Marconi VRC52A combined radio - brightness increasing - UK Vintage Radio Repair and Restoration Discussion Forum

DoctorWho · 23rd Oct 2005, 10:54 am

I wonder if I could please ask for some further advice please regarding my Marconi VRC52A combined radio and TV set?

I have noticed a fault with the set as it gets warmer. When initially switched on the set displays a very good picture, when it's been on for a few minutes the contrast has to be turned down, but once it's been on for 20 minutes or so the screen is very bright and you can see flyback lines, reducing the contrast and brightness even to minimum does not eliminate it, the picture itself starts to become "wobbly" for want of a better term, increasing the contrast brings back a stable picture, but the picture then pulsates or flickers from a perfect picture to one with very high brightness/contrast. Increasing the brightness makes it pulsate/flicker even faster.

Can anyone suggest what is likely to cause this? Is it likely to be the CRT? If the set is left for an hour or so and then powered up again the picture is perfect for up to 20 minutes, but the same thing happens again when it's been on for a while.

I'd be very grateful for any input here or advice as to what this may be? If it is likely to be the CRT can anyone suggest where I may find a suitable replacement, it's an Emiscope TA/10 10" Tetrode.

Peter.

Panrock · 23rd Oct 2005, 2:52 pm

Quote:

Originally Posted by peter_scott

I suspect it's nothing to do with your CRT. More likely something around the area of the sync separator and video amp.

Quite, and I'd poke around looking for any overheating resistors, distressed-looking capacitors, and particularly decoupling electrolytics, in that area.

Steve

DoctorWho · 23rd Oct 2005, 9:46 pm

Thanks for the help here. From your advice I suspect I should be looking around the Z66 (V10) and Z77 (V7 & V8) and D77 (V11). Is this sort of fault likely to be caused by a valve fault, or a resistor? I have already replaced all of the capacitors in the set, so hopefully they should all be alright. I'll order in a few valves, check out the resistors around these valves and see what I can find. I'll let you know how it goes. Thanks again for the help, I am VERY relieved to hopefully have a sound CRT. Which is the video amp valve in this set, is it V7 & V8?

Peter.

DoctorWho · 24th Oct 2005, 8:44 am

Hello Peter,

Many thanks for the help. I think this set is the same as the HMV1805 & HMV1806, the only difference being the radio chassis (so far as I can see). The valves are pretty much the same as those you quote, except for V10, which is a Z66. I'll check out the resistors associated with these areas of the circuitry and let you know what I find.

I do now actually have the correct service data for this set, it took some tracking down. I csn scan it in but I do not have the software to convert it to a PDF file for Adobe Acrobat Reader. If anyone wants a copy of the data to make it available for other members then please let me know and I'll scan in all of it and E-mail it across so that everyone can benefit from it. It's particularly useful with the radio chassis as well as this is very different from the HMV version.

Thanks again for the help. I'll get to work with the test meter and report back.

Peter.

Heatercathodeshort · 24th Oct 2005, 7:52 pm

Hello Peter,
Now then, if we think about the time delay before the fault occurs, we have a clue. A valve will warm up to its nomal temperature after about five minutes use. Any intermittent fault in the valve will usually show up within this time. The rest of the chassis will take a lot longer to 'warm up' and this is why I feel that the fault is caused by a capacitor or maybe a resistor. I have had a look at the circuit and you may find that C27 an 8uf electrolitic, connecting the video output from the anode of V6 Z66 to be suspect.
R34 the anode load of the video output valve is 4.7K [pin 3 V6] may be going high with increasing heat. Thats a start anyway.
I managed to pick up a copy of 'TELEVISION RECEIVING EQUIPMENT'
for a few pence at COWBIT. I will put it in the post tomorrow. It will cover the era of sets you are working on as it was published in 1947. Consider it an early Christmas gift! I don't want you to blow yourself up....... Let us know what you discover. Regards. JOHN.

DoctorWho · 24th Oct 2005, 8:33 pm

Hello John,

That is extremely kind of you, I don't know what to say. I'll just have to find you something that i can send you in return, thank you very much indeed.

With regard to what you have said about C27, I have replaced it with a brand new capacitor from Maplins, however the new capacitor is rated at 450 vols DC whereas the original was rated at 2,000 V DC, similarly the service data says that this capacitor should be rated at 2,000 V DC. Is this likely to be the cause of the problem? My only problem here is that, if this be the case, where ever do I find an 8uf electrolytic rated at 2,000 V? Any ideas on this greatly appreciated.

