sounding great, but the other still showing a bit of
sputtering and popping that just wasn't going
away. It was worst at power-on and settled down
a little over the next few minutes, but never
disappeared entirely. Transients like that are a bit
tough to track down using a scope since they're
so quick. I traced forward from the input, stopping
each time I saw spikes on the scope
corresponding with pops in the speakers, and
replaced them each one at a time. With each
replacement, the noise cleared up a bit until it
was completely gone. Three more transistors were
bad: Q306, a 2SC1345; Q312, a 2SC458; and
Q310, a 2SA777.
After the last replacement, I fired it back up
and it sounded crisp and clean just like it should!
Problem solved. Time to finish up!
With the electrics sorted, it was time to go
through final checks. This amplifier has a very simple
design, so there's no power supply reference or DC offset
adjustments to make; just the idle current adjustment
across R329/R330 for 25 mV ±1 mV.
Then, I ran some power curves. The combination of
top-of-the-line audio grade electrolytic capacitors, new
output transistors which are being run well below their
maximum ratings, and good old fashion hi-fi design meant
this one really shined. This little amplifier delivered below
0.1% THD through its rated power (20W) but continued
to meet its power spec all the way to maximum output at
30W, while still staying below 0.5% THD.
This is pretty impressive — 50%
more available power, and 1/5th
the rated distortion through its
factory power output. Vintage hi-fi
amplifiers are often rated
conservatively from the factory, but
that's a significant improvement.
With that, this H/K A-401
amplifier was ready for use as a
daily driver at the heart of a nice
little vintage stereo system. At only
30W/channel, it's not going to be
setting any stereo competition
records, but that's plenty of power
for filling a room at a comfortable
listening volume. It's got a great
minimalist design, too, which makes
it very attractive and definitely
worth having fixed up.
This was a pretty fun project
which took some good
troubleshooting. There was a
defective driver transistor causing bad distortion, a failed
insulator causing a dead short from the power supply, and
three other intermittent small-signal transistors which were
introducing noise into the signal path.
Vintage hi-fi repair is a fantastic hobby, and this
project only took me about eight hours from start to
finish. Replacing the electrolytic capacitors took about two
hours, and the rest of the time was spent on slow and
careful diagnostics through each stage. At the end, there's
a beautiful functional silver front stereo amplifier to bring
your music to life. Most vintage stereo repairs involve
some detective work, but it's well worth it. NV
July 2015 51
Out with the old, in with the new!
Lots of replaced parts in this project.
The A-401 back in its natural environment, powering a
great little vintage system.