Strange issue with an AKF11 CRT

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DutchAcorn
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Strange issue with an AKF11 CRT

Post by DutchAcorn » Thu Nov 15, 2018 8:05 pm

My AKF11 CRT, (rebranded Philips monitor) has suddenly died on me, but not quite. It sometimes flickered before but yesterday the image suddenly became very vague, distorted and smaller.
akf11 picture.jpg
When left on for a while an unpleasant smell is emitted from somewhere in the monitor though I can't see smoke emitted.

Inside it seems some kind of insulator material has oozed out, but I don't know if that is normal?
inside.jpg
Any ideas?
Paul

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Re: Strange issue with an AKF11 CRT

Post by paulv » Thu Nov 15, 2018 10:35 pm

Beer goggles and reduced size makes me think the tripler/lopt are failing.

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Re: Strange issue with an AKF11 CRT

Post by sbadger » Fri Nov 16, 2018 11:48 am

The insulator ooze looks normal, it's a usually a factory glue to keep winds and convergence magnets in place.
as for the picture, yeah agree, for the focus to go like that sounds like the LOPT.

I think the AKF11 is essentially a Philips CM8833 (mk 1), and new lopts are available. (ebay , donberg etc)

EDIT: part is probably a HR6489
Last edited by sbadger on Fri Nov 16, 2018 11:53 am, edited 1 time in total.
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Re: Strange issue with an AKF11 CRT

Post by DutchAcorn » Fri Nov 16, 2018 5:50 pm

Hmm, usually a lopt failure results in total failure, no power led light and a high pitched whine?

Lopts are expensive. :|

Is there a way to measure if a lopt is ok? As it is a transformer you should be able to measure the resistance of the coils?
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Re: Strange issue with an AKF11 CRT

Post by 1024MAK » Fri Nov 16, 2018 6:31 pm

That’s fault looks like a capacitor failure on one of the main power rails.

If you can, I suggest testing the voltage levels on the main power rails.

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Re: Strange issue with an AKF11 CRT

Post by DutchAcorn » Fri Nov 16, 2018 9:50 pm

1024MAK wrote:
Fri Nov 16, 2018 6:31 pm
That’s fault looks like a capacitor failure on one of the main power rails.

If you can, I suggest testing the voltage levels on the main power rails.

Mark
Thanks Mark, I'll check the caps. I don't know where the main power rails are but I'd guess it can't be that hard to find.
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Re: Strange issue with an AKF11 CRT

Post by 1024MAK » Fri Nov 16, 2018 11:11 pm

Work out which version chassis it is and see if you can find a service manual and schematic. That will then tell you what the power supply arrangements are.

Typically in most mains powered CRT displays, the incoming AC mains is converted to DC via a bridge rectifier and high voltage electrolytic capacitor. If the mains voltage is 240V AC, the DC voltage will be about 339V.

If the set has a DC/DC switch mode PSU, this is then converted to a 100V DC to 150V DC main “HT” rail (HT meaning high tension, another way of saying high voltage). Plus other auxiliary supply rails.

The main HT rail is then used to drive the line output transformer (LOPT) via a transistor. Often additional supplies are derived from auxiliary secondary windings on the LOPT to feed more DC rails.

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Re: Strange issue with an AKF11 CRT

Post by AndyF » Sat Nov 17, 2018 9:33 am

Perhaps I was lucky ( ? ) but over the last 15 odd years I'd had 5 or 6 of these through my hands, usually kept and used them for a few months as they tended to do sterling service. =D>

The only issues I ever had was one was slightly out of focus (when placed side-by-side with another one) although at least 3 if not 4 of them suffered from broken power switches, would not 'latch on' which was annoying. I don't know if it was bad luck, previous owners being a bit too enthusiastic with the switch or just a cheapo switch fitted originally. I never did find a replacement I merely bridged it in so it was necessary to unplug its power lead to turn it off, never was a concern for me to do that.
Last edited by AndyF on Sat Nov 17, 2018 9:33 am, edited 1 time in total.
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Re: Strange issue with an AKF11 CRT

Post by DutchAcorn » Sat Nov 17, 2018 4:35 pm

Ok, I used a module that looks like the power supply from a similar monitor.
114E31AD-5443-4139-8D21-85D45DA1A6DE.jpeg
It worked, but the symptoms remained.

Then I saw there was some smoke coming from the top of the tube, from behind the plastic cup:
5FB4CCB4-9BC1-4CCD-9BEC-4AB77F9678AC.jpeg
So (after discharge) I removed it and used sand paper to improve the connection. I actually got the image back, but only for a minute or 2. Then it collapsed again and it has not come back...

There is crackling and (very light) smoke coming from behind plastic cup..
FAB3E004-6FD5-45C1-9050-B1D60DFD5D5D.jpeg
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Re: Strange issue with an AKF11 CRT

Post by 1024MAK » Sat Nov 17, 2018 4:47 pm

Only the metal inside of the “connector” should be conductive. All the surrounding area covered by the rubber cup should be clean. A breakdown of the insulation will lead to tracking across the surface to the metal coating on the outside of the rear of the tube.

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Re: Strange issue with an AKF11 CRT

Post by DutchAcorn » Sat Nov 17, 2018 9:12 pm

I cleaned the cup and tbe surroundings. It crackles less, but the image has not improved (test rom screen):
D86CF519-7447-484D-9F54-08B5D4F0AF5B.jpeg

There is a lighter vertical line and the colours are a bit off.
Last edited by DutchAcorn on Sat Nov 17, 2018 9:13 pm, edited 1 time in total.
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