I did check R34 as advised and it has indeed gone high and so this has now been replaced. I took the opportunity to check all of the surrounding resistors in the RF unit and also the power chassis, I found a number that had gone high.

From what you've said I really do wonder about C27, would you suggest that I obtained a replacement for this? If so can you suggest anywhere that manufactures capacitors of this voltage range?

Thanks again for all of your help.

Peter.

Heatercathodeshort · 24th Oct 2005, 8:43 pm

Hello again Peter,
My service manual shows this as a 200volt component and I can see no reason for it to be 2000v. You may have a miss print. You are very welcome to the book. Just a Christmas card will be fine! Check the resistor values around the output valve [V6] and let us know. Regards JOHN.

DoctorWho · 24th Oct 2005, 8:57 pm

Hello John,

Thanks for the clarification. I just checked the service manual again and it definitely says 2,000 V. I did think that was rather high for a TV set, but I presumed they had put it there for a reason, but it sounds as though it's a mis-print from what you've said.

One other thing I did notice. In the RF unit there are a number of 0.001uf capacitors which are small flat brown units and I presumed that these were mica? I left these alone as it's generally commented on that mica capacitors are alright, however in other sets where these have been wax I have replaced them. Should I replace these?

Thanks again for the book, that's really kind of you and very much appreciated.

Peter.

oldeurope · 25th Oct 2005, 9:19 am

Quote:

Originally Posted by Heatercathodeshort

Hello Peter,
Now then, if we think about the time delay before the fault occurs, we have a clue. A valve will warm up to its nomal temperature after about five minutes use. Any intermittent fault in the valve will usually show up within this time. Regards. JOHN.

Good morning,
I don't agree with you. Isulation problems in valves take much longer
to come up. Look at EL/UL41 for example, the g1 gets positive after
20 minutes or longer!

Kind regards
Darius

peter_scott · 25th Oct 2005, 12:50 pm

Quote:

Originally Posted by DoctorWho

The only thing I am now not sure about is the 0.001uf capacitors in the RF unit, there are a lot of them, small flat units, they look very much like mica units. Should these be left or are they better replaced?

Peter.

Hi Peter,

I'd leave them well alone and do as Darius suggests, namely look for the voltages that change from normal working to fault condition. The fault is not at RF. In general it's not good to replace components unnecessarily because you loose as lot of the original character of the set. I know this doesn't matter if you only look at the outside but given that these sets are in the enthusiast's domain you will definitely reduce their value.

HTH

Peter.

Heatercathodeshort · 25th Oct 2005, 7:41 pm

Quote:

Originally Posted by Darius

Good morning,
I don't agree with you. Isulation problems in valves take much longer
to come up. Look at EL/UL41 for example, the g1 gets positive after
20 minutes or longer!

Kind regards
Darius

Hello Darius,
I completely agree about your comments with the EL41/UL41 and some other high dissipation output valves but the octal types used in the VC52DA do not suffer from this problem as a general rule. My remark was purely from service experience of the types used in 50's/60's television receivers.
Lets hope with a bit more information we can sort the problems out with Peters set. I feel its in the video stages and like you would recommend he leaves the R.F. stages alone and concentrates on the CRT voltages and the operation of the video output stage.
The set is a real monster and very heavy. The chassis is like a battleship and poor Peter must have a real struggle on his hands. Circuit attached.
Kind regards and well done with your TS1105 and your standards converter. JOHN.

Heatercathodeshort · 25th Oct 2005, 8:13 pm

Hello Peter,
Just jumped out of the bath after a day driving a bulk loader.....Whilst soaking I had a thought. When you had your sync fault I advised that you changed over the Z66's as a test. Did you change them back again? Sync seperator valves tend to work o.k. even when practically useless as they have a very simple job to do and are under run as a rule. The video amplifier is the complete opposite and if a poor Z66 was in this position it would show up faults that would not be apparant when the same valve was used as a sync sep. Just a thought but these faults generally have a simple answer. Redards. JOHN.

DoctorWho · 25th Oct 2005, 8:49 pm

Hello John,

You're right about this set being built like a battleship, of all of the sets in this house, both radios and TV's, and even a manual telephone switchboard I think the Marconi VRC52A is the heaviest of the lot!! What makes it even more amazing is that a friend originally found this for me at a car boot sale and carried it from his car to the garage all by himself, I've never worked out how he did it!!

