Kenwood KR-2400 Receiver
This Kenwood was a crusty mess, and several components were fried, but I was happy that I could bring it completely back from the dead.
I bought this receiver for $20 with no knowledge about what could be wrong with it, other than the owner saying "it doesn't turn on". It was pretty crusty looking as well, so I knew I had a challenging project ahead of me. It's rated for only 16 watts/channel, but I couldn't argue with the price and I already knew it was going to a good home in the end.
I was hoping it just needed a new fuse, so opened it up to check whether the fuse was blown—sure enough it was. After ordering some new fuses, I tried again, but they blew as soon as I turned it on. After a bit more research, I began to test various components on each circuit board starting with the power supply. In addition to this, I knew I should also test the output transistors on the amplifier board, because they are usually the first parts to fry if the power supply is acting up.
I was able to determine that one out of four output transistors was toast, but also two diodes on the power supply board. Looking at the service manual, the diodes (Dk 1-4) were listed as part number V03C, and I had help from a friend to source an equivalent replacement. I ended up ordering part number 60S10-TP from Digikey, which were actually kinda overkill for this purpose, but it didn't matter. For the transistors, I wanted to replace all four with modern equivalents, just in case it affected the sound, I wanted everything to be even. This receiver uses a complimentary NPN and PNP pair of transitors for each channel, which I understand is pretty common—I am new to this. The service manual listed part numbers 2SC789 (Qe 19 & 20) and 2SA489 (Qe 21 & 22). For replacements, I went with 2x BD243C and 2x BD244C, again, with help from a friend.
While I was at it, I figured I should recap the amp and power boards too. After my last project, an entire recap of a Kenwood KA-8006, this was easy.
With all the parts replaced, the receiver powered on again without blowing a fuse, but nothing actually worked. I began to trace voltages out of the power supply into each board, and found a number of bad connections and grounding issues. Each board in this receiver is connected with removeable plugs, so you can easily dismantle the entire unit without having to snip or desolder wires. This made it extremely easy to work on. The downside is that most of them had a build up of oxidization that resulted in many hours of troubleshooting and headaches. Finally after cleaning all of the plugs and reflowing a few connections, it came back to life. And as always, I replaced all of the bulbs.
I took more photos of the repair process but for some reason they've been lost. I think I may have accidentally deleted them unfortunately.