Beomaster 3000-2 heavy distortion from right channel

[Wumpi]

I have a full distortion from right channel.

From the voltage and signal trace I did  I discovered that there is no output voltage and signal  from 496 diode.I changed the diode but still nothing

I swapped the transistors tr45 46 47 50 from the other channel but still nothing.

I swapped the input signals cables which goes to amplifier pcb and again nothing changed

I put new power supply capacitor and didn’t do anything

Tr46 has voltage on emmiter and base but not in the collector

 

Here is the schesmatic marked with no output

I can’t  find any solution to my problem

Any help would be appreciated.

Thanks

 

[Mark-sf]

It’s possible you have a broken trace at the anode of the diode. Try a jumper from it to the base of TR47 and the collector of TR46. Don’t know what else to suggest based on what you are reporting.

[Wumpi]

Hello.

Thanks for your interesting.

There is no voltage on anode leg of diode.i swapped two times with new but still nothing.

The trace is ok

I did a bridge between input and output of diode and I measured a significant voltage drop on the collector of tr50.Without the jumper the voltage was around 28 volt.

With jumper it went to 5 volt!!

Base volt was exactly 0.5volt

Any thoughts  on that?

 

[Mark-sf]

The diode is blocking current flow from TR48 collector to base of TR47 as it conducts in the other direction due to the higher voltage that is supposed to be on the base. Therefore, it cannot be bypassed with a jumper.

You say there is 0 volts on the anode, is it the same at the collector leg of TR46 and the base leg of TR47? If you power down, what is the ohms reading to ground from the anode for each channel?

[Wumpi]

Exactly it’s 0 volt on two transistors as you describe..I check the traces and everything is fine.i will make the meausurement on diode tomorrow to see the results

Btw I tried to put voltage from base of tr46 on tr47 and i read 3 volt on collector of tr50.Without this voltage the collector goes around 29v.

Why is this drop assuming that the base should have this voltage?

THANKS

[Wumpi]

Anode right channel 568 kohm.

Left channel 590kohm

[Wumpi]

Finally I fix.

It seems that tr47 was damaged and from the swap proccess  from left channel  I accidentally put the same transistor..

I replacced it with bc161.

Now i must set the correct supply voltage.

The base voltage in right channel is 27.5 and left is 22.2

Thanks for your support

[Dillen]

Stop swapping components. It will only up the risk of more trouble.
The idle current settings and measuring ponts are given in the service manual.
It’s a voltage measured across the emitter resistors with no speakers attached and volume at zero (hence the term idle current).
7,5 mV if I remember correctly – see the manual.
But if a transistor burns, there is something else wrong – the burning transistor is not the reason.
And if you replace one or more semiconductors in an output stage, you MUST set the idle current again at first power up.
Bring the Beomaster up slowly on a variac while monitoring the idle current – power down immediately if something looks wrong.

Did you replace the electrolytic capacitors in the output stage? If you did, did you fit low-ESR types? You shouldn’t, as that will often be asking for trouble.

Martin

[Wumpi]

Hello.

Which emmiter resistors are you referring?

No I didn’t replace the output capacitors.

The manual aI read doesn’t refer anything about bias procces setting.Not either any mv number set.I replaced tr47 bc311 with bc 161.But the base voltage would  come around 27 volt as I remember.

[Dillen]

Mark-sf wrote:

The base of TR45 should be adjusted to 27.2

– No.
You adjust the idle current to a given current flow at the output stage emitter resistors as stated and shown in the manual.
The voltages stated in the schematics are not for alignment purposes.
They are merely to help diagnosing in that you can see if a voltage is far off.

In a working circuit, the voltage(s) stated at TR45 will change with temperature, as will the voltages stated later in the circuit, because that is what
the circuit is built to do.
TR45 measures the temperature on the cooling fin and adjusts the final
output stages accordingly.
A transistors amplification increases with temperature, so if nothing grabs
and controls it, it would run astray until something starts burning.
That’s also why idle currents always are to be set with the amp (output stage/cooling fins) at room temp.

Martin

[Wumpi]

The output voltage on the resistor is  around 28v

So 28÷0.15=186

Are you sure this the right proccess?

[Dillen]

The idle current should be expected to be around 7,5-15mV or something within that range.
The service manual will specify what’s correct for your model.
But the idle current is not the primary problem in your amplifier. If it’s wrong, it’s because something else is amiss.

Which resistor are you measuring across?

Martin

[Wumpi]

I am getting back in this forum cause this amplifier makes me crazy.

Tr 47 keeps blowing all the time.i have checked the voltage and its all good

The trimmer pot is working fine after i desoldered it and check it out.

It’s on middle position set..

What should i check to find out the problem?

Please guide me..

Thanks

 

[Author]

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