02-19-2014, 06:41 PM
The bias point on these old OTL transistor amps is not well controlled, as there is no DC feedback from the output as Morzh previously mentioned. Centering the output stage voltage really depends on the match between the top and bottom sections of the circuit. Sometimes I have seen a variable resistor in place of one of the 330 ohm resistors so the bias point can be accurately set to half the supply.
If the series output 1500 uF cap is dried out, this could also cause low output in the bad channel, although the signal would most likely be distorted when you tried to turn the volume all the way up.
In any case, the low audio problem may not be associated with the output stage voltage. Have you checked that the signal level is identical into each channel's power amp input? The low audio problem may be located further upstream. You can try switching the wires from the volume control slider into each input and see if the low audio is then on the same or opposite channel.
If the series output 1500 uF cap is dried out, this could also cause low output in the bad channel, although the signal would most likely be distorted when you tried to turn the volume all the way up.
In any case, the low audio problem may not be associated with the output stage voltage. Have you checked that the signal level is identical into each channel's power amp input? The low audio problem may be located further upstream. You can try switching the wires from the volume control slider into each input and see if the low audio is then on the same or opposite channel.
![[-]](https://philcoradio.com/phorum/images/bootbb/collapse.png "[-]")
