts "tracks" the output.
You have your work cut out with this one, it's all done in the name of efficiency and power rather than audio quality I suspect.
Good luck
Regards Karl
Thanks for that Megajocke. It's difficult without the thing in front of you 🙂
R157 and R158, not open circuit are they ?
I can't really get my head round this one without actually seeing it.
The way it looks to me is this.
The collectors of the outputs are linked as in any amp of this configuration. The output would be taken from here via the speaker to the "zero" line at the PSU.
This amp though calls that point ( the collectors) speaker negative.
I think what you need to do is this, see first what DC voltage is across each 3300mfd cap with no speakers attached. I would expect them to be equal pos and neg. If they are not then look around the circuitry at R157/8 etc. Are the rails to the Op Amp OK. They are floating-- look at the returns from the 15 volt zeners, they go to the "output", point P1 which is the collectors of the outputs. The supply to the OpAmp i.e. +/- 15 volts "tracks" the output.
You have your work cut out with this one, it's all done in the name of efficiency and power rather than audio quality I suspect.
Good luck
Regards Karl
Hi,
If one is uncertain of the topology it is much easier to grasp the concept when seeing a simpler schematic, look at this older QSC schematic as an example.
Also search here for "flying rails".
I have the EP1500 schematic so mail me and I will send a copy.
But as DJK says, EP-1500 is similar to QSC RMX-1450.
The differences I've seen regards layout and choice of OP-amp so the RMX-1450 schematic should be enough.
If one is uncertain of the topology it is much easier to grasp the concept when seeing a simpler schematic, look at this older QSC schematic as an example.
Also search here for "flying rails".
I have the EP1500 schematic so mail me and I will send a copy.
But as DJK says, EP-1500 is similar to QSC RMX-1450.
The differences I've seen regards layout and choice of OP-amp so the RMX-1450 schematic should be enough.
The resistors R157 and R158 are for providing more current to the opamp circuitry at higher output levels. R133 and R135 is what supplies current at idle or low output levels. The opamp is NOT floating and output collectors are connected to real ground. This has the advantage of ground-referenced drive to the output stage - the opamp only has to swing a couple of volts to give full output current. Also, the output transistors can be mounted on a non-live heatsink without insulators. This gives a bit better thermal performance.
R118, R119 (R218, R219 on the other channel) are what sets the DC level on the output. If one of them is out of tolerance there will be DC on the output. When speaker is connected this will happen over the caps too. I'd suspect either these or possibly the supply for the opamp having a problem. If the problem is DC imbalance and not caps installed backwards, that is.
That older schematic posted by 4fun is a bit easier to read if the topology is unclear! 🙂
R118, R119 (R218, R219 on the other channel) are what sets the DC level on the output. If one of them is out of tolerance there will be DC on the output. When speaker is connected this will happen over the caps too. I'd suspect either these or possibly the supply for the opamp having a problem. If the problem is DC imbalance and not caps installed backwards, that is.
That older schematic posted by 4fun is a bit easier to read if the topology is unclear! 🙂
i'm thinking you have a shorted output device. that would put 156 volts across one set of caps once a speaker is connected. yes this design is kind of "backwards" because the power supply floats with the audio, and the amp "walks" the power supply in reference to ground. you will measure normal power supply voltages with no signal and no load, but if you have a shorted output device, all of that will change when you connect a load, because you are now providing a reference point for the output terminal, and that reference point is 78 volts from where it should be, and that puts full rail-to-rail DC across one pair of caps..
since all of the collectors are grounded, just check from each emitter to ground. another thing that could do the same without shorted outputs is that the drive to the output transistors is latched to a rail.
since all of the collectors are grounded, just check from each emitter to ground. another thing that could do the same without shorted outputs is that the drive to the output transistors is latched to a rail.
Hi Megajocke ,
"Real Ground" or "Pseudo Ground", depends on your point of view or your point of reference should I say.
Experience says this fault is probably caused by a component that runs hot or is stressed in some way. Sounds simplistic, but when you find the fault, the evidence will have been there all along.
"Real Ground" or "Pseudo Ground", depends on your point of view or your point of reference should I say.
Experience says this fault is probably caused by a component that runs hot or is stressed in some way. Sounds simplistic, but when you find the fault, the evidence will have been there all along.
RE: Blowing Capacitors
I removed all the capacitors and checked the line voltage across the diodes and this checked out at 115v on channel 1 and 116v on channel 2. The voltage remained stable so I installed the capacitors back and tested the voltage again. The voltage went back up to 135. However, this time I did not connect the speaker to the output. With the gain log pots turned down, I checked the output voltage across the speaker output terminals and the voltage was 20 - 26v unstable. This is a clear indication that the capacitors are bad as the voltage across the terminals should be between 0.01 -0.08v which would be a good indication that the caps are ok. I ordered a new set of Capacitors but I wanted more performance so I ordered 4700uf versions. They were a little big so I had to modify the chassis to incorporate the new sized caps.
After installing the new caps I checked the voltage again across the speaker terminals. These tested out @ 0.05v for CH1 and 0.08v for CH2. The next day I tested out the amplifier in both stereo and bridged mode. WOW!! 😎 The performance is incredible even with the slight capacitance increase. The dynamic range of the amplifier has significantly improved with detailed sound reproduction on the upper frequency range from 1khz to 16khz. The low end frequencies are significantly defined with reduced THD. Thanks to all who contributed to helping me on this thread.
I removed all the capacitors and checked the line voltage across the diodes and this checked out at 115v on channel 1 and 116v on channel 2. The voltage remained stable so I installed the capacitors back and tested the voltage again. The voltage went back up to 135. However, this time I did not connect the speaker to the output. With the gain log pots turned down, I checked the output voltage across the speaker output terminals and the voltage was 20 - 26v unstable. This is a clear indication that the capacitors are bad as the voltage across the terminals should be between 0.01 -0.08v which would be a good indication that the caps are ok. I ordered a new set of Capacitors but I wanted more performance so I ordered 4700uf versions. They were a little big so I had to modify the chassis to incorporate the new sized caps.
After installing the new caps I checked the voltage again across the speaker terminals. These tested out @ 0.05v for CH1 and 0.08v for CH2. The next day I tested out the amplifier in both stereo and bridged mode. WOW!! 😎 The performance is incredible even with the slight capacitance increase. The dynamic range of the amplifier has significantly improved with detailed sound reproduction on the upper frequency range from 1khz to 16khz. The low end frequencies are significantly defined with reduced THD. Thanks to all who contributed to helping me on this thread.
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