Sansui 771 Stereo Receiver Troubleshooting Example and Tips

My Sansui 771 Stereo receiver has been with me for more then 35 years .  This article will review a problem that came up 5 years ago and outline the steps I took to fix it.

While I am unsure of the exact year, my Sansui 771 was manufactured in the mid 1970s with mostly discrete components.  The schematic for the 771 and many classic stereos ise available from hifiengine.com which makes troubleshooting much easier.

The problem I had was two fuses blowing in the power supply and no output from the amplifier at all.   Like most of its peers in this mid-price range, the Sansui 771 delays engaging the speakers when it is powered up by several seconds and that was not happening.  

First, locate the fuses that are blowing on the schematic and familiarize yourself with the general type of circuitry in that vicinity which is a receiver power supply (red highlight), a voltage regulator (yellow highlight) and a circuit to drive the speaker relay (green highlight) in the picture above.

In my case, fuse F06 (0.8 A) and fuse F07 (2.0 A) shown in red on the picture below which is a drill-in on the left hand side of the diagram:

Use compressed air to clear any dust of the area and take a close look around for failed components – especially anything that looks burned.  In my case, nothing caught my eye – or my nose!

Fuses are to protect, and there always a chance that there was some kind of power surge so replacing them is a reasonable first step.  Unfortunately, these fuses were of an odd form factor and I had nothing on hand that would fit.  So I soldered leads to standard fuses and wired them in and gave it a shot.  No luck – instant failure.

What I do in these situations is to wire in a resistor in place of the fuse – typically a few hundred ohms and of sufficient power dissipation size to get some current downstream and then use my DMM and oscilloscope to see what is going on.  It only took a few moments to see what was taking out F07 : Capacitor C02  - a 1000 microF at 50 V – was not acting like a capacitor. 

One of the first lessons to take away when working on older Classic Stereo gear is that electrolytic capacitors fail - suddenly, or slowly, and with or without drama.   Because the Sansui 771 is mostly discrete components, it was easy to unsolder C02 and confirm its failure on the bench.  Again, the form factor of that component didn’t match anything I had on hand but I wired in a replacement.   After some basic tests, I replaced F07 with a fuse and could now see regulated DC down stream.  However - fuse F06 still failed ,still no sound and the speaker relay was still not engaging.