Stereo Repair

My hobby is electronics and the repair of most home appliances. Doing repairs “Under the table” nets me a nice income, really all those years of training paid for by Uncle Sam should be used, and I do enjoy a challenge from time to time. Well My most recent challenge is a Stereo Amplifier, It Is a Knight KA-40.

This piece was given to me to keep from having to pay for it to be recycled. It came with with several large boxes, with other items and parts.  I really did not want it in the beginning.

The Knight KA-40 was sold from the mid 60's to the early 70's as a kit, the buyer assembled and then used the amp. Considering that this is 50 years old, came from an attic, was a kit, and has vacuum tubes in it finding parts would be difficult. The common ailment I found while researching this model was the voice coils would burn out.

So I opened the case and find the fuse is still good, I used my home made ESR Meter to test the capacitors in the power supply and they are long dead. I checked the voice coils, and found one was open. There is not a chance of finding another coil available, The options at this point is to try to repair the coil, wind a new coil, or hope to find a replacement......

I decided to gently unwind the outer wrapping of the coil. On the 3^rd turn I found a break! I tested with my multimeter and had continuity! This coil was repaired! So after a good cleaning, replacement of all the electrolytic capacitors, and replacing a few screen resisters Also I found about 80 cold solder joints from either age or was just bad to begin with. I replace the fuse, plug the unit into my Variac, set the voltage low and turned the power on. Slowly I increased the voltage and the amp came to life. Plugging in an older Cassette player I soon had music drifting through the shop. Some work on the cosmetics and I'll have a good Stereo Amp. Sometimes we are just fortunate to have simple repairs.

The pictures are the before  repairs started pics.  I still need to work on cosmetics.   Total cost to repair this Amp was  15.75$ US.