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Monday 15 April 2019

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.
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4 comments:

  1. Anonymous28 April 2023 at 17:58

    Hello, i have the same problem. Thank you for this guide and which transistors did you change? Is it four transistors on a big radiator?

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  2. Dave28 April 2023 at 18:18

    The four I changed are actually in the picture under that text - two black and two green - they are quite small and situated on the board with all the diodes on (at the opposite side to the diodes)

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  3. Paul Bsf.25 September 2023 at 17:58

    Thanks so much Dave for this post! You were an inspiration for me to start and try to fix my old KA-4040R.
    I had no idea about electronics, but your post inspired me and with some tutorials on Youtube I replaced those two leaking capacitors and 28 diodes (it's a whole story). In my country I had difficulty finding 2 capacitors of a certain good brand to replace the original ones; I changed them twice until I stopped at the Samwha brand (made in Korea, I understand that they are counterfeited less than others...). I replaced the diodes with some 1N 4148 ones. I didn't replace those transistors you wrote about, I didn't bother, I didn't know what to look for/order.
    The initial problem of my applicator was not as big as yours, some channels worked ok, others had a certain noise (maybe Thudding? I apologize, I'm not a native English speaker, I don't understand certain words), especially when changing channels. The problem became more acute over time, and the channels that were working ok, started to have that disturbing noise. After replacing the 2 capacitors and the diodes, the noise disappeared. But (maybe I hadn't noticed this because of the noises) the left channel now sounds quieter than the right one. I tried all the inputs (CD, AUX, Tuner, Tape A and B), except Phono, I changed the speakers between them, but it is obvious that the left channel sounds quieter. Well, the left channel does not sound much lower than the right channel, but it is clearly felt.

    If you'd be so kind, Please give me some advice, maybe from your experience you have an idea what the problem could be and what I should replace. I don't know how to measure components, thanks to your post I got started and bought some measuring devices from China (Aneng AN9002, BSIDE ESR02, FINIRSI-1c15, sorry I don't want to advertise them).

    I would like to give a new life to this amplifier that I see can carry more pretentious speakers like Wharfedale or B&W. Thank you very much !

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  4. Dave26 September 2023 at 00:04

    Hi Paul and thanks for leaving a comment! I'm so glad you found the post helpful - from your description is sounds like your amp was just about ready to pop like mine did. As for your question, the reality is that I am far from an expert in these matters and couldn't really say what it was that I did that sorted the sound levels out on my amp as I changed the diodes and those transistors I mentioned at the same time, and gave all the pots on the front panel a good clean. Whatever it was that I did, it was good enough that the amp is still working fine now (even though I still intend to change all the capacitors over at some point!). All I could really suggest is that you clean your pots (I recommend the Service 10 cleaner) and if the problem persists after that, maybe consider changing the transistors I mentioned. You can find the transistors in the corner of the main audio board at locations Q21, Q22, Q23 and Q24. The transistors are 2SD882 (at Q21 and Q23) and 2SB772 (at Q22 and Q24). If you can get the original NEC branded versions that would be better but any make will probably do. They are straightforward to change. And if none of that works, I'm not sure what else I could add.

    Good luck!

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