Picture Flicker/Pulsating - 1940's Marconi - what would cause this? - UK Vintage Radio Repair and Restoration Discussion Forum

DoctorWho · 7th Sep 2005, 1:26 pm

I'm just curious about a rather unusual thing which happens on oen of my TV sets. It's not really a serious fault, I'm more curious as to the reason for it happening.

The set concerned is a 1940's Marconi, it works well and displays a good picture, however if the brightness or contrast are advanced too far the picture flickers or pulsates, as soon as the brightness or contrast are lowered the problem goes away.

Any ideas what is likely to cause this and how it can be cured? It's not a major problem as I just keep the brightness down, but I would like to know why it happens and how it can be cured.

Peter.

DoctorWho · 7th Sep 2005, 5:47 pm

I've already replaced all of the capacitors, I did wonder if the EHT rectifier valve would be worth replacing though? I don't have an oscilloscope I'm afraid, but I'll have a check of the voltages reaching the tube. I'm not sure I have any way of checking the EHT voltage though, how is this usually checked?

Peter.

Heatercathodeshort · 7th Sep 2005, 7:25 pm

Hi Peter,
If the focus and picture size is ok at low brilliance but tends to 'flick' in and out of focus possibly with an increase in picture size your EHT is breaking down under high beam current. If the set you have the problem with is mains derived EHT [Your VC52DA?] it may be due to a failing EHT rectifer possibly a U33.
Another possibility is a part disconnected final anode connection INSIDE the CRT. This is often only a thin layer of conductive paint and can break down with rectifier failure. Often a series resisitor is wired in series with the final anode lead to prevent this under overload conditions. Check this also. TAKE GREAT CARE WITH THE EHT CIRCUIT. It won't take any prisoners if you come into contact with it. If the CRT is faulty you can often hear a slight 'popping' sound from around the area of the CRT's EHT connector with the brightness turned up.
If the set is of the 1807 [PLEASE NO!] series with flyback eht the rectifier in this case is a U35. As the current supplied is VERY small with this model there can be other reasons for the symptoms described....Many reasons.! Worried for your safety....JOHN.

DoctorWho · 7th Sep 2005, 9:29 pm

Hello John,

Thanks for your input, I greatly appreciate your advice. The focus and picture size is indeed fine.

It is indeed the Marconi which does use mains derived EHT, interestingly the service data says that the EHT rectifier should be a U33, but it has a U22 fitted, I've ordered a replacement U33 so hopefuly thay may do the trick.

I'll have a look and see if it has a series resistor fitted, if not is this a modification I could make myself to the circuitry to add one? There are some resistors wired around the area of the two 0.02 EHT condensers, do these perhaps perform this function?

Thanks also for the timely reminder about the mains derived EHT, I am treating that with great respect and have no intention of putting my hands inside the set whilst switched on, however you can't be too careful with this sort of thing.

If I can eliminate the EHT circuitry and it turns out to be the CRT then I suspect that may be a difficult one to replace. Any ideas where I can find a suitable 10" CRT? I haven't heard any popping sounds from the CRT.

Thanks again for your help. I'll let you know how things progress once the U33 has been replaced.

Peter.

DoctorWho · 10th Sep 2005, 11:33 am

Well a new U33 has solved the problem!!

Thanks for the help.

Peter.

oldeurope · 10th Sep 2005, 4:55 pm

Hi, the U22 needs more heater current. Maybe the heater voltage breaks
down with the U22. Did you change the sockets too? They don't look
the same.

Would like to see a few pics of your work. The sets you have are definitely
interesting to see.

Kind regards
Darius

DoctorWho · 10th Sep 2005, 5:06 pm

Hi Darius,

Well the U33 certainly seems to have solved the problem. Thankfully I didn't have to change the valve holder, both have a standard 4-pin base and so, as far as the valve holder is concerned, they are interchangable.

