Emphaser EA2250xt

[Achim1409]

Got this amp. The previous owner damaged allready a few parts, but OK now. It doesn't produce audio. Rail is OK, +15,-15 is present.

Audio signal is present at pre stage. No voltage at all on the bias transistor. I have no diagram, and dont know where to look.

 

[Perry Babin]

Is this close?

 

[Achim1409]

No, it's different. Optocoupler is OK. Q117 and the transistor at the other channel is no voltage. Only the audio signal.. Very strange. Can't find the problem

 

[Perry Babin]

Is Q117 used for muting?

If so, if you lift it, do you get audio?

 

[Achim1409]

No, I lifted allready, It does nothing. Somewhere I missing a voltage.. I don't believe, that all this transistors are negative power supplied.

 

[Perry Babin]

Do the differential amplifiers have both positive and negative bias voltage?

 

[Achim1409]

The op amps at the preamp have both 15v volategs.

 

[Perry Babin]

That doesn't mean that the differential amplifiers have voltage. Since you said that the diagram wasn't useful, I have to ask general questions.

 

[Achim1409]

Can you draw it inside the picture, which are the diff amps?

 

[Perry Babin]

Find the equivalent of R142 and R143. What's the voltage on those resistors.

The resistors on the emitters of the differential amplifier transistors are 220 ohms in this amp. They are generally low value. 100 ohm are also common. << just to help in finding the right resistors.

 

[Achim1409]

It's a 2.7k with 4.8v across. The other resistor is 100 ohm, as you mentioned.

 

[Perry Babin]

What's the DC voltage on terminal 1 of each of the resistors that correspond to R142 and R143?

 

[Achim1409]

Input of rr142 is 5.8v,output is 0.7v

 

[Perry Babin]

R143?

 

[Achim1409]

Channel 1,resistor 1 +5.8v, resistor 2 - 5.8v
Channel 2,resistor 1 + 5.8v,resistor 2 0v

 

[Perry Babin]

Find why you don't have negative voltage on resistor 2 of the second channel.

 

[Achim1409]

A cap was shorted, removed the cap, now 5.8v, 0.7v

 

[Perry Babin]

Do you have audio?

If not, with Q117 lifted?

 

[Achim1409]

No, only very weak with gain full in. Q117 has the audio signal on the single pin, the other two has 0v and no signal. Even with 117 lifted

 

[Perry Babin]

Do you have audio on the inputs to the differential amplifiers?

Have you operated all pots and switches through their entire range while listening for audio to see if it every played through (even just for an instant)?