With regard to the Z66's, no I haven't swapped them back, in fact it had slipped my mind that I swapped them over, thanks for reminding me about that. I'll see if that makes a difference.

I shall certainly leave the RF unit alone, it all seems to be working perfectly, I replaced the few resistors which had gone high, there was one small capacitor which did look sorry for itself, a 47 mmfd (I presume that's 4700 pf) which I replaced, but all of the others remain original.

I'll let you know how things progress.

Thanks again,

Peter.

DoctorWho · 27th Oct 2005, 4:24 pm

I've found the fault on the Marconi, I don't think anyone will believe what it was! I did as advised and took voltage readings before the fault developed, then I took them again when it had started showing the first signs, however the moment I took the voltage reading the problem vanished. It seems that the problem was a badly connecting CRT socket! There should be a spring either side, and one was missing, so this wouldn't have helped, but it's now been cleaned up and a new spring fitted and it's fine. You were right about it being simple, so simple it was staring me in the face.

Thanks again to everyone who helped with this.

Peter.

23rd Oct 2005, 10:54 am	#1
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Marconi VRC52A combined radio - brightness increasing I wonder if I could please ask for some further advice please regarding my Marconi VRC52A combined radio and TV set? I have noticed a fault with the set as it gets warmer. When initially switched on the set displays a very good picture, when it's been on for a few minutes the contrast has to be turned down, but once it's been on for 20 minutes or so the screen is very bright and you can see flyback lines, reducing the contrast and brightness even to minimum does not eliminate it, the picture itself starts to become "wobbly" for want of a better term, increasing the contrast brings back a stable picture, but the picture then pulsates or flickers from a perfect picture to one with very high brightness/contrast. Increasing the brightness makes it pulsate/flicker even faster. Can anyone suggest what is likely to cause this? Is it likely to be the CRT? If the set is left for an hour or so and then powered up again the picture is perfect for up to 20 minutes, but the same thing happens again when it's been on for a while. I'd be very grateful for any input here or advice as to what this may be? If it is likely to be the CRT can anyone suggest where I may find a suitable replacement, it's an Emiscope TA/10 10" Tetrode. Peter.

23rd Oct 2005, 9:46 pm	#3
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Is this likely to be a CRT fault, or something else? Thanks for the help here. From your advice I suspect I should be looking around the Z66 (V10) and Z77 (V7 & V8) and D77 (V11). Is this sort of fault likely to be caused by a valve fault, or a resistor? I have already replaced all of the capacitors in the set, so hopefully they should all be alright. I'll order in a few valves, check out the resistors around these valves and see what I can find. I'll let you know how it goes. Thanks again for the help, I am VERY relieved to hopefully have a sound CRT. Which is the video amp valve in this set, is it V7 & V8? Peter.

24th Oct 2005, 8:44 am	#4
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Is this likely to be a CRT fault, or something else? Hello Peter, Many thanks for the help. I think this set is the same as the HMV1805 & HMV1806, the only difference being the radio chassis (so far as I can see). The valves are pretty much the same as those you quote, except for V10, which is a Z66. I'll check out the resistors associated with these areas of the circuitry and let you know what I find. I do now actually have the correct service data for this set, it took some tracking down. I csn scan it in but I do not have the software to convert it to a PDF file for Adobe Acrobat Reader. If anyone wants a copy of the data to make it available for other members then please let me know and I'll scan in all of it and E-mail it across so that everyone can benefit from it. It's particularly useful with the radio chassis as well as this is very different from the HMV version. Thanks again for the help. I'll get to work with the test meter and report back. Peter.

24th Oct 2005, 7:52 pm	#5
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Is this likely to be a CRT fault, or something else? Hello Peter, Now then, if we think about the time delay before the fault occurs, we have a clue. A valve will warm up to its nomal temperature after about five minutes use. Any intermittent fault in the valve will usually show up within this time. The rest of the chassis will take a lot longer to 'warm up' and this is why I feel that the fault is caused by a capacitor or maybe a resistor. I have had a look at the circuit and you may find that C27 an 8uf electrolitic, connecting the video output from the anode of V6 Z66 to be suspect. R34 the anode load of the video output valve is 4.7K [pin 3 V6] may be going high with increasing heat. Thats a start anyway. I managed to pick up a copy of 'TELEVISION RECEIVING EQUIPMENT' for a few pence at COWBIT. I will put it in the post tomorrow. It will cover the era of sets you are working on as it was published in 1947. Consider it an early Christmas gift! I don't want you to blow yourself up....... Let us know what you discover. Regards. JOHN. Last edited by Heatercathodeshort; 24th Oct 2005 at 7:55 pm.

