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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 10th Jun 2019, 8:18 pm   #1
ljdavek
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Default Philips CM11342/55G video monitor

Hi, hopefully someone can help I have a RGB Philips video monitor, and it stoped working. All it does now when I turn it on is give out a high pitched whistle. when I turn it off the whistle drops in pitch then fades off that’s all it does?!. Can anyone give me a starting point of where to start looking??

Dave
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Old 10th Jun 2019, 9:13 pm   #2
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Default Re: Philips video monitor

Do you have a model number? The fault you're describing sounds very much as though this could be a CM8833 and sadly the LOPT is a well known cause of this.

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Old 10th Jun 2019, 9:23 pm   #3
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Default Re: Philips video monitor

Yes agreed the Lopts on these and Commodore branded versions were a well known cause of this fault, oddly we used to be able to buy the Philips original transformer cheaper than any of the replacement branded ones.
I can remember buying original Phillips ones for about £6 plus VAT trade, some of the copies could be three times as much!, usually it is the other way around.

Edit: IIRC there were about three different common Lopts across the various model ranges, the AT number on the transformer was most important when ordering replacements, because of course they all looked visually alike.
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Old 10th Jun 2019, 11:37 pm   #4
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Default Re: Philips video monitor

I would hapily guess that it is a Phillips grey Lopt.
They were just as bad in VGA screens of quite a few makes.
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Old 11th Jun 2019, 9:48 am   #5
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Default Re: Philips video monitor

Hi the monitor is the CM11342/55G , Iím about to take the back off!!!
AT2079 is whatís on the LOPT
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Old 11th Jun 2019, 12:40 pm   #6
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Default Re: Philips video monitor

That's not the full number, only the basic style.
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Old 13th Jun 2019, 11:53 am   #7
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Default Re: Philips video monitor

The AT2079 used to fail in large numbers in many Philips TVs and monitors of that period. As Maarten says, the rest of the number is vital to get the correct LOPT. HR Diemen is your best bet.
CPC used to have their own line of LOPTs which were a complete failure. The one for the Philips monitors connected a HT pin to chassis! Cutting this off got things going, but the secondary voltages were clearly not right.
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Old 29th Jun 2019, 11:25 am   #8
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Default Re: Philips video monitor

So so far Iíve checked the big transistor that drives the LOPT and itís ok!?

Also checking the switching transistor in the power supply and thatís still a transistor as well... any thing else to look for?
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Old 29th Jun 2019, 8:23 pm   #9
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Default Re: Philips video monitor

Just uploaded some photos of the board !!
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Old 29th Jun 2019, 8:58 pm   #10
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Default Re: Philips video monitor

In a repair tips book I have it says to check R3412 (33K Ohms) for open circuit.
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Old 29th Jun 2019, 9:24 pm   #11
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Smile Re: Philips video monitor

Hi so you mean it could be a resistor!!!! Hope it is the one thatís problem!!
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Old 29th Jun 2019, 9:26 pm   #12
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Default Re: Philips video monitor

Isolate pin 3 of the LOPT, if the squealing stops then it's highly likely to be faulty. From your picture it looks to be an AT2079/37591, it's equivalent is an HR7533.

John.
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Old 29th Jun 2019, 11:33 pm   #13
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Default Re: Philips video monitor

Ok checked the 33k resistor and itís fine I replaced it anyway lol.. checked the transistor near it that was fine ... could the 7412 regular be a problem as was thinking of pulling that out to check... if not how do I find out witch pin is pin 3 to isolate? And il try that next!!!

Dave.
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Old 29th Jun 2019, 11:35 pm   #14
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Default Re: Philips video monitor

Ok iv looked at the photo now I can see pin 3 .!! Would de soldering it be ok and some how put tape round it? To isolate?
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Old 30th Jun 2019, 12:40 am   #15
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Default Re: Philips video monitor

Have you checked the line output transistor and fly back diode are not short circuit?
They will cause that symptom.
Those grey line output transformers usually don't completely trip the mains regulator in the way yours is doing.
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Old 30th Jun 2019, 9:01 pm   #16
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Default Re: Philips video monitor

Quote:
Originally Posted by ljdavek View Post
Ok iv looked at the photo now I can see pin 3 .!! Would de soldering it be ok and some how put tape round it? To isolate?
As long as pin3 doesn't have an eyelet fitted to the PCB (a sort of metal sleeve) its should be possible to remove all of the solder and isolate it. Although you say you have checked the line output transistor as Refugee says the diode across the transistor could also be faulty.

It would be good to know the resistance between pin3 and it's solder pad to chassis before and after isolating it.

Might also be prudent to check the blue tuning capacitor next to the LOPT for any signs of dry joints or arcing damage. This can show as a bulge on the capacitor body.
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Old 30th Jun 2019, 10:51 pm   #17
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Default Re: Philips CM11342/55G video monitor

Iv checked to lopt drive transistor itís ok where do I find the diode?

Dave
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Old 1st Jul 2019, 12:20 am   #18
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Default Re: Philips CM11342/55G video monitor

Just follow the tracks from C and E of the transistor.
What is the part number of the transistor?
The capacitor should also be in the same position.
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Old 1st Jul 2019, 12:42 am   #19
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Default Re: Philips CM11342/55G video monitor

At this point, I would first test the power supply with a light bulb as a load. I don't have the schematic handy, but take care to interrupt the supply to the line output stage before testing.
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Old 1st Jul 2019, 11:23 am   #20
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Default Re: Philips CM11342/55G video monitor

So Iím looking at the pin out of the LOPT thatís on this post would it be pin 10 I disconnected and connect the light bulb to instead? For testing the power supply?
I am a bit of a novice at repairing CRTs sorry..

Dave.
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