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Kenwood KA-8300 - Power supply board issues addressed

redpackman

redpackman

Active Member
I have a Kenwood KA-8300 I love dearly, along with its little brother the KA-7100.

I read that the KA-8300 can often have issues on it's power supply board and in particular with resistor Rk 6 which tends to overheat and cause problems. I was also having some issues with the relay so I thought I'd go in and replace a few components there.

Got the relay in and then began going at the resistors. Replaced R6 with a metal film 1.8K ohm 1 watt resistor. Also got Rk1+2 with new 120 ohm 1 watt metal film resistors also Rk3 with 220 ohm 1 watt metal film resistor. Looking at the parts list in the KA-8300 service manual available on HiFi Engine I picked out four caps to replace in the same neighborhood with some nice audio Nichicons. The service manual lists two 330 uf 35w electrolytics and two 330 uf 50 w electrolytics. I ordered the Nichicon replacements.

But when I looked at the actual capacitors Ck3-6 in the power supply they were all 470 uf two 35 w and two 50w. They looked to be originals. I left them in. Three questions:

1. Are there two different builds in this model's power supply?

2. What's with the 470 uf vs 330 uf caps?

3. The Rk 6 resistor had shifted in value from the spec 1.8k to 1.136k ohms. What would that have led to now and in the future? I could see the board had been heated (discolored) but the traces hadn't been destroyed as I've seen in some other projects pictures.

With the new resistors and relay and the old caps untouched in place it really sounds nice.

Any input would be appreciated.
 
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470 v 330 - probably a mid production change. I found a blurry picture of my 8300 before overhaul and see 330uf there. "Audio" caps aren't the best choice for power supply in my opinion. I'd shoot for 105 degree Nichicon PW, PM or HE or Panasonic FC.

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Rk6 drops current to the pilot lamp LED. I would expect that value to increase if anything considering how hot it gets. If you measured it in-circuit you were likely measuring a parallel resistance.
 
redpackman said:
I read that the KA-8300 can often have issues on it's power supply board and in particular with resistor Rk 6 which tends to overheat and cause problems.
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Did/Does your unit exhibit these issues ?
 
avionic said:
Did/Does your unit exhibit these issues ?
Click to expand...
My unit still played all right but I could see that the board had become hot like so many other unit’s pictures showed it. so I increased the value of the resistor from 1/2 watt to one watt, but thankfully no damage to my unit yet.
 
redpackman said:
No it was out of circuit laying on the bench, and measured two times.
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I have the same issue you do. Starting to rebuild the power board on my Kenwood KA 9800 which is the same as the 8300 I believe. My RK6 resistor also went from 1.8k to 1.1k (tested out of circuit ) and it’s burnt on the outside of course. Also those four main caps in mine were also 470uf instead of 330uf as listed in the service manual. All original never been serviced before.Wish me luck I’m going in!IMG_1789.jpegIMG_1809.jpeg
 
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Best of luck. Be sure to clean up all the glue and replace those Zeners too. I use my lead crimping tool to get them off the board a few mm. I get the resistors as far off as possible without looking ridiculous.
 
Decided to populate the power supply board that was in there to see if it solves my problem. Have a horrible hum somewhere in the preamp section and it’s not the inputs. The amp section is clean. The hum goes up and down with the volume and it’s pretty substantial. Figured it’s possibly in the power supply somewhere. Just waiting for some ceramic .01uf 500V caps I ordered to show up so I can finish this power supply board. The original rectifier diodes had a common cathode and a common anode arrangement, so to replace that I just did a jumper wire from cathode to cathode on the first one and an anode to anode on the second one which simulates the same thing as the common anode common cathode arrangement. I upped the wattage on all the heavier resistors also and got them off of the board. Don’t really care how it looks. It’s so long as it works. I’ve already got a clean donor board, but I want to see if this one’s gonna work first to see if it fixes my issue. Thanks for all the help everybody. G
 

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