Kenwood KA7300 bias check points - diyAudio
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Old 27th June 2016, 02:55 AM   #1
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Default Kenwood KA7300 bias / DC offset

I made mention to a co-worker about my escapades performed on 3 Toshiba SC335 power amps. He was insistent I take a look at his Kenwood KA7300 integrated and KT7300 tuner. I advised him I knew nothing of tuners and my knowledge of amplifiers was rudimentary at best. I think he wants me to buy them as he now lives in a mobile home and I believe these two pieces have been "dead weight" he's been hauling around.

I have yet to try using it but did do some investigating about the amp and learned that it's bias is not adjustable. Nor can I determine precisely where to measure bias voltage. What I was able to find made me take an informed guess that it was across R12 & R13. After letting the amp warm up for about an hour, I did measured there and both channels were a bit over .9 volts. If those were the right check points, should the voltage be that high?

I also checked the DC offset which was good on the left channel at 5mv after warm up but 96mv on the right.

Last edited by 62vauxhall; 5th July 2016 at 01:57 PM.
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Old 27th June 2016, 04:08 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by 62vauxhall View Post
I did measured there and both channels were a bit over .9 volts. If those were the right check points,
should the voltage be that high? I also checked the DC offset which was good on the left channel
at 5mv after warm up but 96mv on the right.
There's no biasing because it's a power module. The voltages that you measured are in the
Zener regulator circuit for the front end diff pair. Some or all of the electrolytic capacitors
may need replacement. http://www.hifiengine.com/manual_lib.../ka-7300.shtml

Last edited by rayma; 27th June 2016 at 04:13 AM.
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Old 28th June 2016, 03:19 PM   #3
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I did see those power modules in lieu of output transistors. I have since read that Kenwood incorporated them in just a few models. I assume this was due to a manufacturing cost reduction?

1980 was the first year of many spent in retail HiFi. The first time I encountered such a device was in some low end NEC integrated amps then later in cheap Sony receivers. I was under the distinct impression these things had limited performance parameters such as incompatibility with low impedance speakers but were cheaper than output transistors. It surprised me somewhat to see them in this relatively expensive when new Kenwood amplifier.

So with no bias pot to adjust, I must look elsewhere in order to get the right channel DC offset down from the nearly 1 volt it's got now.
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Old 28th June 2016, 03:24 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by 62vauxhall View Post
So with no bias pot to adjust, I must look elsewhere in order to get the
right channel DC offset down from the nearly 1 volt it's got now.
Measure the main power supply voltages +/- 50V. If one voltage is low due to a dried out
C1a/C1b 10000uF 56V capacitor, that might do it.

If the main voltages seem ok, try replacing: C5 33uF/10V; C9 47uF/6.3V; C10 47uF/6.3V.

Others to replace if you want to further refresh it: C3 1uF/50V; C4 47uF/63V; C12 1uF/63V; C13 1uF/63V.
These are not likely to cause the offset, though.

Last edited by rayma; 28th June 2016 at 03:37 PM.
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Old 28th June 2016, 08:27 PM   #5
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Thanks!

Measured at the big caps, those voltages are +/- 47v on one side and +/- 46v the other. I assume that's close enough to 50 to be in the acceptable range?

I'll try replacing those small capacitors but only if the amp's owner offers to sell it to me. I have no wish to start trouble shooting on something I do not own. I have a feeling both it and the tuner have been dead weight he's been hauling around and wants them gone. Since I estimate he'd be looking at about $200 (or more) for a shop to do it plus clean all the toggle and rotary switches, perhaps he'd only want a few bucks.

Still not exactly enamored with those power modules though.
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Old 5th July 2016, 02:30 PM   #6
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The owner of the amp and tuner did not wish to spend any money at all on repairs and told me to take them to the recycler.

Since he abandoned them, I started and am nearly finished a complete electrolytic re-cap. I have still to get three non-polarized ones and will probably leave the four 10,000mF filter caps alone.

With all but the bipolar replaced I checked for DC offset and found it unchanged at 96mv on the one channel. Since I had 2SA970 transistors on hand which are supposed to be an acceptable substitute for QE1 & QE2 I did the swap which brought the offset down to about 45mv.

Perhaps I misinterpreted, but subsequent reading seems to say that power modules such as those within a Kenwood KA7300, will typically produce DC offset of 50mv or higher and in fact, 100mv is nothing unusual.

Since I am/was under the impression that lower is better, can anyone say if it is the nature of the beast for these power modules to produce DC offset in that area?
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Old 5th July 2016, 04:18 PM   #7
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Did you match the input transistors? Your 2SA970 might not have enough gain, either.
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Old 5th July 2016, 08:02 PM   #8
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Did you match the input transistors? Your 2SA970 might not have enough gain, either.
I did match the transistors - sort of. I used a cheap and cheerful technique found on a different website. I'm sure a lot is left desired but it involved ensuring all transistors were at the same ambient temperature, a DMM set for Diode Check and shorting two transistor legs with one lead and touching the other lead to the single leg. The process was recording what the reading was after counting to 2.

I got about 2 dozen of those transistors some weeks ago to obtain matched differential pairs for a different project and measured all of them at the same time then labeled each one with the MV reading I got. I just picked two with equal numbers for one pair and likewise for the second.

I did not actually chose the A970's after comparing specs but rather because a poster recommended those in a thread on a different forum about this exact same amplifier. Since that person is held in high regard by others there for expertise and somewhat of a Kenwood aficionado, I went the recommendation.

But with regards to those power amp modules, do you know if they inherently produce large amounts of DC offset?

I don't think I'm too distressed with the <50mv it's producing after idling overnight.

Last edited by 62vauxhall; 5th July 2016 at 08:10 PM.
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