Dos and don'ts with Pre War TVs + Marconi 706 pickup!

[em536716]

Hello everyone,

Tomorrow, I will be picking up a Marconi 706 Pre War Television. The set is complete but It’s unknown if any work has been carried out Inside, or if the Emiscope Tube is any good!

In preparation for this restoration, I have ordered some very high quality capacitors, all of the right values. Before I start work on it, I wanted to ask the community, what are the dos and donts when it comes to restoring something so rare?

I understand these also suffer from breakdown in the EHT transformers, so I might need to give Mike Barker or Ed Dinning a call.

And yes, I’ve contemplated leaving it be if it’s 100 percent original, but let’s face it. I’m young and want to enjoy my new piece in full!

Will post some photos of the set tomorrow, and provide the serial number.

[mark pirate]

If it were mine, I would keep it as original as possible, carefully restuff capacitors etc.
The first thing is to check out the CRT & mains transformer, as you say, these are the most important components.

Looking forward to seeing the chassis, hopefully it is mostly untouched & has not been got at in the past.


Mark

[Iontrap]

Excellent. I look forward to those pictures!

[high_vacuum_house]

Good evening,
Can’t really add too much though the following is worth mentioning

1) I expect the set to be very heavy, much heavier than it looks! my early GEC tabletop receiver weighs an absolute ton!!

2) EHT is very unforgiving in these mains derived supply receivers. Very low circuit impedance with high value EHT capacitors. Ensure all of the EHT chain of bleeder resistors are in place and haven’t gone O/C otherwise you could get a nasty belt even with the set unplugged.

3) The CRT is difficult to replace so worth disabling the EHT supply and checking such things as the timebases are all is well and running before connecting it up.

Christopher Capener

[Radio Wrangler]

Yes, with a 50Hz EHT supply, halfwave rectified, they have to have plenty of microfarads smoothing it, increasing the danger factor a lot.

David

[em536716]

Thanks all for your responses.

I’m very excited to pick it up tomorrow. I’ve since also discovered its in fact a 707. Ive only seen photos of it, but the tube mask was a dead giveaway.

Thanks for the advice. I have a few more thoughts I hope you could give me your opinions on.

With regarding transformers, is it worth passing some DC current through them to warm up the windings to ensure there isn’t any moisture?

Also, to keep originality, with the smoothing caps. Restuff or reform?

And finally the wooden cabinet. Should I leave it as is or sand it down and make it look brand new?

[em536716]

Thanks for the reminder of mains EHT as well.

I hate to say I actually got bit hard by an EHT smoothing cap. Silly ****** was still charged! After that day I now have a WORKING EHT probe lol...

What type of EHT caps do these employ? Are they still any good these days?

[Ed_Dinning]

Hi Em, it would be well worthwhile leaving it in a warm dry place for a considerable time to dry out before applying power. The mains transformers can be rewound, but beware of the EHT caps as they were not noted for long life. There are some good PIO ex gov types that can be substituted that are of good quality.
Do not attempt to restuff the originals as they may contain pcb's (liquid cancer!)

Good luck, Ed

[Al (astral highway)]

Originally Posted by em536716

What type of EHT caps do these employ? Are they still any good these days?

Visconols.

Ed recently sent me a batch to investigate/ potentially use in a pulse application. These were designed for use in EHT application back in the day, including TV.

They all test within spec after all these years . Those that I have are physically large and well-made and I believe commercial not domestic grade, though.

They are oil-filled (PCB hazard if opened and not to be disposed of).

As others have said, be really careful of that mains-derived EHT. It is easily one-touch lethal.

[em536716]

Originally Posted by Ed_Dinning

Hi Em, it would be well worthwhile leaving it in a warm dry place for a considerable time to dry out before applying power. The mains transformers can be rewound, but beware of the EHT caps as they were not noted for long life. There are some good PIO ex gov types that can be substituted that are of good quality.
Do not attempt to restuff the originals as they may contain pcb's (liquid cancer!)

Good luck, Ed

Thanks for the advice on these Ed. I had planned to let it get to room temp before touching it.

I had thought about this with the smoothing caps. I might have to take a different approach.

Once I get it home, I will start a new thread dedicated to it, and share some more info there with an inspection.

[mark pirate]

Also, to keep originality, with the smoothing caps. Restuff or reform?
And finally the wooden cabinet. Should I leave it as is or sand it down and make it look brand new?

I would restuff the smoothing caps, at over 80 years old, it is not worth the risk of failure.

Unless the cabinet finish is in a bad way, I would just clean & polish it, remember it is only original once!

