Marconi VRC52A Picture Height Fault. - UK Vintage Radio Repair and Restoration Discussion Forum

DoctorWho · 6th Sep 2005, 4:29 pm

A while back I was working on my Marconi VRC52A combined radio and TV set.

I replaced all of the old wax and electrolytic capacitors and when the set was powered up both the radio and the TV set worked very well.

The radio is still fine, but after a little use the picture on the CRT started to lose height, it still has a very clear picture, but it is only about 1.5 - 2" high. The height control does alter things, but not very much.

As the picture was initially fine I suspect that something has started to fail after having been bought back in to service.

I replaced the two 0.03uf @ 7,000 V DC condensers which sit across the EHT and chassis, I also replaced the KT61 and the KT44 to see if they were faulty, and that made no difference. The only other valve I wondered about is V16 the U33 (which currently has a U22), but I do not yet have one of those to substitute.

Can anyone suggest what the fault is likely to be? Is it likely to perhaps be shorted winds on one of the transformers, or is something else likely to have developed a fault?

Thanks for any advice.

Peter.

Danny · 6th Sep 2005, 9:02 pm

Hi resistors tend to change value on these sets check the ones around the frame output and from the boost rail also the boost rail itself could be low.
i had low frame on my set if i remember i had to change the value of one or two resistors to get it right. i read in one of the practical television mags that these sets had several mods to the frame output but it was a couple of years ago and i cant remember which one it was.
Danny

DoctorWho · 6th Sep 2005, 10:22 pm

Thanks Danny. That was indeed the problem, a resistor which should have been 0.9 meg had risen to 15 meg!! Replacement bought back a perfect picture!!

One oddity, if the brightness or contrast are turned too far forward the pictire pulsates/flickers, however as soon as the brightness os lowered it stops. This isn't a problem because it's perfectly bright enough, but I was just curious as to what would cause this?

Thanks again for the help, the set is now running very well.

Peter.

Danny · 7th Sep 2005, 10:17 pm

Hi the pulsating picture is probrably the set drawing to much current from the eht supply i would try a new EHT rec as it could be losing emmision and going high resistnce under high current and then going low again until the current rises i would if no other faults are pressent at normal veiwing levels leave well alone!.

DoctorWho · 8th Sep 2005, 7:41 am

Thanks Danny, I have ordered one so hopefully the U33 will do the trick. As it currently has a U22 fitted is this likely to cause problems?

I'll let you know how it goes when I've fitted the U33.

Peter.

Danny · 8th Sep 2005, 10:34 pm

Hi yes it could be the U22 let us know when you get the new U33 good luck

DoctorWho · 9th Sep 2005, 8:08 am

Thanks Danny, I'll let you know what happens when the new U33 is fitted.

6th Sep 2005, 4:29 pm	#1
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Marconi VRC52A Picture Height Fault. A while back I was working on my Marconi VRC52A combined radio and TV set. I replaced all of the old wax and electrolytic capacitors and when the set was powered up both the radio and the TV set worked very well. The radio is still fine, but after a little use the picture on the CRT started to lose height, it still has a very clear picture, but it is only about 1.5 - 2" high. The height control does alter things, but not very much. As the picture was initially fine I suspect that something has started to fail after having been bought back in to service. I replaced the two 0.03uf @ 7,000 V DC condensers which sit across the EHT and chassis, I also replaced the KT61 and the KT44 to see if they were faulty, and that made no difference. The only other valve I wondered about is V16 the U33 (which currently has a U22), but I do not yet have one of those to substitute. Can anyone suggest what the fault is likely to be? Is it likely to perhaps be shorted winds on one of the transformers, or is something else likely to have developed a fault? Thanks for any advice. Peter.

6th Sep 2005, 9:02 pm	#2
Danny Heptode Join Date: Jul 2003 Location: North Walsham, Norfolk, UK. Posts: 516	Re: Marconi VRC52A Picture Height Fault. Hi resistors tend to change value on these sets check the ones around the frame output and from the boost rail also the boost rail itself could be low. i had low frame on my set if i remember i had to change the value of one or two resistors to get it right. i read in one of the practical television mags that these sets had several mods to the frame output but it was a couple of years ago and i cant remember which one it was. Danny

6th Sep 2005, 10:22 pm	#3
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi VRC52A Picture Height Fault. Thanks Danny. That was indeed the problem, a resistor which should have been 0.9 meg had risen to 15 meg!! Replacement bought back a perfect picture!! One oddity, if the brightness or contrast are turned too far forward the pictire pulsates/flickers, however as soon as the brightness os lowered it stops. This isn't a problem because it's perfectly bright enough, but I was just curious as to what would cause this? Thanks again for the help, the set is now running very well. Peter.

7th Sep 2005, 10:17 pm	#4
Danny Heptode Join Date: Jul 2003 Location: North Walsham, Norfolk, UK. Posts: 516	Re: Marconi VRC52A Picture Height Fault. Hi the pulsating picture is probrably the set drawing to much current from the eht supply i would try a new EHT rec as it could be losing emmision and going high resistnce under high current and then going low again until the current rises i would if no other faults are pressent at normal veiwing levels leave well alone!.

8th Sep 2005, 7:41 am	#5
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi VRC52A Picture Height Fault. Thanks Danny, I have ordered one so hopefully the U33 will do the trick. As it currently has a U22 fitted is this likely to cause problems? I'll let you know how it goes when I've fitted the U33. Peter.

8th Sep 2005, 10:34 pm	#6
Danny Heptode Join Date: Jul 2003 Location: North Walsham, Norfolk, UK. Posts: 516	Re: Marconi VRC52A Picture Height Fault. Hi yes it could be the U22 let us know when you get the new U33 good luck

9th Sep 2005, 8:08 am	#7
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi VRC52A Picture Height Fault. Thanks Danny, I'll let you know what happens when the new U33 is fitted.