Marconi 703 Mastergram Television Help Request. - UK Vintage Radio Repair and Restoration Discussion Forum

DoctorWho · 22nd Apr 2007, 11:02 am

Well the Marconi 703 Mastergram has been working beautifully, but I did notice that one of the old wax condensers was not happy, it had melted a bit and had beads of wax hanging from it.

I decided that, to ensure that nothing was damaged, that I would replace the few wax condensers on the Sync chassis.

I very carefully removed the chassis and replaced the wax condensers and the electrolytics which had not been replaced.

V6 (MS4B) was removed to allow access to one of them, and was then carefully replaced. I had labelled up all of the wires and so I made exspecially sure that they all went back in their rightful places.

Before doing this the picture was perfect, now, sadly, I have a vertical line from the top to the bottom of the tube.

I'm now not sure what to look for and where the problem is likely to be. Any advice here would be greatly appreciated. I wished I had left the original condensers in place, but I was a bit worried when I saw one of the old wax ones looking unhappy. I'd like to get the set up and running properly again, s any help would be very welcome.

Thanks in advance.

All the best,

Peter.

DoctorWho · 22nd Apr 2007, 11:37 am

Just to clarify, I replaced C12, 10, 22, 21, 16, 2, 14, 20, 23 and 25. I've attached the appropriate part of the pictorial diagram.

Obviously the scanning coils are working in one direction, but not in the other, as they are presumably scanning from top to bottom, but not from side to side.

I've double checked that all leads have gone back in the right places, and they have, similarly all valves are tight in their sockets and top caps are connected.

I've no idea what to look for, as I did the work so carefully, and it's not so easy to understand the circuit diagram as they are not written out in the easier format used on later sets.

I'd certainly be grateful of any suggestions as to what to look for, or to try, or to point out what I've done wrong.

Thanks in advance.

Peter.

Panrock · 22nd Apr 2007, 11:59 am

Hi Peter,

You did the right thing in replacing those waxies in the timebase. Even when they're functioning so-so, they can give below-par performance on the scans.

You've lost the horizontal scan or 'high synch' as it's called here. If you're absolutely certain there no wire, or connection to a wire, in bad condition, then (with the set disconnected from the mains) I suggest you go through the whole of the line scan circuit with an ohmeter, checking if the readings seem logical. Particularly pay attention to any wound components.

If all seems well, then check the voltages are where they should be on the valve pins.

Don't worry, this is a pretty simple circuit and you'll soon get to the bottom of it...

Steve

DoctorWho · 22nd Apr 2007, 12:22 pm

Thanks Steve, I have to admit that I was nervous doing this one, as it's more complex that other sets I've worked on. I'm glad that you feel I did the right thing in replacing the waxies, that makes me feel better for tackling it

My worry was that something, such as a transformer, was going to be damaged if one of them shorted out.

I'll take the Sync chasis out again and go through it on the table as you suggest, with an ohm meter. I did approach this whole job extremely carefully, making extra sure that I was replacing like for like, and in the right place. The replacement electrolytics (such as 50uf @ 25 vols etc) are much smaller than the originals, but I made sure that the polarity on each one was correct.

I'll give it a good check over and then let you know later on what I discover. Out of interest which valve would you suggest I take readings from? If I can't find a fault visually, and then with a test meter, I'll try and see if I can work out a way of running the chassis out of the cabinet, as it's pretty impossible to get at whilst in the cabinet.

Are there any of the condensers that I have replaced which you would initially suspect? if so, I'll check that part of the circuit first.

Thanks again for your help, I felt really sad that I'd done this job, but now I at least feel better about that.

All the best,

Peter.

Heatercathodeshort · 22nd Apr 2007, 7:58 pm

Hello Peter,
From what you say I think you have made the classic mistake of changing all the capacitors in one go, without testing the set between each component change.
I know its time consuming but it's the only way to stop human error in its tracks before you get in too deep. My guess is that you have put a wire on the wrong tag.

As Steve says it is advisable to replace wax capacitors in the timebases as leaks here will cause, at the least, linearity problems.

Most 'waxies' used in cathode bias positions with just a very low voltage across them will perform ok even if a bit leaky. Believe it or not, my 900 has most of its original components and performs well for hours on end.