Next time I get the sets apart I'll certainly take some more pictures of the insides, for now though I can attach a picture of the outside of each set to the appropriate thread. Here's the Marconi:

7th Sep 2005, 1:26 pm	#1
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Picture Flicker/Pulsating - 1940's Marconi - what would cause this? I'm just curious about a rather unusual thing which happens on oen of my TV sets. It's not really a serious fault, I'm more curious as to the reason for it happening. The set concerned is a 1940's Marconi, it works well and displays a good picture, however if the brightness or contrast are advanced too far the picture flickers or pulsates, as soon as the brightness or contrast are lowered the problem goes away. Any ideas what is likely to cause this and how it can be cured? It's not a major problem as I just keep the brightness down, but I would like to know why it happens and how it can be cured. Peter.

7th Sep 2005, 5:47 pm	#2
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Picture Flicker/Pulsating - What would cause this? I've already replaced all of the capacitors, I did wonder if the EHT rectifier valve would be worth replacing though? I don't have an oscilloscope I'm afraid, but I'll have a check of the voltages reaching the tube. I'm not sure I have any way of checking the EHT voltage though, how is this usually checked? Peter. Last edited by Mike Phelan; 18th Jun 2007 at 11:51 am. Reason: Tidy-up for archive

7th Sep 2005, 7:25 pm	#3
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Picture Flicker/Pulsating - What would cause this? Hi Peter, If the focus and picture size is ok at low brilliance but tends to 'flick' in and out of focus possibly with an increase in picture size your EHT is breaking down under high beam current. If the set you have the problem with is mains derived EHT [Your VC52DA?] it may be due to a failing EHT rectifer possibly a U33. Another possibility is a part disconnected final anode connection INSIDE the CRT. This is often only a thin layer of conductive paint and can break down with rectifier failure. Often a series resisitor is wired in series with the final anode lead to prevent this under overload conditions. Check this also. TAKE GREAT CARE WITH THE EHT CIRCUIT. It won't take any prisoners if you come into contact with it. If the CRT is faulty you can often hear a slight 'popping' sound from around the area of the CRT's EHT connector with the brightness turned up. If the set is of the 1807 [PLEASE NO!] series with flyback eht the rectifier in this case is a U35. As the current supplied is VERY small with this model there can be other reasons for the symptoms described....Many reasons.! Worried for your safety....JOHN.

7th Sep 2005, 9:29 pm	#4
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Picture Flicker/Pulsating - What would cause this? Hello John, Thanks for your input, I greatly appreciate your advice. The focus and picture size is indeed fine. It is indeed the Marconi which does use mains derived EHT, interestingly the service data says that the EHT rectifier should be a U33, but it has a U22 fitted, I've ordered a replacement U33 so hopefuly thay may do the trick. I'll have a look and see if it has a series resistor fitted, if not is this a modification I could make myself to the circuitry to add one? There are some resistors wired around the area of the two 0.02 EHT condensers, do these perhaps perform this function? Thanks also for the timely reminder about the mains derived EHT, I am treating that with great respect and have no intention of putting my hands inside the set whilst switched on, however you can't be too careful with this sort of thing. If I can eliminate the EHT circuitry and it turns out to be the CRT then I suspect that may be a difficult one to replace. Any ideas where I can find a suitable 10" CRT? I haven't heard any popping sounds from the CRT. Thanks again for your help. I'll let you know how things progress once the U33 has been replaced. Peter.

10th Sep 2005, 11:33 am	#5
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Picture Flicker/Pulsating - What would cause this? Well a new U33 has solved the problem!! Thanks for the help. Peter.

10th Sep 2005, 4:55 pm	#6
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Re: Picture Flicker/Pulsating - What would cause this? Hi, the U22 needs more heater current. Maybe the heater voltage breaks down with the U22. Did you change the sockets too? They don't look the same. Would like to see a few pics of your work. The sets you have are definitely interesting to see. Kind regards Darius Last edited by Mike Phelan; 18th Jun 2007 at 11:52 am. Reason: Tidy-up for archive

10th Sep 2005, 5:06 pm	#7
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Picture Flicker/Pulsating - What would cause this? Hi Darius, Well the U33 certainly seems to have solved the problem. Thankfully I didn't have to change the valve holder, both have a standard 4-pin base and so, as far as the valve holder is concerned, they are interchangable. Next time I get the sets apart I'll certainly take some more pictures of the insides, for now though I can attach a picture of the outside of each set to the appropriate thread. Here's the Marconi: Attached Thumbnails