Broken B.K.E. OMP MF-450 (output messed up)

[Thinkcat]

Hello

I have one 450W mono module built by B.K. Electronics. The power devices say SML BUZ900 or BUZ905, G9614BY or N9532BY. I guess the thing was bought some time in 1998. The capacitors are light blue, so this seems to be quite an early model.

I ordered this for a friend at that time. He used it, driving subwoofers, for maybe a year or two. Then it started to malfunction, and he gave it back to me for repairs. I have lost contact with him, and all these years the amplifier has stayed gathering dust in my bedroom closet.

The symptoms are that all output is kind of distorted. At first one hears nothing. Then, turning up the volume, out comes something that sounds like someone making farting sounds by pursing his lips. Turning up the volume, one can get as much of this as one wants.

Nothing on the circuit board seems loose. Nothing seems burnt. Both fuses are ok (visual inspection and measurement), and the power supply caps do charge up to +80V and -80V. If I want to ever have this thing work again, what should I go on looking for? I know how to use a soldering iron, and I know how to take apart the amplifier. I will post pictures on request.

[destroyer X]

Post pictures and schematics, this way people will come to see and will help....others will come to help and will see.... well.... everybody loves to see amplifiers..we are all fanatic.

regards,

Carlos

[Fanuc]

Hello,

This design is based on the infamous Hitachi lateral Mosfet amplifier for PA applications. The noise you mention sounds like DC on the output. The output fets and drivers can blow on these. BK doesn't even drive the 6 output fets with followers! no wonder!. I have got a BK OMP600 amp. 300w rms. per chl. simliar design to yours.

Check the output relay also.

You should be able to get schematics from BK electronics.
http://www.bkelec.com/MODULES/mf450.htm

I would check the drivers and fets first.

Regards

Kevin

Originally posted by Thinkcat
Hello

I have one 450W mono module built by B.K. Electronics. The power devices say SML BUZ900 or BUZ905, G9614BY or N9532BY. I guess the thing was bought some time in 1998. The capacitors are light blue, so this seems to be quite an early model.

I ordered this for a friend at that time. He used it, driving subwoofers, for maybe a year or two. Then it started to malfunction, and he gave it back to me for repairs. I have lost contact with him, and all these years the amplifier has stayed gathering dust in my bedroom closet.

The symptoms are that all output is kind of distorted. At first one hears nothing. Then, turning up the volume, out comes something that sounds like someone making farting sounds by pursing his lips. Turning up the volume, one can get as much of this as one wants.

Nothing on the circuit board seems loose. Nothing seems burnt. Both fuses are ok (visual inspection and measurement), and the power supply caps do charge up to +80V and -80V. If I want to ever have this thing work again, what should I go on looking for? I know how to use a soldering iron, and I know how to take apart the amplifier. I will post pictures on request.

[Thinkcat]

Originally posted by Fanuc
Hello,

This design is based on the infamous Hitachi lateral Mosfet amplifier for PA applications.

You mean there is a known bad precedessor for these amps? Where could I read more about it?

I have this 450W mono module I ordered for the friend I mentioned. I also have two 300W mono modules that I ordered for myself. Of these two, one has done fine always when I have used it. Last time was maybe 5 or 6 years ago. After that I built myself some Leach amps, and I've been happy with those since.

The other 300W module was plagued by an unstable quiescent current. There was maybe a bad contact in the potentiometer that is used to set it. Sometimes the module was barely warm like the other one, but sometimes it felt very hot after a while of just idling. I finally soldered the potentiometer off and put another one in its place, but something went wrong because there now appeared intermittent but violent pops and cracks in the output.

The transformer of the 450W one is manufactured in April of 1996. The 300W modules are about as old, maybe one year older. So if there is no quick and simple fix, I am giving up. And if there was a quick and simple fix, are these bound to go the same way soon anyway? I'm not even sure if these are worth saving. Or what would anyone else think?

[nigelwright7557]

Originally posted by Thinkcat
Hello

Then, turning up the volume, out comes something that sounds like someone making farting sounds by pursing his lips. Turning up the volume, one can get as much of this as one wants.

I would start by checking the output transistors.
If they are OK I would start to have a look around the driver board. Could be a fault in a constant current soruce or the LTP or the VAS or the bias. Its a matter of poking around with your scope or multimeter until you find something that doesnt look right.

RS components still do the BUZ device as i bought some a while back. In the end I dropped it for the IRFP240/9240 series.