Philips CM8833 Overbright Picture - UK Vintage Radio Repair and Restoration Discussion Forum

DavesTV · 17th Apr 2013, 5:59 pm

Good Day all,
So pleased to have found this board !
My trusty Philips CM8833 when first turned on has a very bright picture(No flyback lines though) for about 10 - 15 minutes then progressively gets darker till normal.
During this time the contrast pot has no effect on picture at all.
I suspect an electrolytic cap but does anyone have an idea which one ?
RGB supply to tube base board seems good and have replaced all power supply secondary smoothing caps with no improvement.
I have no experience working on these and the tube is still in very good condition so would like to sort it.
My second CM8833 flashes bright and dark so I expect the same cap is busy dying.
They are getting a bit long in the tooth now,but work so well.
Any ideas welcome,tia

hamid_1 · 18th Apr 2013, 1:27 am

Hello and welcome

The Philips CM8833 was a computer monitor from the 1980s. I'm not too familiar with the circuit but there's a method of fault-finding that I've had success with. You'll need a source of heat, like a fan heater or hairdryer, and some freezer spray like this : http://www.amazon.co.uk/Servisol-Fre.../dp/B000U1BU2G

First of all, open the monitor and try gently heating the PCB with a hairdryer. If it now starts working perfectly after a minute or two instead of taking 15 minutes for the fault to clear, it's temperature related. With the monitor switched on and running, spray some Freeze-it onto one of the suspect electrolytic capacitors, then wait a few seconds. If the fault comes back, you've found the bad capacitor! The Servisol spray comes with a straw attachment enabling you to aim the spray at a single component, so you should be able to find the fault exactly. You can confirm it by warming up and freezing the capacitor again.

FIXITNOW · 18th Apr 2013, 8:21 am

these suffered from horrendous dry joints all over
it might be worth opening and reflowing

Welsh Anorak · 18th Apr 2013, 1:28 pm

Hi
I think I've had the A1s drift on these monitors, either due to the decoupling cap or the flyback transformer itself. I'd mark the position of the A1 preset and give it a couple of rotations to clear any dirt (with the set off) the return it to the correct position and see if there's any improvement.
I'd hang a meter on the supply to the RGBs to see if it varies in sympathy with the fault.
Glyn

Nicklyons2 · 18th Apr 2013, 1:41 pm

Is the 'picture' actually there but very bright OR does the brightness completely obliterate any actual picture info? Also is this very bright picture tinted any particular colour?
As mentioned above, Philips went through a period when they specialised in dry joints so that's well worth a try.
Also - make sure tube base is clean , vac it off really well and then 'swish' it around in a cup of 'meths' - let it dry thoroughly; 'leaky' or faulty spark-gaps may be pulling the Cathodes down.

DavesTV · 5th Jun 2013, 4:29 pm

Hi Chaps,

Thanks for the helpful replies.

It seemed to be a combination of faults, as Nicklyons2 mentioned ,I found dozens of dry joints under the screening plate on the main PCB. This did not sort it though,after careful inspection and preparing to spray the TDA3505 IC with deep freeze spray, I noticed some thick sticky liquid across pin 19 and surrounding pins. Pin 19 happens to be the pin connected to the wiper of the contrast pot! After cleaning and drying with a hairdryer all seemed good.

Do not know how this liquid got onto the TDA3505 IC as it is right in under the tube??

However 2 days later when using the monitor again,the bright pic greeted me when set was cold. This time I removed the IC and found stickiness on the PCB under the IC and a little between the pins where my toothbrush could not get.

After cleaning this thoroughly all is now sorted.

Thanks again for your input.

Cheers, Dave.

