My baby is sick... [Kenwood ka 8100]

_Attila_

Asking & asking & aski...
Since I started with vintage audio for a couple of years, I wanted to get a ka-8100. For me, it looks perfect, and I heard it sounds perfect!
So finally I got my Ka-8100 yesterday evening. Boy was I happy! Terrific look!
After letting it come to room temperature, I tried it, and well, no sound on the left channel. Or very very low. But when you turn up the volume knob, it's coming back at more or less 1 - 2 o' clock position. I thought ok, will just have to clean the pots and it'll be fine. After cleaning no change, so I opened up everything this morning. Re-deoxy and co. Still no change. I opened the potentiometer for the volume. Bad discovery: the one for the left channel (the rearmost one) is missing some wipers and also has some bended ones. Also the traces are black (or really dark grey). :tears: :tears: :tears:
I think my only option is to find a new potentiometer. Or does someone have a fix for this?

So if anyone of you have a spare one, or know where to get one, I would be really really happy! Any ideas how to fix the problem are wellcome too! I waited so long for being able to get a 8100, and now this... :pity:
 
I just added 2 pics, so you can see what I mean...
The first one shows the tracks, the second one the bursts :sadwave:

I could cry...
 

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I'm not sure if the assembly is the same as the 9100 or 8300, but I have both and if you need to replace the whole assembly give me a PM maybe I can help you out.

John Mik
 
I have a pot form a ka-9100. I believe it is the same pot. 100K x 2 and 250k x 2 stack.

it is dirty and needs cleaned up, but it specs out and came from a working unit. If you are interested send me a private message with your mailing address. You can send me what you think it is worth and postage.
 
Additionally, check (and clean) the preamp jumpers at the back of the amp.

Congrats on getting a KA-8100, I love mine!
 
Thanks! I love her too. I tried without the preamp, very nice soundig amp! I just have to test how she does with my LS-770. Amazing loudspeakers, but I'm not sure if I like them or not... Very special sounding ones!

Jumpers already checked and cleaned :yes:

Now I'm just waiting for the pot to arrive, and in the meantime I purchased a soldering station :D
 
The replacement pot arrived yesterday! :banana:
Many many thanks to SicMan for the part!
I'll try to find some time today for doing the swap!

But 2 questions before I start:
- can I replace the cables going from the pots (volume and balance) to the boards with other cables? Let's say cables from a computer PSU? Or is that a bad idea sound-wise? The cables now have a solid core, the cables from the PSU would be a little bigger in diameter and are multi-leads type.
- it seems that someone already tinkered (not sure if it is the right word in english) on the unit :thumbsdn: : the negative leads don't look original and are badly soldered... :thumbsdn: again! If there is someone with a KA 8100 here: how does the negative lead that is going from the pots to the ground look like? One piece straight from the pots to the pin behind them (the pin is in a 3 pins group, half covered by the radiator assemby), or in 2 pieces, running to a screw in the chassis behind the pots, right (seen form the front of the amp) and from here to the pin in the 3 pins group?

Thanks!
 
Back to life since Saturday, final reassembly after cleaning and adjustment on Sunday: bias: 13.9 mV and 14.1 mV (should be 14 mV), offset: 0.1 and -0.2 (fluctuating), and now I know why everybody say to change the trimpots!
She sings wonderful! :banana:
One more time: many many thanks to SicMan! :thmbsp:

Now I just cant' decide if I like my LS-770 or not :scratch2:
 
Congratulations Attila.
I have two of them, got both in really bad shape and I had to learn so much servicing them. One had the volume pot broken, the other one of the zeners for the preamp PSU, had to change all condensers (not the big ones), both amps had the outputs burned. Have you changed the condensers on yours? I've made comparisons between the two - the one with new condensers has much better sound that the one with old ones. Fluctuating bias or offset is probably not a problem of the trimpots but because of the thermal instability. Should be less fluctuating after 30 mins or more. Older amps are made so.

