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8 posts

Wannabe Geek


Topic # 97034 9-Feb-2012 13:51
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Hi, fellow members,

I have been a member for over one year now, however, this is my first time attempt of trying to ask a question about a broken Philips Micro DVD Theater MCD700 which my friend had only enjoyed it for only one and a half years before it had gone dead (a burnt transformer coil).

I like to find out how things work; I like to open up broken devices to find out what has gone wrong. And if possible try to fix it. So I asked my friend to give it to me.

I opened it and found there was no power getting into the system. So this is simple, I took the doughnut shaped transformer out of the place, and searched for the rewinder on the web. And found the only one rewinder in New Zealand is located in Christchurch, I am in Auckland. And the cost plus shipping will be over 100 bucks, while a brand new system available in the market is only $120. So this is not practical to try to fix it.

So I studied on it further just for fun, and I found the system needs 4 kind of different voltages-- 20.5v, 18v,16.5v and 2.2v all are AC power. I saw there are rectifiers on the PCB. And then I searched in the eBay for any electronic parts that might help. I found a voltage regulator module that can reduce a voltage range from 24v - 40v DC to any voltage between 1.5v DC to 20V DC, and its price is affordable, three US dollars each. So I bought 4 of this item and use the power supply from my notebook, which converts 240 V AC to 24 V DC. Now I hope this assembly will work for the system.

I have got everything I know connected, then I turned on the power, and it really worked. At this stage, I controlled the system using the remote, and listen to the sound by an earphone. Then, I hooked it up to the TV set and they worked as I expected. However, when I hooked it to the two speakers that come with the system, the sound was distorted and creaky, totally unacceptable. Another problem I have found is that the front panel where the status of the system is displayed is not bright enough, too deam and not readable

So I turned to Geekzone to seek for help. Can anyone out there give me some more ideas or hints if something else I can try to solve the two problems mentioned above.

Regards.
Kuei

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  Reply # 580043 11-Feb-2012 00:02
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It's possible the laptop power supply can't supply enough power as in watts.
There should be a sticker on the MCD700 showing the power requirements of the unit. There should also be one on your laptop power supply. In order for every thing to work correctly the rated wattage output of the laptop power supply must be greater than the wattage power requirements of the MCD700.

If the the MCD700 does not input power rating it should have a input current (Amps). Just multiply this figure by 240. This will give you a pretty close approximation of its power requirements.
P = I x V (Power watts (P) = Current Amps (I) x Voltage (V) )

Before someone jumps on me, I know this power calculation is not strictly correct for AC current but it is close enough for the needs of this post Smile

Dave

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  Reply # 580066 11-Feb-2012 08:21
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+1
It sounds like the Laptop Power Supply hasn't got enough grunt. The individual regulators also need to have sufficient wattage output to drive their circuitry.
You don't make it clear how you have connected the supplies. You say it needs 4 different AC voltages but you are connecting DC volts. Please clarify.



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Wannabe Geek


  Reply # 580133 11-Feb-2012 11:42
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Hi, B1GGLZ, The reason I used DC as this player's power supply was because I could only found the voltage regulator which changes DC to DC volts on Ebay listings and also because they are light and affordable. Before I moved the doughtnut shaped transformer out of place, I noticed there is a sticker on the coil. The sticker shows the distribution of the 4 different volts to the color coded cables, that was how I connected the four volt regulator. Frankly to say, I am green to electronic repairing. All the knowledge I have is the basic learned when I was in high school.

Hi, djtOtago, Thank you for your hints. the player has a sticker220-240v AC 50Hz 32W. The laptop power supply: Imput-100-240v AC 50-60Hz 1.8A; Output- 24v DC 2.5A
When I listen to the music by earphone or by way of TV set it functions totally well, but when I want the music to be played through the speakers that come with the machine, then the sound is terribly unacceptable. Oh yes, the stickers attached on the back of each one of the speakers show this: MCD700 8 ohm 25A 

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  Reply # 580138 11-Feb-2012 12:03
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kuei: Hi, B1GGLZ, The sticker shows the distribution of the 4 different volts to the color coded cables, that was how I connected the four volt regulator.


In that case you are connecting DC to the input of the rectifiers (presumably bridge rectifiers?) which require AC input. At best you are getting half the required voltage. At worst, nothing on the rectifier output. You need to connect the DV voltages to the output point of the rectifiers but after removing the rectifiers.



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Wannabe Geek


  Reply # 580178 11-Feb-2012 14:26
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<<<< At best you are getting half the required voltage. At worst, nothing on the rectifier output.>>>
Awsome! Thank B1GGLZ, your diagnosis has cleared my misgiving about the what it has changed to be after the DC V is fed to the system that requires AC V. Now I have some ideas to carry on with my project which I name it "learning by experimenting".

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  Reply # 580180 11-Feb-2012 14:50
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Just don't harm yourself in the process. Watch out for the big capacitors which can retain a fair whack of energy if you still have the original power supply in the unit. As above, whilst you may have the correct DC voltages via the laptop power supply, you may not have the current capacities required to drive the various components on the board.

If this project costs you anything, you'd be better spent purchasing a new unit. Sometimes there are head units only on trademe where people have lost/damaged the speakers etc. Your speakers are 8 ohm, so should hopefully be fairly transportable to a new unit.



