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Old 25th March 2008, 02:20 PM   #1
AndrewB is offline AndrewB  United Kingdom
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Default Help to fix Mission PCM II

I have a Mission PCM II, owned since new.
Problems started a few months ago, the display would go on or off at random, but still played fine.
Then it got random about whether it would play or not - but once working it continued as long as it remained powered up.
Now the trasnport drawer will open, but won't close using the open/close or play buttons - it will close using a light finger push - and won't respond to any other buttons.
I have unplugged and checked all the internal connectors - all were clean and shiney. Also visually checked the boards for poor joints or corrosion but can't see anything obvious.

The PCB under one voltage regulator is blackened - could this be a result of the component failing? There are other signs of heating around many components, but none are black like this.

Could this failure cause the symptoms I described? Any other suggestions as to what else I should check?
I can email higher res pictures if this would help.

Thanks in advance,
Andy
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Old 25th March 2008, 03:04 PM   #2
Puffin is offline Puffin  United Kingdom
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Andy. Have you checked the voltage going to the regulator ? Check the others and see if you can see the type eg 7805 =5v, 7815 =15v 7812 =12v etc. (negative regs are - 7805 etc)

If you don't get any reading or an odd reading for the bleckened one then it will need replacing. You should check to see if any caps in the chain are shot.

For testing purposes the regs 3 legs (viewed from the front) is In, Gnd, Out.

Higher res pics would be of help.
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Old 25th March 2008, 03:23 PM   #3
poynton is offline poynton  United Kingdom
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Location: A life on the ocean waves when I'm not at home in N. Wales (but I'm not Welsh so no sheep jokes!)
Please post photos of the supply area.


I'm trying to remember what Philips model this is based on. Anyone?


Andy
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Old 25th March 2008, 05:07 PM   #4
AndrewB is offline AndrewB  United Kingdom
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Great response already, thanks!

The regulator is a MC7906CT.

Searching old threads suggests the PCM II is based on the Philips CD660 - uses a TDA1541A S1 and SAA7220P/B.

Here is a close-up of the regulator.

I'm a newbie when it comes to checking components - I've replaced 'memory' caps in 2 tuners and replaced a timer IC and LCD driver IC on a speed display board, but that's my limit so far. I have a digital multimeter, and willing to learn, so please excuse any dumb questions!
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Old 25th March 2008, 05:07 PM   #5
AndrewB is offline AndrewB  United Kingdom
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Default Here's the bottom view

Underside of PCB where MC7906CT is mounted
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Old 25th March 2008, 05:07 PM   #6
AndrewB is offline AndrewB  United Kingdom
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Here is the back end of the board - the regulator is just hidden under the loose grey/black 5-pin connector (sorry - took the pic before finding the black bits).

Hopefully you can also see the brown discolouration around quite a few of the resistors. Is this normal for old boards, or does it suggest that everything has been getting hotter than it should?
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Old 25th March 2008, 05:27 PM   #7
poynton is offline poynton  United Kingdom
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Location: A life on the ocean waves when I'm not at home in N. Wales (but I'm not Welsh so no sheep jokes!)
That regulator is a -6v.

It looks hot enough to have melted the solder and also the pcb trach looks to be cracked. It could be the photo angle.

Scrape off the black stuff.

Andy
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Old 25th March 2008, 05:54 PM   #8
Puffin is offline Puffin  United Kingdom
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Andy. Sorry, edit to my post - regs are designated 7905, 7915 etc
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Old 26th March 2008, 07:56 AM   #9
AndrewB is offline AndrewB  United Kingdom
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PC at home crashed yesterday so had to spend the evening trying to fix that .....

Local shop had every other type of 79xx regulator in stock except a 7906 - they'll have one for me later this week, so I will replace it anyway and see what happens.
I'll keep you posted.
Thanks again.

Andy
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Old 8th April 2008, 09:09 AM   #10
AndrewB is offline AndrewB  United Kingdom
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Finally found time to look further.
A meter across the regulator (7906) shows Vin = 13V, Vout=7V
Data sheet says it should be 8-21 V input and 5.7 to 6.3 V out.

Local shop still hasn't managed to get a replacement - any ideas of where to get one without a huge carriage cost?

I also checked the tracks and get typically 0.1 to 0.2 ohm from the regulator solder 'blob' to the next component along the track (without scraping the connection clean).
The 47uF caps across the regulator appear to charge (using a continuity check across the regulator, I get a reading then it stops).

Any more ideas? The controller chip on this board is an MAB8461P/W127.

Andy
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