Time to fix the oldies... the Pioneer SM-B200A Stereo Amp with Tunner

I got this amp many years ago. But I hadn't touched it until recently. The previous owner told me it had some problems and because of these I bought it cheap. I hadn't check what went wrong and the only things I knew was the output and power transformers were good.

It is a 60s tube amp, having two radio tuners.  You can hear the news from AM broadcasting on the left speaker while you enjoy the music on the right channel.   It is a very distinctive feature of this kind of tuners. The design is for the country having a stereo broadcasting during the 60s by sending stereo signal through AM and FM.

It is the SM-B200A circuit

One of the problems of this amp is that it can play music normally for the first 15 minutes and after that, the power tubes turn red hot.  By looking at the circuit diagram, this amp is biased by -ve voltage and it is not a self-biased design.  And the bias voltage is rectified by a selenium diode.  This diode is a highly suspect to cause the problem.  Selenium diode is not that stable after 50 years and the stack of the selenium wafer will loose due to the thermal expansion/contraction cycle. And it explains why amp works perfectly for first few minutes but fails after heating up. After the diode gets warm, it gets expanded and increases its reversal voltage.  In the end, the bias voltage turns to more positive and the current increases gradually.  After changing this diode with modern silicon diode, the amp runs normally and backs to life again.  But....it is just the beginning.

The Red Selenium Diode - loosen screw causes the malfunction

During the 1960s, most of the amp were made with carbon composition resistors or wire resistors.  Wire resistors should be ok and they are pretty reliable.  However, carbon composition resistors (CCR) are not.  The value of this kind resistors will draft along with time.  The moisture will get into the package and heat generated will promote the value drafting.   Measuring the value of the CCRs near to the power amplifying section and I found most of the CCRs were going bad. However, the CCRs in other section are still OK. It is a rule of thumb to check the CCR in old equipment if it's running hot before.

The value of the CCRs changed from 4.7k to 5.2K

The hum level of this amp is acceptable but still high.  This is a sign of deterioration of power capacitors.  However, the 20ufx3 and 20-20-40uf multi-section cap are too expensive and so I create a printed circuit board to hold the capacitors which can be fitted to the original mounting hole.

Not only replacing the old components, I was also trying to improve the wiring.  Originally, the filament wires were not twisted or twisted not enough.  After I rewired the filament wires and I can hear/measure the different before and after.

Spent a day and all the bugs were fixed. It is a time to test the amp with the most sophisticated testing equipment - my ears. (Sure I tested the amp with oscilloscope/signal generator before the listening test. I am the true believer that the good amp should have a good measurement result. But the amp with good measurement result doesn't mean it is a good amp!)

It is a really a good little amplifier even though it only has 11W per channel.   The tuners are also good and sensitive even I haven't re-calibrated them. It provides a typical tube sound and good for jazz and vocal.  The frequency response is about 30Hz to 19,000Hz -2dB.