doesn't go through the phono
amplifier — there's only the switch
between the tape output and the
tuner input. As expected, everything
was fine on the amp's front end. This
meant the problem had to be in the
In a push-pull amplifier design, if
the transistors aren't conducting
evenly you'll hear the distortion
audibly. I put the scope on the
speaker terminals to see what the
output signal was doing while
playing some music, and it was
showing a clearly one-sided trace,
and only the highest peaks at that.
This helped me narrow it down to the negative side of the
From the input to the final amplifier, I traced through
the circuit using the oscilloscope until I found the point
the signal became distorted. It turns out that Q8 — a
2SC1509 transistor for that side — was defective. It was
conducting, but just barely. I pulled it and replaced it with
a new old stock part, and that cleared the distortion right
up. The sputtering and popping in that channel remained,
though. Time for more digging!
The Plot Thickens
While poking around with the oscilloscope to figure
out where the noise was coming from, another problem
July 2015 49
Ready for first power-up.
Partial schematic of the inputs and phono pre-amp.
Oscilloscope trace showing the
bad channel. Trace (1) showing
the output of the defective
side — only responding to high
peaks and only in the positive
direction, while trace ( 2)
shows the other channel's
Q8, the defective 2SC1509 transistor, center.