From the Rain City Audio Repair Blog:
I recently got to work on a pretty big, fairly rare Sony receiver from 1973-74: the STR-6200F. It’s a powerful receiver featuring 60W per channel of power available, with quite a few inputs and outputs, and a strong FM tuner built in.
The owner brought it in complaining of poor sound quality. Not particularly surprising given it’s age!
First up was component replacement. All the on-board capacitors were replaced with new Nichicon Fine Gold electrolytics. This receiver had seen some minor service in the past: there was evidence of soldering, and one transistor on the FM discriminator board was replaced at some point.
This receiver was laid out pretty well. Most of the boards were easy enough to slide out, flip up, and service from beneath.
Time for the lower boards:
The power supply was a little tricky to re-work, but not too bad. Sony soldered many of the leads directly to pads on the bottom, so you had to be careful to make sure it didn’t come apart.
The power amplifier board was similar.
Time for the main caps. Two smaller ones, and two huge ones.
The new capacitors used screw terminals, so I soldered spade connectors to the ends of the old wires for a strong, durable connection.
Time for a first power-up. Not smoke, and it sounded okay, but not at all like it should. Time to check the adjustments. First, the power supply reference voltage. It was pretty close, but I adjusted it anyway.
Rather than provide test point jacks, the service manual called out junctions of resistors for probing the voltage. Not my favorite method but it works. Then onto DC offset, which was WAY out of spec. One channel reading 73.1 mV, the other 100.2 mV. Both reset to zero, though.
Bias was also way off, starving the transistors of power and introducing some distortion. Both channels call for 25 mV across their emitter resistors, but it was more like 5 mV per channel. I adjusted both up to 20 mV, which provides plenty of power but will run a tiny bit cooler.
Checking the temperature of various components on the driver board with a laser thermometer while playing into a dummy load.
The sound was much improved – and the measurements showed it, too!
With all this, the amplifier itself sounded fantastic, but the FM left something to be desired. This is a pretty well-regarded FM tuner, so something wasn’t right. I hooked up the FM test signal generator, distortion analyzer, and dummy load and measured the REC OUT port and found that it was receiving with fairly high distortion, indicating the alignment needed to be adjusted. Then proceeded to align the FM through distortion analysis, adjusting the various cores in the order described in the service manual. Instead of aligning curves on an oscilloscope, though, I made each adjustment for minimum measured distortion. This is faster, more accurate, and more reliable than an oscilloscope alignment, thanks to modern technologies.
Adjusting the front end:
Centering the discriminator:
And finally the discriminator’s DC balance:
There we go! Much better, exactly where I’d expect it to be. The sound really cleaned up, too, now it’s bringing in clear highs and powerful bass over the air just like it should.
All told, 76 components were replaced in this overhaul, followed by amplifier adjustment and FM alignment.
Another classic preserved! This Sony is going to sound fantastic for a long time to come, and it looks great!