Help with Crown XLS 402 input board...

[Nails]

Help guys..........Any techs out there in audio land...

Recently my trusty XLS 402 keeps blowing "both" op amps TL072...... on the input board...I,am getting a reading of 16.2-.3 volts across the rear XLR inputs..as I believe there should be 1v thereabouts..reading the specs on the TL072 tey should be powered at about 18-20v which is taken from the 80v main board supply......a lot of head scratching...... I,am at my wits end with it also.... this would be the last resort before I,ll take it to a service shop and get the guys there to have a look at it...Cheers.

ATB

Nails

[indianajo]

Well, one thing you can do to stabilize things so you can look at it with a DVM, or even better an oscilloscope, is replace the fragile TL072 with something more rugged. If the ones you have are 8 pin Dual Inline Package, then you can replace it with ST33078 or JRC 4560 or 4562 for equivalent noise performance. Those don't have FET inputs. If you just want to check it out, DIP versions of the 4558 will work in the socket. They hiss a little bit. I think R**** S**** has 4558 in bigger cities.
You can also check from upper right to lower left on the pcb, if you can reach it, for no more than 18 VDC. Upper left is the top side, the one with the dot, or the left of the end that has the cutout or notch in it.
You could also have oscillation problems or shorted input capacitors allowing DC to get in from the signal source and blow it up. Didn't look at a schematic to see if this model has input capacitors or not, but it solves problems of damage from outside if it does. My Peavey CS800S PA amp had ceramic capacitors the size of rice grains on the input, that were exploded and burned, probably by somebody plugging a guitar amp output in the PA amp input, instead of the speaker where it belongs. Lots of 1/4 phone plugs with various functions float around on stage in an amateur band, and it is dark up there when they try to setup.

[Nails]

Originally Posted by indianajo

Well, one thing you can do to stabilize things so you can look at it with a DVM, or even better an oscilloscope, is replace the fragile TL072 with something more rugged. If the ones you have are 8 pin Dual Inline Package, then you can replace it with ST33078 or JRC 4560 or 4562 for equivalent noise performance. Those don't have FET inputs. If you just want to check it out, DIP versions of the 4558 will work in the socket. They hiss a little bit. I think R**** S**** has 4558 in bigger cities.
You can also check from upper right to lower left on the pcb, if you can reach it, for no more than 18 VDC. Upper left is the top side, the one with the dot, or the left of the end that has the cutout or notch in it.
You could also have oscillation problems or shorted input capacitors allowing DC to get in from the signal source and blow it up. Didn't look at a schematic to see if this model has input capacitors or not, but it solves problems of damage from outside if it does. My Peavey CS800S PA amp had ceramic capacitors the size of rice grains on the input, that were exploded and burned, probably by somebody plugging a guitar amp output in the PA amp input, instead of the speaker where it belongs. Lots of 1/4 phone plugs with various functions float around on stage in an amateur band, and it is dark up there when they try to setup.

Thanks Joe...I,ll have a look at those pin voltages first..
Here,s the schematic link.....XLS 3U input and output boards..down the bottom of the page...cheers..

[Nails]

Originally Posted by indianajo

You can also check from upper right to lower left on the pcb, if you can reach it, for no more than 18 VDC. Upper left is the top side, the one with the dot, or the left of the end that has the cutout or notch in it.

We seem to have 16.8-.9v across those ones pin 4 & 8....the same voltage at the input stage at the XLR pins......

[indianajo]

The voltage from pin 8 to pin 4 seems correct. I hope the voltage out the XLR input disappears when you pull the IC out of the socket and turn the power on. If you don't have op amp sockets, install them. I'm using tyco phosphor bronze contact ones from newark.com, seem to work fine. Got my 33078's there, too. Rugged little beasts of an op amp, I made any number of mistakes with them on the disco mixer upgrade and didn't blow any.
Your schematic link didn't make the post. I copy the window at the top of my browser then paste in the text I am typing. Downloaded 2 sales brochures from crown but no schematic yet.
If your design has input caps you might want to check to see if they are shorted out. If it doesn't, you may want to check your signal source to make sure it is not producing DC offset, or supersonic oscillation (with an oscilloscope). I got my B&K 2120 oscilloscope for $40 off craigslist, but the probes are $55 each from mouser.com. Supersonic oscillation from the disco mixer didn't blow up the inputs on my peavey, but it sure makes the fan run really fast on the amp. The peavey CS800s has JRC4560 op amps. Interchanging with 33078 made no difference in sound.

