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Kenwood KA 8004 bias problem

Gronard

Gronard

New Member
Hi,

I have just bought my first vintage Kenwood amplifier: KA 8004

The amp seems to be working fine and sounds good but when I tried to check and adjust the bias setting according to manual I am getting really strange values.
According to the service manual the bias should be set to 20mV by adjusting Vre1 (measure on terminal 2 and 4) and Vre2 (measure at terminal 1 and 3).

Measuring at 2 and 4 gives a low voltage about 5mV (unstable) adjusting Vre1 does not change anything.
When measuring at 1 and 3 I measure 36V :-( adjusting Vre2 does not change anything.

When I measure at the speaker terminals I get the same results.

Any ideas on what could be wrong?

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Thanks,

Bernard
 
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This is the schematic diagram of the power amp that I found in the service manual.
I can see the 35.5V I measure on the speaker terminals mentioned at Qe6.
Could someone help me to identify where the problem could be?


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I have done some further investigation and I have found that resistor R25 is dead.
R26 measures 150ohms, According to the manual it should be 330ohms. I also checked some other resistors and found more different values.

Turns out that my service manual is for main amp board X07-1120-01 and my print board has X07-1220 printed on it :-(
 
I have found a new schematic for my version X07-1220 of the main amp board and have noted all the modifications on the parts list.
Close to the faulty resistor there also is a small green component called THe1 and THe2: 5T-32L. On one of these the outside is chipping and it does not look ok.
I suppose these are thermistors but I cannot find any information or replacement for this. Does someone know about these components? Is there a way I can test if they are still good?

Thanks.
 
Last edited:
Gronard said:
I have done some further investigation and I have found that resistor R25 is dead.
R26 measures 150ohms, According to the manual it should be 330ohms. I also checked some other resistors and found more different values.

Turns out that my service manual is for main amp board X07-1120-01 and my print board has X07-1220 printed on it :-(
Click to expand...
Make sure that you remove one leg of the resistor to remove it from the circuit to get an accurate reading. You can compare the 2 channels
with each other to find out how they compare with each other. :)
 
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