Steve has it spot on. You will have to check the basic voltages and check to see if the frame oscillator is running. Has one of the scan coil wires popped off the external cage connections?
Is the line dead straight or does it have a slight 'kink'?
If kinked the fault will be on the secondary side of the output transformer. Its going to be man-made. I reckon. so I suggest you go down that path.

Be very careful, the EHT is mains derived as you know, and quite a large potential exists on the CRT base due to the electrostatically focused tube.
Don't mess with the scanning coils [saddle] as the fault is likely to be a simple one.
Regards
John.

DoctorWho · 22nd Apr 2007, 8:32 pm

Hello John,

Thanks for your advice, it's always appreciated. You are 100% right about the problem - what a twit I am!

The 703 is now fine. It was entirely due to my error. When I replaced one of the 0.5 condensers I missed a tiny lead beneath it to one of the transformers, it was only when I carefully traced the circuit diagram through that I discovered this. I moved the lead to the correct place and the picture is now better than ever! Whilst I was at it I also replaced the wax condensers in the radio and it's now picking up a lot more stations and without the interference, so it was worth the time to do it.

One thing I did want to ask. Do I need to replace the two 0.1uf condensers across the EHT, or should they be all right? There's also a 1uf in the same area between the EHT earth and the CRT; does that need replacing? If so, with what voltage rating?

Thanks again for your help. It's good to learn as you go along, and I'm certainly learning! I'm very glad I did this now as, when I examined the sync chassis there was quite a collection of wax beads where the condenser/s had been dripping; there were also similar in the radio chassis, so I'm pleased that it's now done, as I can use the set sparingly but without having to worry about damage being done to anything else from a leaking condenser.

I agree with you totally about checking between each component change, the problem, though, with this set was that you couldn't really access the components inside the set, and you couldn't connect it up outside of the set, so I didn't have a lot of choice, however I will follow your advice in the future where ever possible.

Thanks again for your advice.

All the best,

Peter.

Heatercathodeshort · 23rd Apr 2007, 7:25 am

Hello again Peter,
I'm sending you a tube of superglue so that when you have fixed it the back can be securely fixed....
Seriously, though, do not attempt to replace the EHT smoothing capacitors.
They were manufactured by B.I.C [British Insulated Callenders] the guys that build power stations and high voltage components for heavy industry and transmitters. Unlike the TCC 'Visconol' these capacitors were made to an unbelievable specification and I have never seen a single failure. All the pre war EMI sets I have seen have contained their original capacitors.

They are very unlikely to fail and in the extremely unlikely event of one going S/C, the worst that can happen is a ruined U16 EHT rectifier and a blown fuse. No modern replacement component would be as reliable as the originals.

Perhaps I should substitute the superglue for boxing gloves? It's very difficult to wield a soldering iron thus clad....
Congratulations on recovering the set. It's a very easy mistake to make and I guarantee I will do a similar thing when carrying out my next restoration. All the best, John.

DoctorWho · 23rd Apr 2007, 9:11 am

Thanks John, your post gave me a good laugh

Thanks also for the information about the EHT smoothing condensers, they can stay well and truly where they are

Thanks again for your help. I hope you don't do something similar on your next restoration, that was such a daft thing to do, still, I guess that's a good way to learn.

All the best,

Peter.

Heatercathodeshort · 23rd Apr 2007, 7:19 pm

Hello Peter,
No need to feel daft about your mistake. Its very easy to do and I can assure you that the most experienced repair guys have made and will continue to make mistakes such as yours.
It's very easy to connect a component to the wrong tag or reverse an electrolytic capacitor.
I once replaced a mains switch in a Thorn 1500, and it was not until I connected the last wire to the switch and watched in disbelief as the valves warmed up, that I had done so without disconnecting the set from the live mains.....
Take a few digi pictures of the area you are going to work on. It often solves a mystery.. Regards John.

DoctorWho · 24th Apr 2007, 11:18 am

Thanks for your advice John, I'm glad to know others have done similar things. What a frightening thought to have worked on a set which was still connected up to the mains glad you were not hurt!

That's a good tip about taking some digital pictures, I'll keep that in mind for the future.

All the best,

Peter.

