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Old 3rd Jan 2008, 11:15 pm   #1
Hunts smoothing bomb
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Default Philips K70 - LOPTx?

Hi all,

I can't believe it! I think the inevitable has happened.
I was servicing my K70 in the shed tonight when I was setting up the tube cut off point, I unplugged the luminance lead from the CRT panel, turned off the green gun and then all of a sudden the most horrible screeching sound started coming from what sounded like the LOPT.

I turned off straight away but couldn't see anything untoward so I turned the green gun back on, plugged the luminance lead back into the CRT panel, swung the chassis out so I could see the LOPT valves and switched on, (keeping my hand on the mains switch) as soon as the valves warmed up the screeching came back, I also looked at the ammeter and the set was drawing just over 2 amps, it usually takes 1.5 This set has a funny LOPT stage in the fact it uses both a PL509 and also a PL504 somehow, when the LOPT is screeching the anode of the PL504 glows red

Looking inside the cage I could not see any marks on the LOPT or damage.
I know I shouldn't have done this but I defeated the interlock switch so I could run the set with the cage open and spent two seconds inspecting the area, There would have been no EHT though as I noticed the heater of the GY501 was not lighting and also there is no visible arcing or smoke.

Well, thats about it so I will take a back seat now and desperately await your advice.

Thanks
Lee

P.S. I have a spare Chassis and LOPT for this set but was going to see what you professionals say before undergoing nightmare surgery.
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Last edited by Hunts smoothing bomb; 3rd Jan 2008 at 11:29 pm.
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Old 4th Jan 2008, 1:11 am   #2
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Default Re: philips K70

Have you got the circuit?

It basically works in the same way as any other line timebase/output stage. The PL509 works in the same way, so we can act in the same way.

1: Measure the volts on the Control Grid. If it's at a -ve voltage, go on. If 0v, go back.
2: Try replacing the valves. All of them.
3: Capacitors etc.
4: LOPT.

If the LOPT has gone, it may be worth getting it rewound as a spare.

Don't run it for long like this, and please don't run it with the screening can disabled.

Cheers,

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Old 4th Jan 2008, 9:37 am   #3
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Default Re: philips K70

(2) is the most likely - take the GY out, run it and see what happens.

The reason for the PL509 and PL504 is to reduce the number of turns on the LOPTx by using the PL504 to pass some of the current. Later K70s had two 509s.

Apart from the dodgy print, one of my favourite hybrid CTV chassis - picture and sound are better than any of that period.

Field engineers didn't like them, as they did not understand the way-out circuitry!
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Old 4th Jan 2008, 4:55 pm   #4
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Default Re: philips K70

Hi all,

Thanks for your replies, I will have a whole day tomorrow to play with it so will have a look at what you have all suggested.
I must admit I have only worked on B&W valve sets and not really attempted work on CTV's before and it does look a little daunting to say the least.

Steve P, yes I have the original Philips service manual with schematic and full adjustments procedure, don't worry I will not run the set without the can in place anymore.

If it does come to a faulty lopty will I have to reset beam limiter, boost volts and other line associated supplies on replacement?

If I do is there a procedure that these steps should be carried out in to avoid any damage to other parts of the set?

Thanks again.

Lee
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Old 4th Jan 2008, 6:44 pm   #5
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Default Re: philips K70

All a colour set is is a B&W set with a decoder added on. Might be a simplistic view to some, but the saying 'Get it right on Black and White' holds true. Bigger line output and power stages I admit, but the principles are basically the same.

Look here: www.oldtellys.co.uk

The fact that the fault occured while you were playing with it points to a MMF. (Man Made Fault.). The advice in the posts above holds true.

It shouldn't need any resetting if it needs a LOPT change. As far as using it like this goes, not much. Take readings and switch off. Check print and wiring too. Like Mike says, the print can be dodgy.

Cheers,

Steve P.
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Old 4th Jan 2008, 7:12 pm   #6
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Default Re: philips K70

Hi.
Dont hastily jump to the LOPTX, although I never liked hybrid Philips CTVs the K70 had a good tranny.
If you remove the GY501 please make sure the top cap is well away from exposed metalwork and be careful!
Did the set go peak white or was any gun fully on? This may have put an excess load on the GY501 and caused the valve to fail also there may have been a flashover that possibly blew something in the feedback circuit causing the line frequency to drop causing the noise you hear.
Good luck.
Trevor
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Old 5th Jan 2008, 3:03 pm   #7
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Default Re: philips K70

Hi everyone,

A little tricky to get to the boost cap, disconneced one end and tested and it has a very small leak (makes the neon lamp just about glow on my tester. Is it suspect?

I haven't got a replacement to hand, is it safe to run the set without the boost cap connected or will the untamed spikes damage anything?

Thanks
Lee
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Old 5th Jan 2008, 3:42 pm   #8
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Default Re: Philips K70

Replace it before you switch on, Lee. Any leak at all is much too much!
The line output stage will not run at all without the boost cap, and there would almost certainly be some damage, probably to the valves.
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Old 5th Jan 2008, 3:57 pm   #9
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Default Re: philips K70

Thanks for the advice Mike, kinda guessed this so went out to maplin.

I couldn't believe it, they had one in stock the same value, 0.1uf @ 1250vDC.

Putting it all back together now!!

Fingers crossed.

Cheers
Lee
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Old 5th Jan 2008, 4:17 pm   #10
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Default Re: philips K70

I love this place, Full of brilliant sound advice.

The K70 lives on!!!
One new boost capacitor later and we're back in business!!

Thanks to everyone's suggestions in helping me find the problem with this set.
There are still some niggly little things with it, so I will set it all up according to the service manual and then I maybe back to pick all your brains!!

Cheers
Lee
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Old 5th Jan 2008, 4:33 pm   #11
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Default Re: philips K70

Wonderful - performance to enjoy! Come back with any problems.
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Old 6th Jan 2008, 7:01 pm   #12
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Default Re: philips K70

Hi all,

Posted piccies and a little write up in success stories: https://www.vintage-radio.net/forum/...ad.php?t=23961

Cheers
Lee
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Last edited by Mike Phelan; 28th Aug 2008 at 3:49 pm.
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