Kenwood KT-45 Tuner Schematic/Service Manual

[zeneslev]

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

 

[bob91343]

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.

 

[zeneslev]

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.

 

[DigitalJunkie]

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?

 

[zeneslev]

``

Thanks for the replies. I'm going to set up my o-scope and capture these signals in a bit.

 

[zeneslev]

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.

 

[zeneslev]

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.

 

[binbash]

Hello, i'm new in the forum, i dont speak english, sorry for traduction.
This post is old, but i have a KT-45 with the same problem, do you have the service manual, i dont find it in google, try several searches. What is the capacitor that fail in the power supply? or what is the transistor associated with it?, very thanks from Uruguay, a little country in South America

 

[campinaverde]

Hello, i'm new in the forum, i dont speak english, sorry for traduction.
This post is old, but i have a KT-45 with the same problem, do you have the service manual, i dont find it in google, try several searches. What is the capacitor that fail in the power supply? or what is the transistor associated with it?, very thanks from Uruguay, a little country in South America

I also had the same problem on my Kenwood KT-45.
Do you found any service manual for this tuner, binbash ?
Thanks from Brazil.