Kenwood KA-4040R - faulty channel (fixed)

Almost exactly 10 years ago I picked up this Kenwood KA-4040R Amplifier.

I'd put a "Wanted" ad on our local Freecycle page - I fancied a go at fixing a broken amplifier - so I specifically asked for something that was broken. So, this amp was given to me along with the CD player and Tuner (all separates). The fault on the amp was that one channel was not working. At this stage I knew nothing at all about how amps work (and pretty much still don't), but I set to work with the only relevant tools that I had - a multi-meter and an ESR meter for checking capacitors. Within a short while I had found what I thought was a dead capacitor. Having de-soldered it and checked it again, it seemed to be fine. So I reinstalled it and the amp worked! Easy fix, and the amp ran fine for the next ten years, right up until a couple of weeks ago. The left channel started playing up again. This coincided with a lot of scratchy interference when adjusting the volume, and a quiet "thudding" sound on the left channel. This thudding was louder on certain inputs (CD, Tuner, Aux, Phono), but not really noticeable on the second tape input. As I was mulling this over something in the amp went "pop". I unplugged it all and removed the cover. This is what I saw:

This is a 3300uf 6.3v capacitor on the "EQ" board which has spilled its guts into the amp.

I bought a couple of replacements and fitted them but I was not hopeful that this, and cleaning up all the cap juice, would sort the issue.

I was right - with the new caps fitted there was clearly some sort of current leak as there was a very rapid thudding on all inputs and within about 30 seconds of switch-on the speaker protection relay fired. On a positive note, the left channel did now have audio, albeit quiet and drowned out by the thudding. I did some more checks. I found that about half of the diodes which appear to serve the input connections had fried. So I replaced them, along with some transistors that I'd had a hunch about.

The original diodes are no longer available but the consensus on the internet was that IN4148 diodes would serve as a suitable replacement. I managed to find some new-old stock transistors so replaced like-for-like.

And whilst I had it in bits, I took the front panel apart and cleaned all the pots with some Super 10.

So, 28 diodes (I replaced them all...) and four transistors later, and a liberal dose of switch cleaner, I now have a fully functioning amplifier again. Cost - less than £10 to fix. I am extremely pleased ;) But I am now toying with replacing all the caps in the amp - they are 27 years old and are unlikely to be in the best of shape. I like the amp and it would be a shame if it got wrecked by a failing cap.


More soon, no doubt...


D.