Behringer BXL 1800A

[oc disorder]

Have a Behringer bass amp a very early model by the look of it with a blown IRFP9 140N mosfet.
Instead of taking the fuse out it exposed and fried the copper trace that leads to the main supply diodes.
I managed to track down 1 replacement in Aust (seems hard to get ) and also got the compliment IRFP 140N.
The 2 MJE 350/340's seem ok so I'm hoping the front end of the circuit is ok.
The small output board is "Model LPA1180".
Its a small wedge style bass amp supposed to be 180 watts.
Right at the back right are 2 pins labeled "X1 2P" which appear to be the place where the quiescent current is measured.
What I would like to know is the suggested reading one should get at this point as if I stuff up.... I have no replacement mosfet I can easily obtain. Thanks

[Enzo]

Those test points are across R1 - a 0.1 ophm resistor. Set the bias for 5mv between them with the amp COLD. Don;t warm it up.

The two gate resistors for the MOSFETs are 47 ohm R2,3. Make sure neither burnt open.

D1-4 are all 15v zeners in pairs of back to back. They serve to clamp the gates if excess signal appears. Make sure none are open or short. (probably are OK)

If a gate resistor is burnt then check 220 ohm R6,7. They are in the collector circuit of the two MJE xstrs.

Did you find you had to remove the couple cement resistors to access the mounting screws of one of the MOSFETs?

[oc disorder]

Thanks Enzo for your reply.
"Did you find you had to remove the couple cement resistors to access the mounting screws of one of the MOSFETs?"
Well I confess no I was a bit brutal removing the whole unit but I managed
to loosen that retainer clip enough to slide out the whole unit.
I did remove the 2 main caps to see the extent of damage to the tracks.
Had to cut the power wires that run through the tiny hole in the cabinet.
Then it was a matter of wrestling with the pre-amp connector plug to pass that through the hole.
Those wire wound "cement" resistors did look like they were hand soldered
so I'm not sure if someone had been there before or if it was a standard production procedure.
Interesting to see how it goes as I still have the BX 1200 in the too hard basket (actually now have 2 units) I think my smd soldering could have been neater soldering in the smd TLO 74 (IC 10 ? can't remember) although it didn't fix the problem and the amp sounded the same although I did feed an external signal direct into the power amp which confirmed the fault still lies in the pre-amp. However I shall save this for another post when I get time.
At least they both seem to have the same fault as I have the luxury of two preamps and power amps !
Thanks again for the info I still don't know how you find the time to help so many and keep your own business going !
Maybe you should have a "Dear Enzo" column in a suitable magazine.
cheers
OC

[oc disorder]

Well having no luck with the unit.
The .1 ohm resistor was o/c (heh) the MJ340/350 are fine as are the 15v zener's.I also checked the KPS 06 out of circuit which is also fine.
Had to replace R7 a 220 ohm resistor that seems to connect to the MJ350. The two 47ohm gate resistors were ok.
Getting DC on the output seems to be the supply rail used a 100w light bulb in series which lit up so I didnt hang about and check voltages.
I made a right botch around T4 MJ340 trying to get it out thats near where it got extreemly hot I found later it was eaiser to remove one leg of the quarter watt resistors near the MJ350 to test it for gain and breakdown wish I had thought of that before I pulled the 340 !
So I'll put it with the others for a while and maybe try to check all the tracks
and through holes . The 140's fets are not shorted but the way its going maybe they are open as I took a couple of secs to turn it off.
As its an old model I may try the local rep and see if its possible to at least get just the power amp schematic but I may get no reply.
The leds lit up on the preamp so I guess the regulators are ok.
I'll replace the 340 with a new one and not clip the leads too short so I have some termination points to hard wire it where the track is damaged.
All the smd diodes seem ok I dont know about the V4580M IC made by Cool Audio...http://www.coolaudio.com/files/publi..._DATASHEET.pdf .
Is this DC coupled? Probably drag this thread out again in a few weeks
when my patience is restored !

[Enzo]

Here, this is real similar to the amp module.

[miguelplus]

this is the schematic

[oc disorder]

Hi still having problems with this which I've just returned to.
Removed the fets and disconnected the 220 ohm resistors where pin 7 of the 4580 feeds the 2 340/350 transistors.
On power up pin 7 swings positive then swings negative and stays there
pretty much full supply (-15v). I assumed (wrongly) the chip had a problem
replaced it with a generic 4580 (btw the original chip was wider than the replacement ! ) and got the exact same result.Feeding in a signal - output waveform looks like a bunch of m's riding on the -15v rail see attached.
My thought process is as the output of the IC is direct coupled to the amp
and its dc that explains why I had almost full dc (40v) on the output.
I'm now guessing it may be due to the limiter or mute circuit fault
but I can't seem to get meaningful readings/waveforms from any of the other pins.
Next I plan to check all the components diodes etc around the opto and first
half of the IC. I thought by simplifing the circuit the fault may be obvious
but obviously missing something here !

[Enzo]

Like all solid state amps, this one is heavily fed back. That is what the lone from the output up and over to pin 5 is (through R29.)

When you disconnect all the output stages, the amp is no longer a loop and it has no weay to correct itself. SO it slams over to the rail.

You can't run the amp that way and get anything meaningful.

Are T2,T3 the outputs missing as well? if so, try putting a load on the output or just ground it. See if that stabilizes it somewhere.

For that matter what is at pin 1 of the IC?