I’m having some issues with a Beomaster 1000. It’s an early one with white keys and AD149 transistors. It worked the last time I used it back in 2014, then sat on a shelf. I replaced the PSU capacitor and output capacitors before powering it on. Now I’m seeing extreme distortion and low volume on both channels. The voltages are fine from the driver and main transistors. The rail voltage is sitting at 30v. The quiescent current seems on the low side, around 9mv. The output capacitors sees about half of the rail voltage, 15.5. Both L and R channel get’s louder if the balance potentiometer is switched to each respective side. Any idea where to look next? The values of the three capacitors were 2000uF, I replaced these with 3300uF like they are in later versions. I find it weird that both channels are equally faulty, I’ve checked my soldering and wiring multiple times without finding any fault.
I would try injecting a signal at the wiper of the volume control for each channel with the volume at max and then take your distortion measurements. This will pinpoint whether you have an amp or preamp issue. I’m betting on the latter from your discription.