Kenwood KT-45 Tuner Schematic/Service Manual - diyAudio
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Old 24th January 2011, 07:14 PM   #1
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Default Kenwood KT-45 Tuner Schematic/Service Manual

Hi all. I have a Kenwood KT-45 Tuner that seems to not power up. Does anyone have a service manual for it or schematic? Where can I find info to help me fix the tuner?

Thanks
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Old 24th January 2011, 10:54 PM   #2
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If it doesn't power up, you don't need a service manual. Look for continuity through the power transformer primary - fuse, switch, solder connection. If you get line voltage at the transformer primary, do you get anything out of the transformer? Do the windings measure proper low resistance? If there is a line voltage selector, it could be not making contact.
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Old 24th January 2011, 11:33 PM   #3
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There is no fuse on the primary. The transformer seems good since I can read a voltage with my multimeter on the secondary at 17V. There is a switch which functions properly, connecting the secondary to a bridge rectifier right next to a huge filter cap. I don't see any burn marks or bulging capacitors on the board. I have an image of everything here. The switch is in the top right, the display is connected to the main board, and the transformer is on the right with the secondary coming to the main board near the huge cap.

Here is a closeup of the power supply.



Whats wierd is that there is a large IC on the left side of the board (description link here) which I measure a voltage from the Vdd pin to ground of about 2.1V when it should be 5V.

Here is another picture of the main board.

I am not sure how to proceed without a schematic.
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Old 25th January 2011, 12:00 AM   #4
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You might try tracing the Vdd line back to the PSU section,and finding out what's feeding it. (maybe Q34,or something around there?)

Do you have a DC voltage across the leads of that large filter cap?
Have you tested the rectifier diodes?
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Old 25th January 2011, 01:36 AM   #5
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Default ``

Thanks for the replies. I'm going to set up my o-scope and capture these signals in a bit.
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Old 25th January 2011, 05:46 AM   #6
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I've tested the diodes and they are all good. I've also measured the unregulated "DC" signal across the large filter cap and it appears to be 21V with some ripple as shown here (scope software wouldnt let me show the offset from 0V)

I am not sure of the circuitry of the rest of the PSU. I would expect a voltage regulator of some sort but maybe the rest of the transistors in the PSU make up the regulator rather than having a one chip solution (this is the 80's after all). Q34 is the biggest transistor in the PSU, marked as a C2167. The rest of the PSU seems a bit complicated for me to analyze.

It is difficult for me to get a path from the Vdd pin of the chip I mentioned before back to the PSU. Whats strange is that the supply goes through a diode before going into the Vdd pin, im not sure why.
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Old 25th January 2011, 09:30 PM   #7
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I found a service manual and after testing, it turns out that a capacitor was bad. It was internally shorted with no signs of damage. The cap was in the PSU and the shorting caused one of the transistors to become off-biased because the capacitor was now shorting the base to ground.

Thanks guys.
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