22nd Apr 2007, 11:02 am	#1
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Marconi 703 Mastergram Television Help Request. Well the Marconi 703 Mastergram has been working beautifully, but I did notice that one of the old wax condensers was not happy, it had melted a bit and had beads of wax hanging from it. I decided that, to ensure that nothing was damaged, that I would replace the few wax condensers on the Sync chassis. I very carefully removed the chassis and replaced the wax condensers and the electrolytics which had not been replaced. V6 (MS4B) was removed to allow access to one of them, and was then carefully replaced. I had labelled up all of the wires and so I made exspecially sure that they all went back in their rightful places. Before doing this the picture was perfect, now, sadly, I have a vertical line from the top to the bottom of the tube. I'm now not sure what to look for and where the problem is likely to be. Any advice here would be greatly appreciated. I wished I had left the original condensers in place, but I was a bit worried when I saw one of the old wax ones looking unhappy. I'd like to get the set up and running properly again, s any help would be very welcome. Thanks in advance. All the best, Peter. Attached Thumbnails

22nd Apr 2007, 11:37 am	#2
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi 703 Mastergram Television Help Request. Just to clarify, I replaced C12, 10, 22, 21, 16, 2, 14, 20, 23 and 25. I've attached the appropriate part of the pictorial diagram. Obviously the scanning coils are working in one direction, but not in the other, as they are presumably scanning from top to bottom, but not from side to side. I've double checked that all leads have gone back in the right places, and they have, similarly all valves are tight in their sockets and top caps are connected. I've no idea what to look for, as I did the work so carefully, and it's not so easy to understand the circuit diagram as they are not written out in the easier format used on later sets. I'd certainly be grateful of any suggestions as to what to look for, or to try, or to point out what I've done wrong. Thanks in advance. Peter. Attached Thumbnails

22nd Apr 2007, 11:59 am	#3
Panrock Nonode Join Date: Feb 2005 Location: Worcestershire, UK. Posts: 2,533	Re: Marconi 703 Mastergram Television Help Request. Hi Peter, You did the right thing in replacing those waxies in the timebase. Even when they're functioning so-so, they can give below-par performance on the scans. You've lost the horizontal scan or 'high synch' as it's called here. If you're absolutely certain there no wire, or connection to a wire, in bad condition, then (with the set disconnected from the mains) I suggest you go through the whole of the line scan circuit with an ohmeter, checking if the readings seem logical. Particularly pay attention to any wound components. If all seems well, then check the voltages are where they should be on the valve pins. Don't worry, this is a pretty simple circuit and you'll soon get to the bottom of it... Steve __________________ https://www.radiocraft.co.uk

22nd Apr 2007, 12:22 pm	#4
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi 703 Mastergram Television Help Request. Thanks Steve, I have to admit that I was nervous doing this one, as it's more complex that other sets I've worked on. I'm glad that you feel I did the right thing in replacing the waxies, that makes me feel better for tackling it My worry was that something, such as a transformer, was going to be damaged if one of them shorted out. I'll take the Sync chasis out again and go through it on the table as you suggest, with an ohm meter. I did approach this whole job extremely carefully, making extra sure that I was replacing like for like, and in the right place. The replacement electrolytics (such as 50uf @ 25 vols etc) are much smaller than the originals, but I made sure that the polarity on each one was correct. I'll give it a good check over and then let you know later on what I discover. Out of interest which valve would you suggest I take readings from? If I can't find a fault visually, and then with a test meter, I'll try and see if I can work out a way of running the chassis out of the cabinet, as it's pretty impossible to get at whilst in the cabinet. Are there any of the condensers that I have replaced which you would initially suspect? if so, I'll check that part of the circuit first. Thanks again for your help, I felt really sad that I'd done this job, but now I at least feel better about that. All the best, Peter.

22nd Apr 2007, 7:58 pm	#5
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Marconi 703 Mastergram Television Help Request. Hello Peter, From what you say I think you have made the classic mistake of changing all the capacitors in one go, without testing the set between each component change. I know its time consuming but it's the only way to stop human error in its tracks before you get in too deep. My guess is that you have put a wire on the wrong tag. As Steve says it is advisable to replace wax capacitors in the timebases as leaks here will cause, at the least, linearity problems. Most 'waxies' used in cathode bias positions with just a very low voltage across them will perform ok even if a bit leaky. Believe it or not, my 900 has most of its original components and performs well for hours on end. Steve has it spot on. You will have to check the basic voltages and check to see if the frame oscillator is running. Has one of the scan coil wires popped off the external cage connections? Is the line dead straight or does it have a slight 'kink'? If kinked the fault will be on the secondary side of the output transformer. Its going to be man-made. I reckon. so I suggest you go down that path. Be very careful, the EHT is mains derived as you know, and quite a large potential exists on the CRT base due to the electrostatically focused tube. Don't mess with the scanning coils [saddle] as the fault is likely to be a simple one. Regards John.

