Philips 405U repair questions. - UK Vintage Radio Repair and Restoration Discussion Forum

DoctorWho · 15th Mar 2008, 9:58 pm

I have a Philips 485U set which is probably one of the first sets I bought and which has worked well, but I think the CRT is going low. The chassis was fully restored and the set worked well, but in recent months, after the set had been on for a while, the contrast would disappear and whilst you could increase the brightness and see the visible line structure, increasing the contrast just blacked the picture out. Reducing the contrast allowed you to see a faint outline of the picture with flyback lines, but no real contrast.

A while back you would get a fair picture to begin with, but it would soon disappear and go to the condition described above, however it now does this as soon as it is switched on, and it is impossible to get any decent watchable picture. There is plenty of brightness, but advancing the contrast control just blots the picture out, turning it back lets you see a very faint picture with extremely poor frame and line lock. I have previously checked the CRT voltages, and they have all been correct, and given that this has been getting worse over the last few months am I right to suspect the CRT?

The set is fitted with an MW31-18 CRT which does not have an ion trap. I have a spare MW31-16 which incorporates an ion trap, so am I right to presume that I can fit the MW31-16 and ion trap without the need for any modifications to the base connector or different heater supply?

I have been working from Trader service sheet 993. Any thoughts/advice here would be appreciated, and a confirmation that I can just swap these CRT's over without having to alter anything would be appreciated.

Thanks in advance for any advice.

Peter.

FERNSEH · 15th Mar 2008, 10:54 pm

Hi Peter,
Before you change the CRT it might be a good idea to check the UL46 video amplifier valve. You can use an UL41 for test purposes.
The valve V13 (UL46) is described as the vision detector, it operates in the same manner as in the TF384. The negative bias for the vision detector/amplifier is developed across a series resistor chain in the HT negative return to mains neutral.

DFWB.

FERNSEH · 16th Mar 2008, 12:59 am

Hi Peter,
The sound output valve V8 is an UL41, the frame output valve V21 is an UL46 or UL41. Try one of those valves in the video position.

DFWB.

15th Mar 2008, 9:58 pm	#1
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Philips 405U repair questions. I have a Philips 485U set which is probably one of the first sets I bought and which has worked well, but I think the CRT is going low. The chassis was fully restored and the set worked well, but in recent months, after the set had been on for a while, the contrast would disappear and whilst you could increase the brightness and see the visible line structure, increasing the contrast just blacked the picture out. Reducing the contrast allowed you to see a faint outline of the picture with flyback lines, but no real contrast. A while back you would get a fair picture to begin with, but it would soon disappear and go to the condition described above, however it now does this as soon as it is switched on, and it is impossible to get any decent watchable picture. There is plenty of brightness, but advancing the contrast control just blots the picture out, turning it back lets you see a very faint picture with extremely poor frame and line lock. I have previously checked the CRT voltages, and they have all been correct, and given that this has been getting worse over the last few months am I right to suspect the CRT? The set is fitted with an MW31-18 CRT which does not have an ion trap. I have a spare MW31-16 which incorporates an ion trap, so am I right to presume that I can fit the MW31-16 and ion trap without the need for any modifications to the base connector or different heater supply? I have been working from Trader service sheet 993. Any thoughts/advice here would be appreciated, and a confirmation that I can just swap these CRT's over without having to alter anything would be appreciated. Thanks in advance for any advice. Peter.

15th Mar 2008, 10:54 pm	#2
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Philips 405U of repair questions. Hi Peter, Before you change the CRT it might be a good idea to check the UL46 video amplifier valve. You can use an UL41 for test purposes. The valve V13 (UL46) is described as the vision detector, it operates in the same manner as in the TF384. The negative bias for the vision detector/amplifier is developed across a series resistor chain in the HT negative return to mains neutral. DFWB.

16th Mar 2008, 12:59 am	#3
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Philips 405U of repair questions. Hi Peter, The sound output valve V8 is an UL41, the frame output valve V21 is an UL46 or UL41. Try one of those valves in the video position. DFWB.