Nuts and Volts - July 2015

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!

Finishing 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