Hi

I have just recapped and renovated a BeoMaster 3000-2 (type 2402).  All has gone well (like others I have done ) but at zero volume from all input sources I hear fairly loud (10watt ??) random crackes and pops on the left channel .  I am certain that problem lies in the power amplifer because a) it occurs with the volume on zero and b) moving the balance to the right channel has no effect either i.e. the noise can still be heard.  I have heated the board with a hair dryer and cooled each component belonging to the left channel in turn looking for the faulty component but the problem does not seem to react to temperature.  Monitoring the signal path with a scope has not brought much because the pops and crackles appear as transients all over the place.

Do any of you experienced repairers have any suggestions as how to narrow the problem further?

Regards

Richard

Hi Richard,

this effect happens, when old parts come to life again... Some caps are still more dead than alive. I think, there are still some of them in the power supply or in the (left) amp.

Old (electrolyte) caps are not save when used on its highest voltage, they become worse and make shortcuts... that's what you can hear. 

But this could also be a dying diode in the rectifier of the power supply. 

You have a adjustable transformer? Maybe try a lower voltage... 180-190V? Then you could test this effect.

Martin

Some of the small signal transistors seem to develop noise. I've had many cases of radios that, after 20 years of standing unused, recapping and general service, play fine for a couple of hours and then start to crackle and pop. In most cases, one of the small signal transistors were acting up. Especially BC182's seem prone to do this and the ones I've seen until now were unaffected by temperature changes.

Where the older germanium transistors, especially AC126, start to buzz constantly and at a constant level, the "newer" silicon transistors, like the ones used in your Beomaster, tend to crackle quietly with an occasional louder pop.

I agree that caps can cause the strangest faults but in your case, I think I would focus on a small signal transistor. If you don't have a lot of new transistors lying around, you can swap left and right channel transistors, one by one. Since the fault in your Beomaster is not affected by the volume control setting, you can start at the tonecontrols and work outwards towards the output stage.

Martin

@Dillen: Martin, when you replace a "defect" BC182 by a new one, does it also happen?

I think, even a "new" BC182 is 20 years old and should have a similar defect. Or do you think, there is something like a "start" necessary for the aging process?

Martin

I don't know what causes the transistor to develop this fault.

I don't have any new BC182's so I always put in a substitute. But with the AC126, it doesn't seem to matter whether it's a new component or a used one. Even a NOS (New Old Stock) AC126 can fail. 

Martin

Spot on Martin 

It was TR56 (BC212B L) that was causing the problems.  Thanks for the tip...

Richard