With a set of such rarity & value, any restoration should be sympathetic.
I have an HMV radio from the same period that a previous owner refinished, although it has been well done, it just does not look right for something that old.
It stands out like a sore thumb next to my other 1930's sets.


Mark

[em536716]

Originally Posted by astral highway

Visconols.

Ed recently sent me a batch to investigate/ potentially use in a pulse application. These were designed for use in EHT application back in the day, including TV.

They all test within spec after all these years . Those that I have are physically large and well-made and I believe commercial not domestic grade, though.

They are oil-filled (PCB hazard if opened and not to be disposed of).

As others have said, be really careful of that mains-derived EHT. It is easily one-touch lethal.

Thanks for the response.

I’m currently working on a Murphy V114. I’m quite familiar with visconols, as my set employs one for the EHT smoothing.

[em536716]

Originally Posted by mark pirate

I would restuff the smoothing caps, at over 80 years old, it is not worth the risk of failure.

Unless the cabinet finish is in a bad way, I would just clean & polish it, remember it is only original once!

With a set of such rarity & value, any restoration should be sympathetic.
I have an HMV radio from the same period that a previous owner refinished, although it has been well done, it just does not look right for something that old.
It stands out like a sore thumb next to my other 1930's sets.


Mark

Thanks Mark.

Just seeing what a good way to approach the set should be.

I tend to agree with your idea. I don’t think making something look 100 percent brand new is genuine, but I’d like to at least keep some originality to it. The marks and scuffs tells it’s story, and all!

[mark pirate]

I don’t think making something look 100 percent brand new is genuine, but I’d like to at least keep some originality to it. The marks and scuffs tells it’s story, and all!

I have had great success with bringing back the finish on cabinets, just using off the shelf stuff, I am certainly not a skilled refinisher, but was well pleased with the end results that can be achieved.

See this thread for the restoration of a set that had been forgotten in a loft since 1960.
https://www.vintage-radio.net/forum/...d.php?t=123306


Mark

[em536716]

Originally Posted by mark pirate

I have had great success with bringing back the finish on cabinets, just using off the shelf stuff, I am certainly not a skilled refinisher, but was well pleased with the end results that can be achieved.

See this thread for the restoration of a set that had been forgotten in a loft since 1960.
https://www.vintage-radio.net/forum/...d.php?t=123306


Mark

Thanks for sharing Mark.

What a great read! I’ll have to try this technique on my cabinet once I’m ready.

Unfortunately I don’t really know the history of my set, as I’ve purchased it from a museum up north!

Let’s hope my CRT is as bright as the one in your Pye, or near enough anyway. I expect ion burn!

Cheers,
Ed

[mark pirate]

Let’s hope my CRT is as bright as the one in your Pye, or near enough anyway. I expect ion burn!

Ion burn is all part of the charm of these old sets, it is quite amazing how you adapt to it.
The Pye is still performing well, I watched the original 39 steps film on it last week, it produces a far better picture when compared to the small LCD set in the kitchen!


Mark

[beery]

Hi Ed,
I've a number of points, some based on my experience with my HMV 905:-

Cabinet
Leave the original finish on it if you can. The EMI sets used serveral coats of spayed on celulose lacquer which had a slight brown tint added to it. Unlike other sets (eg PYE sets) this finish does not crack and craze and is generally very hardy.
On my HMV 905 I used a special furniture cleaner to remove dirt and wax from the cabinet. I was then able to use scratch cover liquids of the required shades and then applied brushing wax, which after allowing it to dry a bit was buffed up to a nice shine. I will have to look up exactly which products I used. My set does have deep scratches on the top, but I decided to leave well enough alone.

If you do have to strip the cabinet, it might be hard to find someone who can respray it. French polishing looks quite good, but the other problem after stripping it is to replace the transfer on the top of the cabinet.



Chassis, where to start
To remove the chassis you have to carefully unplug the CRT socket and then the scan coil plug which is located on the chassis.
Loosen the two terminal screws on the sound output transformer and slide off the two speaker connections.

Remove the control knobs. This can be tricky. The timebase controls use normal grub screws, note that there is a scalloped portion of the outer concentric controls to allow better access to the grub screws on the inner knobs. The outer concenric knobs just slide off. Other controls, such as the wavechange use a self tapping screw that passes through a slot in the control shaft; these screws need to be removed completely.