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Wannabe Geek


  Reply # 580185 11-Feb-2012 15:21
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<<<If this project costs you anything, you'd be better spent purchasing a new unit.>>>
No, it costed me nothing. Because it looks like new, although it has been broken for a couple of years, so my friend found it hard to put them out on the inorganic rubbish collection day by himself. So he gave it to me for free. I have 3 different kinds of DVD players, I don't need it, but just want to use it to learn electronic basics. Thank you for reminder of the danger of the capacitors on the PCB. I will be careful.



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Wannabe Geek


  Reply # 580186 11-Feb-2012 15:21
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<<<If this project costs you anything, you'd be better spent purchasing a new unit.>>>
No, it costed me nothing. Because it looks like new, although it has been broken for a couple of years, so my friend found it hard to put them out on the inorganic rubbish collection day by himself. So he gave it to me for free. I have 3 different kinds of DVD players, I don't need it, but just want to use it to learn electronic basics. Thank you for reminder of the danger of the capacitors on the PCB. I will be careful.

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  Reply # 580192 11-Feb-2012 16:00
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Jaxson: As above, whilst you may have the correct DC voltages via the laptop power supply, you may not have the current capacities required to drive the various components on the board.

+1
This sounds like it may be your problem. If the unit's sticker says 32W then that is probably with speakers at full bore.
Using headphones may only use a few watts which your Laptop supply and/or regulators are capable of delivering and therefore no distortion. Using the speakers is probably needing more current than either the Power Supply or regulators can supply therefore pulling down the volts and massive distortion. Is your Laptop supply rated better than 32W?  Are each of the regulators capable of around 1 Amp each? Remember Power = Voltage x Current.



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Wannabe Geek


  Reply # 580222 11-Feb-2012 18:14
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<<<Is your Laptop supply rated better than 32W?  Are each of the regulators capable of around 1 Amp each?>>>
the sticker attached on the laptop power supply shows 24 V, 2.5 A and doesn't show wattage, but according to the formula P=I X V, the wattage should be 60w, am I right? But this is before it steps down to the four different voltages required. When I was testing how good it is when it works with its own speakers, I take turns to disconnect each of the four powers for their individual amperage at their connecting points let me write it down below:
20.5v--0.00A; 18V--0.29A; 16.5V--0.14A; 2.2V--0.00A. so the 2.2 V and 20.5 V, these two powers got lost after I replaced the the doughnut shaped transformer with this 4 voltage regulators. 
Before I placed order for the four mortgage regulator, I remembered the sailor said each of the board can provide current up to 2A.

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  Reply # 580230 11-Feb-2012 18:41
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There's also the possibility that something else caused the previous power supply to fail, and whilst you have replaced the power supply, the real culprit is still at large...

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  Reply # 580256 11-Feb-2012 20:16
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kuei: <<>>
20.5v--0.00A; 18V--0.29A; 16.5V--0.14A; 2.2V--0.00A. so the 2.2 V and 20.5 V, these two powers got lost after I replaced the the doughnut shaped transformer with this 4 voltage regulators. 
Before I placed order for the four mortgage regulator, I remembered the sailor said each of the board can provide current up to 2A.

OK. 60w is correct. The Laptop supply and the regulators are able to supply sufficient power. The missing current on 2.2v and 20.5v would indicate there are more problems which is probably why the Transformer burnt out although I would expect to find fuses which should have prevented that.



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Wannabe Geek


  Reply # 580306 11-Feb-2012 22:01
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@B1GGLZ:<<which is probably why the Transformer burnt out>>
 
ah! a very important point you have raised for me. My friend told me he loved this toy very much so that two years ago when he found it could not work, he sent it two his friend, who is veryl very knowledgeable about electronics. A few days later, his friend returned the machine and told him. "The transformer was gone and you better buy a new one, which is much cheaper then you order a new transformer and try to repair it".

I remembered when I opened up this device to have a look inside for the first time, I noticed there was a fuse holder located it in place right before the cable enters into the toroidal transformer. I uncovered it and found no fuse inside. I thought I had found the problem why there was no power being fed into the system. But when I got a spare one and put it in place, it's still would not work. So I told my friend about what I had seen, and also asked if he still wanted its back if I could not do anything over it. And he told me "I love it so much so it is very hard for me to get rid of it, So you, throw it away for me". On the next day, I began to unwind the toroidal transformer and found there were a few points among the various kind of wires and the plastic insulation wrap were melt down in the deeper layers of the winding. Before I unwound it with hands, I also checked the continuity of each pair of the four different power of cable and found no continuity.

So yes, you have reminded me a very important points I have neglected. Tomorrow I will proceed to investigate any suspicious points of short circuit. Can you please give some tips of checking for possible short points?

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  Reply # 580381 12-Feb-2012 09:25
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kuei: @B1GGLZ:<>
 Can you please give some tips of checking for possible short points?

Not really. It's hard to say without seeing the unit and a circuit diagram. Unfortunately transistors and IC's die without even the joy of a puff of smoke and any visible damage. You'd also need a tone generator, multimeter and oscilloscope to get very far.



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Wannabe Geek


  Reply # 580391 12-Feb-2012 09:48
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Yeah, that is too far for me. But thank you for your advice.

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