Kenwood KA-3500 Repair And Restoration

This Kenwood KA-3500 integrated amplifier is in pretty good cosmetic shape but is in need of some repairs.

The top cover of the Kenwood  KA-3500 is now removed. This integrated amplifier has a nice internal layout.

I found that during testing I could not set the bias (idle current) on the right channel.

  The front panel will tilt down when you remove three screws from the bottom panel and loosen one screw on each side of the case.

With the front panel tilted down I removed the screws holding the assembly I needed to access and pulled the assembly straight up.

I'll now be able to troubleshoot the bias issue with this Kenwood KA-3500 integrated amplifier.

Always obtain the service manual for the piece of vintage stereo equipment that you are restoring.

The bias issue was caused by a bad transistor. Modern test equipment is helpful when troubleshooting vintage audio equipment.

The bias can now be set according to the service manual specifications for this Kenwood KA-3500.

The filter capacitors check out fine and will stay for now. The ESR number is the more important one. If it's a high resistance it's a sign of trouble.

This Kenwood amplifier has no adjustable DC Offset. It is done automatically by a pair of transistors that must be closely matched. 

This KA-3500 integrated amplifier is 40 years old. I did change out some smaller electrolytic capacitors when I had the assembly out. 

I've about got the amplifier back together. I need to install some tie wraps that I had removed. Always try to put everything back like it was.

Don't forget to clean up the back panel. Clean the RCA jacks and the speaker connectors with deoxit.   

I've hooked up the Kenwood KA-3500 to a tone generator and a Sound Technology 3200A for some bench testing. The channels are balanced.

Both channels show similar distortion numbers as I worked my way up through different frequency's and output levels.

Your ears are the ultimate tool but using an audio analyzer in a controlled environment can help you verify if an amplifier is operating correctly.

This integrated amplifier is ready for some music. This Kenwood KA-3500 repair and restoration is complete.