22nd Apr 2007, 8:32 pm	#6
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi 703 Mastergram Television Help Request. Hello John, Thanks for your advice, it's always appreciated. You are 100% right about the problem - what a twit I am! The 703 is now fine. It was entirely due to my error. When I replaced one of the 0.5 condensers I missed a tiny lead beneath it to one of the transformers, it was only when I carefully traced the circuit diagram through that I discovered this. I moved the lead to the correct place and the picture is now better than ever! Whilst I was at it I also replaced the wax condensers in the radio and it's now picking up a lot more stations and without the interference, so it was worth the time to do it. One thing I did want to ask. Do I need to replace the two 0.1uf condensers across the EHT, or should they be all right? There's also a 1uf in the same area between the EHT earth and the CRT; does that need replacing? If so, with what voltage rating? Thanks again for your help. It's good to learn as you go along, and I'm certainly learning! I'm very glad I did this now as, when I examined the sync chassis there was quite a collection of wax beads where the condenser/s had been dripping; there were also similar in the radio chassis, so I'm pleased that it's now done, as I can use the set sparingly but without having to worry about damage being done to anything else from a leaking condenser. I agree with you totally about checking between each component change, the problem, though, with this set was that you couldn't really access the components inside the set, and you couldn't connect it up outside of the set, so I didn't have a lot of choice, however I will follow your advice in the future where ever possible. Thanks again for your advice. All the best, Peter.

23rd Apr 2007, 7:25 am	#7
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Marconi 703 Mastergram Television Help Request. Hello again Peter, I'm sending you a tube of superglue so that when you have fixed it the back can be securely fixed.... Seriously, though, do not attempt to replace the EHT smoothing capacitors. They were manufactured by B.I.C [British Insulated Callenders] the guys that build power stations and high voltage components for heavy industry and transmitters. Unlike the TCC 'Visconol' these capacitors were made to an unbelievable specification and I have never seen a single failure. All the pre war EMI sets I have seen have contained their original capacitors. They are very unlikely to fail and in the extremely unlikely event of one going S/C, the worst that can happen is a ruined U16 EHT rectifier and a blown fuse. No modern replacement component would be as reliable as the originals. Perhaps I should substitute the superglue for boxing gloves? It's very difficult to wield a soldering iron thus clad.... Congratulations on recovering the set. It's a very easy mistake to make and I guarantee I will do a similar thing when carrying out my next restoration. All the best, John.

23rd Apr 2007, 9:11 am	#8
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi 703 Mastergram Television Help Request. Thanks John, your post gave me a good laugh Thanks also for the information about the EHT smoothing condensers, they can stay well and truly where they are Thanks again for your help. I hope you don't do something similar on your next restoration, that was such a daft thing to do, still, I guess that's a good way to learn. All the best, Peter.

23rd Apr 2007, 7:19 pm	#9
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Marconi 703 Mastergram Television Help Request. Hello Peter, No need to feel daft about your mistake. Its very easy to do and I can assure you that the most experienced repair guys have made and will continue to make mistakes such as yours. It's very easy to connect a component to the wrong tag or reverse an electrolytic capacitor. I once replaced a mains switch in a Thorn 1500, and it was not until I connected the last wire to the switch and watched in disbelief as the valves warmed up, that I had done so without disconnecting the set from the live mains..... Take a few digi pictures of the area you are going to work on. It often solves a mystery.. Regards John.

24th Apr 2007, 11:18 am	#10
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Marconi 703 Mastergram Television Help Request. Thanks for your advice John, I'm glad to know others have done similar things. What a frightening thought to have worked on a set which was still connected up to the mains glad you were not hurt! That's a good tip about taking some digital pictures, I'll keep that in mind for the future. All the best, Peter.