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Old 18th Mar 2007, 11:55 pm   #41
FERNSEH
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Default Re: Philips TV set for 441 lines

The vision IF stages and the video amplifier are now working and a video waveform is apparent at the cathode of the CRT.
The UB41 double diode serves as the vision and sound interference limiters.

The vision interference limiter is causing problems, it is clipping almost all of the video waveform and leaving only the sync pulses.
Nevertheless, some vision modulation can be seen on the screen.

The R10 valve does operate as a damper diode but the higher heater current does rob the line output stage of some the available power. It will have to be a silicon diode until a EA40 is found.

DFWB.
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Old 26th Mar 2007, 1:01 pm   #42
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Default Re: Philips TV set for 441 lines

At last I have a good picture. The 250kohm brightness control has been replaced with a temporary component and although this did effect a slight improvement the picture remained poor quality.
My attention returned to the negative bias supply. It was found that the bias to the video detector/amplifier had diminished to only a few hundred millivolts, it should be -2.5 volts.
The problem was solved when I discovered a resistor in the bias supply was connected incorrectly, reconnecting the resistor to the correct position has restored the bias supply to the video detector.

The brightness control is a type unique to Philips.
I think that similar controls were fitted in many British post-war TV sets.

The attachments show the brightness control and the greatly improved pictures. The frame timebase still needs sorting out. That will be this weeks project.

DFWB.
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Old 27th Apr 2007, 3:50 pm   #43
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Default Re: Philips TV set for 441 lines

I'm still having problems with the frame timebase. It is the only obstacle that is holding up delivery to the set's owner in France.

The pictures from the previous post show that the on screen picture has much reduced height and compression at the bottom.

The frame timebase employs the triode section of an UCH21 triode-hexode as the blocking oscillator and the output valve is a triode connected UL41 pentode.
The sawtooth waveform at the control grid of the UL41 is 25 volts P to P. which I think is sufficient for this receiver.

As the UL41 is connected as a triode the valve has a Ra of 3.5Kohms and the max PA is only 1.55 watts and these figures are very similar to the Mazda 6L18 triode.

So it leaves only two components that are the possible suspects, the frame output transformer or the scanning coils. These components used in the TF384 are similar to those employed in the UK 405 line model Philips 385U and the 441 line TX390/29.

DFWB.
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Old 13th May 2007, 1:16 am   #44
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Default Re: Philips TV set for 441 lines

The good news is that a member of the Forum has kindly lent me his Philips 385U TV set. This evening I carried out some preliminary tests before I connected the set to the mains,. Just as well I did as I discovered one the decoupling capacitors in one of the three heater chains was short circuit and it is likely that many other heater supply capacitors will require replacement.

The 0.001 microfarad heater decoupling capacitors were made by TCC.

I will make the restoration of this set the subject of a a separate topic. I have to determine if the frame timebase works OK in the 385U. Then I will remove the scanning coils and fit them into the TF384.

DFWB.
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Old 27th May 2007, 1:35 am   #45
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Default Re: Philips TV set for 441 lines

Various frame output transformers have been tried in this receiver but there has been no improvement to the frame scan.

Today, the scanning coils were removed from the Philips 385U and installed into the TF384. The results? just the same as before.

The UL41 frame output valve is triode connected which results in the valve Ra being only 3.5Kohms. I will rewire the valve base for pentode operation. I know that the Ra will be a much higher figure but I think it is an action worth trying.
All the other parts of the set are working fine, the picture is bright, the timbase oscillators are stable and the sound section works well on TV and radio. Those reformed HT electrolytic capacitors are holding up well.

DFWB.
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Old 29th May 2007, 10:20 pm   #46
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Default Re: Philips TV set for 441 lines

Hi David.
Theory says the problem lies with the bottom end of the output valve, no doubt you have changed the obvious
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Old 29th May 2007, 10:49 pm   #47
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Default Re: Philips TV set for 441 lines

Hi Trevor,
I've tried many UL41 valves in the set including a NOS UL46.
Connecting the valve for pentode operation did increase the height but the cramping at the bottom remained, Also dfferent frame output transformers have been tried including one from the Philips style 70.

Actually after replacing the scanning coils it does seem after all there is a slight improvement to the frame linearity.

The cathode bias resistor is OK, reading 620 ohms and a negative bias of 3 volts is supplied to the height control, in fact the valve could be considered over biased and this should result in the picture being cramped at the top of the picture.

DFWB
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Old 12th Jun 2007, 12:31 am   #48
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Default Re: Philips TV set for 441 lines

Hi There
I am not familiar with this set but does the frame output valve
have a decoupling capacitor across the cathode bias resistor.
usualy an electrolytic of about 50uF, If so change it.

Rod
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Old 12th Jun 2007, 1:21 am   #49
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Default Re: Philips TV set for 441 lines

Hi Rod,
The UL41 cathode bypass capacitor was replaced early in the restoration of the set. The original Philips capacitor had indeed gone low capacity and in order to maintain originality the capacitor innards were removed and a 100 microfarad 125 C cap fitted inside the old capacitor outer tube.

I now have sufficient height after increasing the sawtooth drive to the frame output valve.

Only a few jobs need to be done to set to complete the restoration and then it will be returned to it's owner in france.

DFWB.
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