17th Apr 2013, 5:59 pm	#1
DavesTV Retired Dormant Member Join Date: Apr 2013 Location: Port Elizabeth,South Africa Posts: 2	Philips CM8833 Overbright Picture Good Day all, So pleased to have found this board ! My trusty Philips CM8833 when first turned on has a very bright picture(No flyback lines though) for about 10 - 15 minutes then progressively gets darker till normal. During this time the contrast pot has no effect on picture at all. I suspect an electrolytic cap but does anyone have an idea which one ? RGB supply to tube base board seems good and have replaced all power supply secondary smoothing caps with no improvement. I have no experience working on these and the tube is still in very good condition so would like to sort it. My second CM8833 flashes bright and dark so I expect the same cap is busy dying. They are getting a bit long in the tooth now,but work so well. Any ideas welcome,tia

18th Apr 2013, 1:27 am	#2
hamid_1 Heptode Join Date: Aug 2009 Location: High Wycombe, Bucks. UK. Posts: 811	Re: Philips CM8833 Overbright Picture Hello and welcome The Philips CM8833 was a computer monitor from the 1980s. I'm not too familiar with the circuit but there's a method of fault-finding that I've had success with. You'll need a source of heat, like a fan heater or hairdryer, and some freezer spray like this : http://www.amazon.co.uk/Servisol-Fre.../dp/B000U1BU2G First of all, open the monitor and try gently heating the PCB with a hairdryer. If it now starts working perfectly after a minute or two instead of taking 15 minutes for the fault to clear, it's temperature related. With the monitor switched on and running, spray some Freeze-it onto one of the suspect electrolytic capacitors, then wait a few seconds. If the fault comes back, you've found the bad capacitor! The Servisol spray comes with a straw attachment enabling you to aim the spray at a single component, so you should be able to find the fault exactly. You can confirm it by warming up and freezing the capacitor again.

18th Apr 2013, 8:21 am	#3
FIXITNOW Octode Join Date: Feb 2007 Location: Ely, Cambridgeshire, UK. Posts: 1,976	Re: Philips CM8833 Overbright Picture these suffered from horrendous dry joints all over it might be worth opening and reflowing

18th Apr 2013, 1:28 pm	#4
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,921	Re: Philips CM8833 Overbright Picture Hi I think I've had the A1s drift on these monitors, either due to the decoupling cap or the flyback transformer itself. I'd mark the position of the A1 preset and give it a couple of rotations to clear any dirt (with the set off) the return it to the correct position and see if there's any improvement. I'd hang a meter on the supply to the RGBs to see if it varies in sympathy with the fault. Glyn

18th Apr 2013, 1:41 pm	#5
Nicklyons2 Octode Join Date: Jan 2013 Location: Barnsley, South Yorkshire, UK. Posts: 1,517	Re: Philips CM8833 Overbright Picture Is the 'picture' actually there but very bright OR does the brightness completely obliterate any actual picture info? Also is this very bright picture tinted any particular colour? As mentioned above, Philips went through a period when they specialised in dry joints so that's well worth a try. Also - make sure tube base is clean , vac it off really well and then 'swish' it around in a cup of 'meths' - let it dry thoroughly; 'leaky' or faulty spark-gaps may be pulling the Cathodes down.

5th Jun 2013, 4:29 pm	#6
DavesTV Retired Dormant Member Join Date: Apr 2013 Location: Port Elizabeth,South Africa Posts: 2	Re: Philips CM8833 Overbright Picture Hi Chaps, Thanks for the helpful replies. It seemed to be a combination of faults, as Nicklyons2 mentioned ,I found dozens of dry joints under the screening plate on the main PCB. This did not sort it though,after careful inspection and preparing to spray the TDA3505 IC with deep freeze spray, I noticed some thick sticky liquid across pin 19 and surrounding pins. Pin 19 happens to be the pin connected to the wiper of the contrast pot! After cleaning and drying with a hairdryer all seemed good. Do not know how this liquid got onto the TDA3505 IC as it is right in under the tube?? However 2 days later when using the monitor again,the bright pic greeted me when set was cold. This time I removed the IC and found stickiness on the PCB under the IC and a little between the pins where my toothbrush could not get. After cleaning this thoroughly all is now sorted. Thanks again for your input. Cheers, Dave.