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

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Old 20th Jan 2019, 9:04 pm   #1
Brian Illsley
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Join Date: Jan 2019
Location: Bognor Regis, West Sussex, UK.
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Default Philips CM8833 whining problem

Hi all,

First post here, im looking for help/advice on a Philips CM8833 that has been stored in my garage for a good long while. Well i pulled it from the garage the other day as i was testing a Neogeo Aes which i bought at a bootsale. Anyway i hooked it up and got a loverly picture and a high pitched whine. I thought due to its age this was proberbly normal. Continued testing and everything was fine. But when i turned off the neo geo i forgot to turn off the monitor.

3 hours later, i realised my mistake and rushed to it. But alas the monior no longer displays a picture. Ok so here is where i need advise, i removed the chassis, (im an experienced arcade engineer and know the correct disscharge procedures) i tested the transistor {HOT} and found that the middle was shorting to ground but after i removed this the short was still there. I checked the poly caps c470 and c469 which also showed direct shorts.

I also tested the damper diaode D467 but this was ok and no short. I tested around the lopt and found that 3 points around this were shorted to ground. I removed the lopt and all shorts went away. So im kinda thinking i need a new lopt, but im worried that i may have missed somethin or that the new one may break when if i put one in. Just for your info i powered the psu board not connected to nothing and its not whining and its giving the right voltages.

Any advice would be great.

Just so you know i know how to discharge monitors but this was to send them away for repair. This is the first time i have ever attempted to fix one myself, mostly a confidence thing 😜
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Old 21st Jan 2019, 12:37 am   #2
yampy187
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Location: Coventry, Warwickshire, UK
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Default Re: Philips CM8833 whining problem

Hi.

It will definitely be the LOPT. They were notorious for failure.

They are still available on eBay ��

Jan
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Old 21st Jan 2019, 3:30 am   #3
mickash
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Join Date: Sep 2007
Location: Derbyshire, UK.
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Default Re: Philips CM8833 whining problem

Brian,

The scan coils can fail on these monitors so are worth checking before spending on a new LOPT.
The rubber wedges between the CRT and scan coils can attack the insulation of the coils windings causing tracking and shorts between the windings. Carefully remove the scan coil unit (paying attention to positioning to enable you to refit it accurately) Check the points where the wedges meet the inner side of the coil assembly. The copper wires should be shiny and not degraded. Any black marks and signs of tracking point to faulty scan coils.
The windings can be repaired if not too badly damaged by bridging and insulating affected windings but it's a bit like microsurgery!

Michael.
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Old 21st Jan 2019, 4:34 am   #4
Maarten
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Location: Haarlem, Netherlands
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Default Re: Philips CM8833 whining problem

Scan coils could fail and it never hurts to check them, but with a dead short to ground the LOPT has very likely self-destructed. This was a very common failure mode for this size (AT2079 I think) in various designs that used them.
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