Kareokee amplifier not working

[Cornboats]

This has been in my pile of not working and needs fixing pile for some time.
It works using 2 x 9v batteries plugged into the supplied flyleads as picture but the 12v Dc power supply input from mains supply doesn't work. There's 12v dc coming in from the jack and 12v between jct of D3 and D5. There's a 0.6V drop across d3 supplying positive voltage regulator KA7809 thro capacitor C25. This regulator has healthy 12v output.
Theres no voltage drop across D5 and no input voltage into negative voltage regulator KA7909. I've changed capacitor C27 25v 450 microfarrads. Esr was 30 and capacitance was all over the place. In doing so the circuit track eyes were broken for the capictor so I've soldered in leads direct from D5 and to Ka7909.

Still no joy- which is why I'm asking you bright fellows for help please. I know nothing about negative voltage rails or why theres here. Can someone help me solve this one please.

[lotas]

Re: Kareokee amplifier not working

Why did you put this jumper, you close the positive to the negative! Delete it. 7809 and 7909 have different pinouts.

[Cornboats]

Re: Kareokee amplifier not working

Thanks for taking the time to reply. I haven't put a jumper in, I've just followed the circuit board track exactly. To +capacitor c27 coming from -d5 M1 diode is long wire. From - capacitor c27 to pin 2 of KA7909 is short wire. I had to run new wires from both sides of the capacitor as the track terminals at the capacitor just disintergrated when I desoldered.

[R_J]

Re: Kareokee amplifier not working

This should fix it, D5 anode needs to connect to the - of C27 not to the + The positive of C27 is connected to common ground

I removed C27 from the picture to show how the trace is likely connected under the cap.

[Cornboats]

Re: Kareokee amplifier not working

I made a school boy error running track to + of capacitor. Please see last 3 photos attached. I removed tracks and capacitor to show existing track, then wired in new track from D5 linking at capacitor -C27 then to input of KA7909.
Still no joy I'm afraid.
Voltage readings taken are;
Across D3 0.73v, D3 + to grd 11.33v
Across D5 11.84v D5 - to + rail 11.84v

I'll remove KA7909 to see if was pulling down voltage.

[R_J]

Re: Kareokee amplifier not working

What voltage is ACROSS C27? If the voltage is 0, disconnect the power and check the resistance across C27 to see if there is something shorted.

[Cornboats]

Re: Kareokee amplifier not working

voltage across c27 is 0 ( this is a new capacitor), voltage across c25 is 11.22v.
resistance across c27 is 3M and across c25 is 4M.

I pulled the KA7909 to see if that affected the readings but there was no difference so I soldered it back.
What puzzles me is that the negative voltage regulator requires a negative input voltage. How is this achieved from a positive dc supply?

Thanks for taking interest.

[R_J]

Re: Kareokee amplifier not working

I think you need an AC adaptor, not a DC one. What is the model of the machine?

[Cornboats]

Re: Kareokee amplifier not working

I think you're right. I just assumed as there was no bridge rectifier in the supply circuit it needed DC and having a battery backup hard wired into the circuit just confirmed my idea.
Model no is DVD K1 made by Accoustic solutions, Northampton, uk.
I'll dig out an ac adaptor, try it and let you know.

[R_J]

Re: Kareokee amplifier not working

It is just using 1/2 wave rectification so only one diode for positive and one diode for negative voltage, it doesn't draw much current so that is adequate.

[Cornboats]

Re: Kareokee amplifier not working

Well done for that. Fitted an ac adaptor and negative voltage to the -voltage regulator is restored. Quality of sound at the speakers is not very good though. Could be the stand alone speakers. I'm going to connect up the amp thro a tv & dvd to see if its any better. There are loads of caps on the board, maybe some of those have dried out.

[Cornboats]

Re: Kareokee amplifier not working

So I checked every single capacitors ESR and found all within tolerance. The only one I changed was C27 which was across power to negative voltage regulator. The ESR on the old one was 23.00, average for this spec is 0.12. The capacitance should have been 470 UF but measured 86.7NF.

The crackling and sound distortion I managed to remove by turning down the echo and microphone volumes. All in all I treat this as a result. Many thanks for the Forums help. More broken and in need of repair items coming out of the box.