[Nails]

Originally Posted by indianajo

The voltage from pin 8 to pin 4 seems correct. I hope the voltage out the XLR input disappears when you pull the IC out of the socket and turn the power on. If you don't have op amp sockets, install them. I'm using tyco phosphor bronze contact ones from newark.com, seem to work fine. Got my 33078's there, too. Rugged little beasts of an op amp, I made any number of mistakes with them on the disco mixer upgrade and didn't blow any.
Your schematic link didn't make the post. I copy the window at the top of my browser then paste in the text I am typing. Downloaded 2 sales brochures from crown but no schematic yet.
If your design has input caps you might want to check to see if they are shorted out. If it doesn't, you may want to check your signal source to make sure it is not producing DC offset, or supersonic oscillation (with an oscilloscope). I got my B&K 2120 oscilloscope for $40 off craigslist, but the probes are $55 each from mouser.com. Supersonic oscillation from the disco mixer didn't blow up the inputs on my peavey, but it sure makes the fan run really fast on the amp. The peavey CS800s has JRC4560 op amps. Interchanging with 33078 made no difference in sound.

Sorry Jo try this one...

Discontinued Amplifier Products

[Nails]

When for broke this morning and changed out all 8 100uf 25v caps...4 each side...
It,s solved the problem of the high input voltage reading on the rear xlr terminals, which stands at 4.8-5.0 from the original reading of around 16-17v....not to sure if the 4-5v sounds right thou.... and also the LT072,s have stopped blowing...

[Nails]

Originally Posted by Nails

Went for broke this morning and changed out all 8 100uf 25v caps...4 each side...
It,s solved the problem of the high input voltage reading on the rear xlr terminals, which stands at 4.8-5.0 from the original reading of around 16-17v....not to sure if the 4-5v sounds right thou.... and also the LT072,s have stopped blowing...

[indianajo]

I don't know if your xls is a 2 rack height or a 3 rack height, so I got another stupid sales brochure from crown instead of the schematic. Looked at your blue-black picture, seems the xlr's inputs are DC coupled and don't have series capacitors. They don't have parallel overvoltage arrestors either, rendering the op amp input subject to all sorts of damage from DC or static from your input device. Peavey has clamp diodes to the power supply on the cs800s inputs to prevent overvoltage on the input from hitting the op amp.
However, if you have stopped having problems, stop there. The output of the op amps is coupled through the feedback loops to the xlr input, so you will have some voltage there, although 5 sounds like a lot of offset voltage. I can't read the feedback resistor value, I believe the input resistors are 10k? One wonders if the offset voltage at the input differes on 2 versus 3 as one is connected to the op amp output by feedback (I can't read the number) and the other is not.

[Nails]

Originally Posted by indianajo

I don't know if your xls is a 2 rack height or a 3 rack height, so I got another stupid sales brochure from crown instead of the schematic. Looked at your blue-black picture, seems the xlr's inputs are DC coupled and don't have series capacitors. They don't have parallel overvoltage arrestors either, rendering the op amp input subject to all sorts of damage from DC or static from your input device. Peavey has clamp diodes to the power supply on the cs800s inputs to prevent overvoltage on the input from hitting the op amp.
However, if you have stopped having problems, stop there. The output of the op amps is coupled through the feedback loops to the xlr input, so you will have some voltage there, although 5 sounds like a lot of offset voltage. I can't read the feedback resistor value, I believe the input resistors are 10k? One wonders if the offset voltage at the input differes on 2 versus 3 as one is connected to the op amp output by feedback (I can't read the number) and the other is not.

Yeah Jo it,s the 3U...you can view the schematics from here...
Discontinued Amplifier Products




There seems to be two diodes coming off the main board...is there a possibility that we are getting over voltage from the board......