24th Oct 2005, 8:33 pm	#6
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Is this likely to be a CRT fault, or something else? Hello John, That is extremely kind of you, I don't know what to say. I'll just have to find you something that i can send you in return, thank you very much indeed. With regard to what you have said about C27, I have replaced it with a brand new capacitor from Maplins, however the new capacitor is rated at 450 vols DC whereas the original was rated at 2,000 V DC, similarly the service data says that this capacitor should be rated at 2,000 V DC. Is this likely to be the cause of the problem? My only problem here is that, if this be the case, where ever do I find an 8uf electrolytic rated at 2,000 V? Any ideas on this greatly appreciated. I did check R34 as advised and it has indeed gone high and so this has now been replaced. I took the opportunity to check all of the surrounding resistors in the RF unit and also the power chassis, I found a number that had gone high. From what you've said I really do wonder about C27, would you suggest that I obtained a replacement for this? If so can you suggest anywhere that manufactures capacitors of this voltage range? Thanks again for all of your help. Peter.

24th Oct 2005, 8:43 pm	#7
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Is this likely to be a CRT fault, or something else? Hello again Peter, My service manual shows this as a 200volt component and I can see no reason for it to be 2000v. You may have a miss print. You are very welcome to the book. Just a Christmas card will be fine! Check the resistor values around the output valve [V6] and let us know. Regards JOHN.

24th Oct 2005, 8:57 pm	#8
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Is this likely to be a CRT fault, or something else? Hello John, Thanks for the clarification. I just checked the service manual again and it definitely says 2,000 V. I did think that was rather high for a TV set, but I presumed they had put it there for a reason, but it sounds as though it's a mis-print from what you've said. One other thing I did notice. In the RF unit there are a number of 0.001uf capacitors which are small flat brown units and I presumed that these were mica? I left these alone as it's generally commented on that mica capacitors are alright, however in other sets where these have been wax I have replaced them. Should I replace these? Thanks again for the book, that's really kind of you and very much appreciated. Peter.

25th Oct 2005, 8:13 pm	#12
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Is this likely to be a CRT fault, or something else? Hello Peter, Just jumped out of the bath after a day driving a bulk loader.....Whilst soaking I had a thought. When you had your sync fault I advised that you changed over the Z66's as a test. Did you change them back again? Sync seperator valves tend to work o.k. even when practically useless as they have a very simple job to do and are under run as a rule. The video amplifier is the complete opposite and if a poor Z66 was in this position it would show up faults that would not be apparant when the same valve was used as a sync sep. Just a thought but these faults generally have a simple answer. Redards. JOHN.

25th Oct 2005, 8:49 pm	#13
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Is this likely to be a CRT fault, or something else? Hello John, You're right about this set being built like a battleship, of all of the sets in this house, both radios and TV's, and even a manual telephone switchboard I think the Marconi VRC52A is the heaviest of the lot!! What makes it even more amazing is that a friend originally found this for me at a car boot sale and carried it from his car to the garage all by himself, I've never worked out how he did it!! With regard to the Z66's, no I haven't swapped them back, in fact it had slipped my mind that I swapped them over, thanks for reminding me about that. I'll see if that makes a difference. I shall certainly leave the RF unit alone, it all seems to be working perfectly, I replaced the few resistors which had gone high, there was one small capacitor which did look sorry for itself, a 47 mmfd (I presume that's 4700 pf) which I replaced, but all of the others remain original. I'll let you know how things progress. Thanks again, Peter.

27th Oct 2005, 4:24 pm	#14
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Is this likely to be a CRT fault, or something else? I've found the fault on the Marconi, I don't think anyone will believe what it was! I did as advised and took voltage readings before the fault developed, then I took them again when it had started showing the first signs, however the moment I took the voltage reading the problem vanished. It seems that the problem was a badly connecting CRT socket! There should be a spring either side, and one was missing, so this wouldn't have helped, but it's now been cleaned up and a new spring fitted and it's fine. You were right about it being simple, so simple it was staring me in the face. Thanks again to everyone who helped with this. Peter.