Electronics > Repair
Dimmer Pack Repair
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paul_g_787:
My Transcension 4 channel dimmer pack CDP405 stopped responding to DMX following a suspected power surge.
My first point of call was the power supply. It has a 1x240V to 2x15V transformer. Then a full wave rectifier into a 7805.
My voltages after the transformer 15VAC on each secondary, and the pulsating DC after the rectifier looks normal. However my 7805 was outputting 9V DC (measured using a BM786).
I replaced the 7805 now getting 4.9 V out but I have a pulse on the DC line. I probed it with the scope.
I have attached an image of the scope display.
What could be causing the pulsing? It doesn't look to me like the usual waveform I would expect for a dodgy capacitor. Perhaps indicating a short somewhere?
Any idea what to check next?
Harry_22:
Hi Paul!
You have 100 Hz pulsations at 7805 output. This means that you have them also at the input.
Check the bulky capacitor.
Harry
paul_g_787:
That would make sense. I will replace it when I come back to this in a couple weeks and post back.
Perhaps it is heat damaged over time from it's close proximity to the 7805....
Might replace the smaller output capacitor too as it is also right next to it. And bolt a small metal strip to the 7805 as a heatsink to take the heat up and away.
The PCB was a bit discoloured under the 7805 too.
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