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Old 5th March 2012, 05:09 AM   #1
hpp3 is offline hpp3  United States
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Default Repairing a Kenwood KA-7300

Hey all, I hope I'm posting in the right section of the forum.
I'm no stranger to the repair bench, but this is more high-end than I have experience with (guitar amps, stompboxes, etc.).

I have a Kenwood KA-7300 with a blown preamp (I think).
Sound still comes through, but it is thin and distorted and the tone controls have no effect.
I opened up the unit, and everything tone-control related seems to fit on one board, so I'm thinking I should start there, with solid-state parts usually on my "most likely to fail" list.
The original transistors are marked "A872" which experience tells me is the venerable 2SA872.

My question is, am I on the right track, or should I test other stuff first?
The power section is working just fine (on the KA-7300, the power section can be tapped independently).
-Eddy
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Old 6th March 2012, 06:18 AM   #2
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well .... this a flagship amplifier of the series ...actually a very good amplifier and worths both repairing and upgrading .

It is a simple fact that most of the capacitors inside are gone You need to replace them all .In your case focus on the secondary power supply area ( that powers tone control and buffer ) look at Rk1 and Rk 2 capacitors Ck5 and Ck6 related zeners DZk1 and Dzk2

verify voltage of 12 and 15 volts is present and also verify all voltage according to the schematic ( i have one if you want it you have to email me )

also you need to seriously clean all pots and switches .... spray will almost be not enough ... but you can start from there

kind regards
sakis
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Old 6th March 2012, 02:59 PM   #3
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Ah yes, I hadn't taken into account the age of the thing and started with caps first.
That would explain why it still passes sound, but very thinly.

I've also wanted to clean the pots and switches, as the input selector switch would sometimes drop the right channel, but have had no success getting the front panel off.
Do you have any advice for removing the central volume control?
It's the only knob that doesn't simply pull off.
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Old 6th March 2012, 03:14 PM   #4
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Check to see if there is a small hole allowing access to a hex screw in the volume knob. Loosen the screw and the knob ought to fall off.

Here is a picture (not mine) in another forum thread I found by doing a GIS of "Kenwood KA-7300 volume knob"

Picture is in here... http://www.audiokarma.org/forums/sho...d.php?t=350193

Last edited by overtheairbroadcast; 6th March 2012 at 03:22 PM. Reason: Added a link to a picture of a KA-7300 volume knob.
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Old 6th March 2012, 06:14 PM   #5
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Aha! Thank you very much for that, my 6mm Allen did the trick nicely.
Now to get at those caps...
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Old 7th March 2012, 02:31 PM   #6
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You have an awesome amp there. Good luck with the re-caping and clean up. Since it is open. how far are you going to go with it? Breaking it down to component boards (though it would be amazing to see, I don't mean individual components) and easily removable parts? Or just go with the re-capping and cleaning?

From the sounds of it, it is a very nice 70's amp to run music through. Enjoy.
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Old 7th March 2012, 07:05 PM   #7
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I'm just going to go with a re-cap and clean for now, there's a lot of real pretty board-to-board wire-wrap connections I don't want to undo.
If there's still something amiss after the re-cap, I'm going to tackle replacing possible burned transistors in the pre-amp sections.
At least I know the power section still works; this unit has a switch that separates pre-amp from power-amp, and I've got my computer sound card plugged into that.

I never knew how much coloration a pre-amp section can give to your listening experience.
Not that I'm complaining; a good pre-amp is often needed to bring out the best in the overall system, but now I'm really juiced to DIY some stand-alone units just to experiment with.

Back to the capacitor inventory...
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Old 7th March 2012, 07:51 PM   #8
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Good luck and happy soldering, post pics when you are done with it all.
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Old 26th March 2012, 04:30 AM   #9
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OK, long story short, kinda...

I chanced to meet up with the lady who gave the unit to me, and she said her husband had it repaired before it was retired to a shelf in their storage unit (it took the repair guy almost a month and a half, so he bought something else in the meantime, and mothballed the Kenwood when it came back).
I talked to her husband and indeed, the repair guy had replaced all the capacitors. So I came back home and hooked up the new speakers I had gotten and it worked perfectly.

So why did I think it was broken? Fast forward to two weeks later; my son and I were experimenting with some chipamp stuff, and I used the old, cheap speakers that had originally been connected to the Kenwood. Sure enough, the woofers were blown.

Apparently, 80 watts fed into cheap 5 watt consumer-grade speakers is OK until you try to do something out of the ordinary (which is another tale...) , and hang me for not suspecting the speakers first.
Oh well... at least I had the opportunity to De-Oxit the switches, so I figure it's good to go for a good while.
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Old 26th March 2012, 10:21 AM   #10
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You can download service manual here
AK Database PDF Gallery - Kenwood/Kenwood KA-7300 Service Manual

so it will much easier for trouble shooting...
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