Replace internal AC tranny for DC adapter?

[Martaine2005]

Hi guys, just joined as I am learning electronics as a NOOB!!
The Question.
Can I replace an internal 220v-9v .5A tranny with an external DC 9v-1.5A adapter?
Obviously it can be done, but I am a little confused as the pcb is already full wave rectified.
So, with a 9v DC adapter going through the bridge rectifier and 2200uf smoothing cap, will it be rectified again..smoothed again? Sorry don't have a scope only a Fluke 7-600.
Also, would there be a polarity issue with the adapter and rectifier?
Hope this makes sense and thank you in advance for any answers..Martin.

EDIT: I have already done this and the speakers work but seem to have distortion on higher volume. Hence the original question. I know the higher amperage wont pose an issue too.

 

[Arouse1973]

Hi Martaine. Welcome to EP. The DC adaptor will work but wont be rectified again because it's already DC. If you connect the DC adaptors output after the rectifier where the capacitor is then it should be fine. You could also connect it to the input of the bridge where the transformer was but you would lose the voltage drop of two diodes. And you could also connect it between the input to the bridge and the 0V of the circuit and you would lose the voltage drop of one diode one diode. The first option I mentioned means you don't lose anything. Do you want me to do a small drawing to help you?
Thanks
Adam

 

[witsender]

Hi Martaine
Yes, it very much does matter which way the DC is connected to the circuit.
A good way to make the repair might be to solder the wires from the external adapter directly to the smoothing capacitor on the PC board.
The 2 legs of the 22000uF capacitor are marked with + and - signs. You need to make sure that the positive (+ve) side of the supply is connected to the + side of the capacitor, and similarly the negative (-ve) side of the supply to the -ve side of the capacitor.

 

[Martaine2005]

Hi Arouse1973
And Witsender..
Thank you both for (The same answer).
Tomorrow I will re-solder directly to the cap and bypass the rectifier.
Incidently, apart from the v drop across the diodes, any other issues with this regarding wave forms for the (two) rectifiers?
one on pcb and one in the adapter? Just a thought....
Thank you both sincerely. Martin.

EDIT: Sorry forgot...
@witsender. NOW the polarity will be important connecting directly to the cap

 

[Arouse1973]

Hi Martin
You shouldn't have any waveforms, it's DC which is a straight line. Apart from any noise obviously.
Adam

 

[Martaine2005]

Thanks Adam...Just was curious. I will bypass the v drop of the rectifier tomorrow and all should be good.
So glad I joined this forum. I love the Tinternet..

 

[Martaine2005]

Update guys... I have been scratching my head all morning but very pleased with myself..
I originally omitted the fact that the item in question is pc ext speakers.

I connected the DC directly to the capacitor with correct polarity.
And the speakers worked!! I thought fantastic!!!!! Until I turned them off via the onboard switch....Hmm, they stayed on...
SO, I had to trace the board to the other side where the switch is and using continuity on my meter, I traced it back to one half of the rectifier. Soldered the + to the diode and the - the neg on the cap. It switched off perfectly.
And for some other GOOD reason, the DC adapter is nowhere as warm as it was connected directly to the cap.
Job done. And very pleased with myself!!!
Thanks guys really appreciate your help..Martin.

 

[Arouse1973]

No Probs See you around!
Adam