LIX
 
Hi lixaudio,

no I haven't change the caps for now. I'm planing to get another 8100 (hopefully in bad shape! :) ) and recap it, so I can do a direct comparison between the two.
May I ask your where you got the outputs?
Fluctuations while setting the bias: the amp was given an one hour (give/take 10 min) to warm up. Barely touching the pots made the bias fluctuate...
 
I'm looking for another 8100, but I would like to find an almost destroyed one. Would like to make modifications on everything, from power supply to output connectors.
For outputs I use MJ15015/16.
Maybe you should really put new trimmers for the bias settings. At least give them a spray of Oszillin.
 
Now let me just say, if you buy a new volume pot new from some parts store you won't be able to use the Loudness switch anymore.

The only way (if you still want loudness and I think also tone) you'll have to get a 8100 or 9100 volume pot.
 
Hi, I had problems with my Kenwood KA-8100. Both the balance and volume controls created problems (scratching, no volume in one or both channels randomly when I was moving either control, etc, etc). Since it has integrated balance and volume on the same axis, it looked unlikely that I would be able to reach everything with deoxit. So I decided to look inside. The pot or pots are basically two cilinders (traditional pots) on the front (balance) and two square boxes on the back (volume) all using the same axis. It cannot be disasembled from the front, since the aluminum vol axis is thicker on the front and will not allow the golden cilinder acting as axe for the balance to come out. So I warmed and lifted the metal tabs holding the back of the square volume pots. This is not the traditional carbon ring you see on potentiometers. I do not know what the metal concentric segments are for and it has four (two of them 180 degrees away) rather than three connectors. Is this broken or in good shape? Does anybody has any experience on how to deal with this? would just cleaning with deoxit on top work (assuming I can continue opening the other ones too)?
20190126_212031.jpg 20190126_200253.jpg 20190126_193956.jpg
 
The volume section isn't a "regular old pot" - it's a printed resistor "attenuator". At least that's how I understand it.

Hard to tell if yours is OK from those pictures, but from what I see it looks fine.

You don't need to disassemble it like that to get cleaner in. Pretty risky I'd say. The bottom side has huge slots to get cleaner (and dirt) in. Be sure and clean all of the switches and pots on the amp and your problem might go away.



P1011603.JPG
 
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thxs gort69 for the picture. I had already disassembled the second square pot before seeing your reply. At least I learned how they make that step motion! (pic below). I wonder why it was sounding bad since I did not see any dirt on the volume pots. From your picture above I saw that I could reach the inside of all the pots from the soldering ends. That was hard to see in my amp from the top with the pots still attached. I was able to insert deoxit through those slots in your picture and assembled everything back. Now volume and balance work flawlessly. There is some slight distortion going on in the amp section, but I guess it will be a problem to solve another day; I guess I should clean every single switch first.

20190126_224046.jpg


As a future guideline to anybody having volume/balance problems in Kenwookd KA-8100 or similar (no need to disassemble pots):
- detach front plate (you will need hex key 1.5mm to use first with vol, then with balance control; three screws on top, two on bottom)
- detach inside plate (two screws on right, two + one on left, three on bottom; loosen two screws on the plastic joiner of input selector on the right back of the amplifier; it is not necessary to remove the electronic board on the front)
- remove nut and washer holding vol/balance pots
- now without desoldering or detaching any cable of the balance/vol pots you may be able to push it back and if you rotate it 90 degrees counterclockwise the slots to insert deoxit on all four pots will be facing upwards.
Sometimes cables prevent you to move the inside plate. If you do not have enough space to work you may also remove the nut of the upper left speaker selector; it will make working with the inside plate easier.
You can also take advantage of the thing being open and insert deoxit to the Bass and Treble pots directly from above.
 
Congratulations Attila.
I have two of them, got both in really bad shape and I had to learn so much servicing them. One had the volume pot broken, the other one of the zeners for the preamp PSU, had to change all condensers (not the big ones), both amps had the outputs burned. Have you changed the condensers on yours? I've made comparisons between the two - the one with new condensers has much better sound that the one with old ones. Fluctuating bias or offset is probably not a problem of the trimpots but because of the thermal instability. Should be less fluctuating after 30 mins or more. Older amps are made so.

LIX
Condensers???
 
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