[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 sounds like TR46 is not being turned on. Are the voltages on its base and emitter as per the diagram?

[Wumpi]

It’s not tr46 cause I swapped from the left channel and still nothing..

The commiter still has no output voltage while base and emmiter has..

[Mark-sf]

You misunderstood my suggestion. If the voltages are not as specified in the schematic the transistor will not turn on even if it is good. What are the actual base and emitter voltages?

[Wumpi]

Hello Mark.

The voltage in left good channel is around 22 on input diode and on right is 27 voltage.more close to factory.

BUT this transistors are only for current protection. As the manual says.When I removed them the left channel had still voltage on diode output.

Btw base voltage 28.6 emmiter 29 collector 0

 

 

[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

[Mark-sf]

Glad you got it sorted out as I was out of suggestions. First try the bias adjustment.

[Wumpi]

How much the bias should be set?where exactly should I look?

The problem is that the replacement transistor burned again after a few minutes..

[Mark-sf]

The 250-ohm trimmer and TR45 governs how much no load bias is applied to each base. It is mounted on the heat sink to sense temperature and also adjust the bias. It’s possible that transister is bad or the adjustment is too high. The circuit calls for a BC311 and since the bias is not separately adjustable for its BC310 mate its possible that your BC161 is a poor match. Sorry, but its been too many years since I worked on one of these.

[Wumpi]

But Bc 161 specs are almost identical with bc 311.

Btw How many miliampere should I adjust?

The manual doesn’t say anything for the adjustment.

In which transistor base should I put the multimeter probes to take meausurement?

Thanks

 

[Mark-sf]

The base of TR45 should be adjusted to 27.2 which should give you 26.6 at the collector of TR48. If the base of TR44 is 29.2V and you still are seeing a drop to 22V then the speaker output voltage before the output cap can’t be 28v. Note that the positive side has an 100 ohm resister to isolate it where the negative doesn’t. You need to further investigate after TR47 for the current draw.

[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]

Don’t get me wrong, it may sound harsh but I mean this in the best of ways when I say that
I have a feeling that you will not be the right person to do this repair.
Your methods will only bring more headache, and it appears your electronic education and experience is limited. Right?

Every educated electronic technician will know what the emitter resistors are and how to
measure idle currents.
The manual may state a mA figure, which is and refers to the actual current,
but a brief look at the emitter resistors value and a little bit of Ohms law will
allow you to measure the current through a resistor as a voltage across it, and
as an added bonus you can measure it without disturbing the circuit by lifting components.
You only need to take the top cover off to measure and adjust.

And I wasn’t referring to the output series capacitors. I was referring to the capacitors
in the output stage itself, as that is where the problem is.
They must be replaced, and it’s good advice to never fit low-ESR types in a circuit like this.

Output stages like this can give even trained tech people a good run for their money.

Martin

[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

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