Beomaster 900K with loud hum on everything

[Owenski]

Hi all, I have a beomaster 900k which seems to have developed another bad fault. The fault this time seems to be a bad loud hum which is present on every wavelength and on the tape and gram buttons. I cannot hear any music or radio stations at all.

I have noticed the big SP1446 transistor on the back looks a bit rusty and the small resistor that is across the two pins of this transistor looks very black in the middle.

I have looked on the internet and I have found the SP1446 can be replaced with a AD149.

Should I replace this SP1446 Transistor first or am I looking in the wrong area?

I have the original B&O circuit digram and I am very handy with a soldering iron

If anyone could help me with this then I would really appreciate it.

Many Thanks

Owen

[Graham G3ZVT]

Is the hum present when the volume is at zero?

Graham

[Owenski]

Hi Thanks for replying.

yes you can turn down the loud hum with the volume knob.

I now have taken the inside radio frame out of the speaker box to look at it on my workbench.

I have just noticed the SP1446 Transistor on the back is getting very hot and I think there is a little smoke coming from ether the SP1446 transistor or the little brown resistor that is linked across the two legs of the transistor.

Any ideas on this?

[Owenski]

Ok I have now done some tests with my multimeter.

Here are the results.

From transformer with bridge rectifier disconnected I get 25.5 volts AC

With he Siemens B30C 1600 bridge rectifier connected I am only getting 12v DC

I have disconnected the DC red and black output wires from the Siemens rectifier so the radio is dead to do these tests.

Now the circuit digram I have is showing 34v AC from the transformer and 24V DC from the output of the bridge rectifier.

I also can not understand why the transistor SP1446 on the back is smoking hot when the DC voltage from the bridge rectifier is so low.

Anyone any ideas on this?

[stevehertz]

I also can not understand why the transistor SP1446 on the back is smoking hot when the DC voltage from the bridge rectifier is so low.

Anyone any ideas on this?

Without getting into the 'actuals' of that particular circuit, the laws of physics say that if the impedance is low enough, then the current in such a circuit will be high (making components hot) even if the voltage is low. In fact, the low supply voltage may well be as a result of a low impedance (low resistance fault) in that circuit. Also, in some power supplies the transistors are too hot to touch yet working as designed. I was working on a hifi receiver today where this is the case.

[Owenski]

Did I tell you that the Motorola SP1446 Transistor seems to be giving off smoke when the radio is turned on?

I am thinking the mains transformer is faulty as according the circuit digram I should be getting 34 volts AC. I seem to be only getting 25 volts AC

[ms660]

""Now the circuit digram I have is showing 34v AC from the transformer and 24V DC from the output of the bridge rectifier.""

On the schematic I'm looking at it's 34volts DC output from the bridge rectifier and 24 volts DC from the output of the regulator transistor.

Lawrence.

[Owenski]

Thanks for the quick reply Lawrence,

I have that same B&O circuit digram that came inside the radio. It is showing me there should be 34 volts AC off the transformer secondary side.

I think there is something up with the transformer as 25v AC from it does not make sense.

With the rectifier connected the DC output is just 12v DC

The 6V illumination output from the transformer seems to be ok.

Would the low voltage on the transformer be causing the large Transistor SP1446 to give off smoke? This Transistor is marked as 2N555 on the circuit digram

Please tell me your thoughts on this?

Thanks

[ms660]

Not sure yet, with respect to chassis measure the voltages on the regulator transistor (SP1446) emitter, base and collector, need exact measurements to at least one decimal point.

Lawrence.

[TrevorG3VLF]

I would suggest that the transformer is alright. There is a large load somewhere that is pulling too much current through the regulator transistor and pulling the transformer output low. The bridge rectifier is also likely to OK, test each diode with a meter.

A loud hum suggests that the reservoir capacitor(s) is dead and possibly shorted. I would disconnect it and see if overheating still occurs. The hum will still be there.

[PJL]

Too much current probably from a fault in the output stage. Don't leave it on as the mains transformer will be the next to fail.