Total posts : 45366
You are getting close to solving the problem.
Measure the emitter voltage and post what you get. Also measure and post the C,B,E voltages for Q1, Q2, and Q3.
From what you have done so far, it is possible that Q4 or Q5 are in backwards or there is something wrong with R30 or R31. Double check this.
Transistors are easy to substitute and the DC bias voltages will not vary much. The use of “not exactly the same from the package” will not cause the trouble you are having.
There is something grossly wrong with the biasing or there may be a bad part. If you post the voltages I asked for, it will really help narrow it down.
What follows has little to do with your problem. Some additional information about Q4 and Q5. This circuit is called a balanced modulator and it is not obvious how it works. Q4 and Q5 both are involved in the modulation, but so are Q2 and Q3. For those who are interested, Q4 and Q5 are multiplying the carrier by the audio to produce the AM sidebands (DSSC) and are acting basically as switches. If anyone wants to pursue this check out the MC1496 application notes.