Chassis screws are located underneath, but note that they attach to four spikey legs that dig in to the cabinet, you will need to lift the chassis slightly, one side at a time to walk it out of the cabinet.
Make sure that you have somewhere ready where you can place the whole chassis including its four feet. This is because some of the tag boards exrend below the chassis slightly and they will break if say the chassis if half hanging off the bench. Tip the chassis un to the end where the transformers are. Make a couple of wooden skids that run the width of the chassis and counterbore or countersink holes so that you can screw the skids into the 2BA tapped holes in the spikey feet. This really helps protect those tag boards. I think mine were made of soft wood 1 1/2" deep by 1/2" high x the width of the chassis.
Obviously cleaning and dusting the chassis is the first thing to do. When removing IF valves (KTZ41), don't mix them up otherwise you will mess up the alignment, this is a good general rule for any TV that does not use miniature valves in the reveiver section.
Check the condition of the wiring that goes from the mains input panel at the rear. This is cotton sleeved rubber, which if undisturbed may well be ok. That part of the wiring in my set was ok, but all of the normal un-sleeved rubber wiring required replacing.

Transformer tests
Obvously observe the usual precautions that you would when applying power to a pre-war radio for the first time. The chassis must be earthed.
When you are happy that the mains side of things is sufficiently safe, you can prepare to power up the radio and HT transformer.
First unsolder the lead that goes to the top cap of the EHT rectifier. Unsolder it from the tag on the transformer of course!. The EHT transformer is the one on the underside of the chassis.
Make sure the unsoldered wire is well out of the way. Remove the EHT rectifier valve. I actually recommend that you disconnect the mains feed to the EHT transformer for now anyway.
Remove the HT rectifier valve.
Switch the wavechange to one of the radio wavebands (ie, not Television).
Check again that the chassis is earthed.
Make sure you are nowhere near the EHT section.
Before mains is connected, make sure that the on-off volume is set to on and that you have a remote way of switching the set on and off such as a safeblock.
Apply power and check for smoke as you would when testing a radio. If all is ok then run for a few minutes. The valves should warm up, but nothing will happen as there is no HT rectifier. Don't run it for hours like this as it will lead to cathode poisoning of the valves. After safely disconnecting the power check to see how hot the HT transformer is (on the top of the deck).

Don't worry about testing the EHT transformer to start with. It will fail! It can be a great help and much safer to have external 4V and 6.3V heater supplies to test the TV section without the EHT transformer (eg. if it has been removed for rewinding). I wound a heater transformer for testing my set, if I still have it then I can lend it to you.

That's about all you can do before you replace the HT smoothing caps. I would recomend restuffing the original caps to keep the set looking original.

Take lots and lots of photos with your phone, so that every wire you move and every capacitor you restuff can go back as it was.

Get the radio working first because this shares alot of circuitry with the Television side of things.

In the same way that you should not run a radio with no speaker load on the output transformer, running the television section with the scan coils unplugged can cause the line output transformer to fail. Remove the line output valve if you need to run the TV section without the scan coils. I did a simple mod to my set which is to use a couple of spare pins on the scan coil plug to provide a link for the HT to the line ouput valve, so no scan coils means no anode or screen feed to the line output valve.

Be very, very wary of the mains EHT, you only get to touch it once and it is game over.

Don't worry about testing the CRT if you do not know how to do it. Better to leave it alone than to risk damaging it. Best to leave it safely in the cabinet until the rest of the set is working.

Please see my HMV905 thread here:-
https://www.vintage-radio.net/forum/...ghlight=hmv905

Cheers
and Happy New Year pre-war set tinkering.
Andy

[beery]

Hi again,
you can get the manual here:-
https://www.thevalvepage.com/resourc...ls/manuals.php

You need to enable unsecured content for this site in your browser for the site to work properly. If it is working you should see it default to service info beginning with the letter 'A'.
Type in HMV and select HMV904 from the list.

Do check out the rest of Jon's site here:-
https://www.thevalvepage.com

Cheers
Andy

[em536716]

Originally Posted by beery

Hi Ed,
I've a number of points, some based on my experience with my HMV 905:-

Cabinet
Leave the original finish on it if you can. The EMI sets used serveral coats of spayed on celulose lacquer which had a slight brown tint added to it. Unlike other sets (eg PYE sets) this finish does not crack and craze and is generally very hardy.
On my HMV 905 I used a special furniture cleaner to remove dirt and wax from the cabinet. I was then able to use scratch cover liquids of the required shades and then applied brushing wax, which after allowing it to dry a bit was buffed up to a nice shine. I will have to look up exactly which products I used. My set does have deep scratches on the top, but I decided to leave well enough alone.

If you do have to strip the cabinet, it might be hard to find someone who can respray it. French polishing looks quite good, but the other problem after stripping it is to replace the transfer on the top of the cabinet.



Chassis, where to start
To remove the chassis you have to carefully unplug the CRT socket and then the scan coil plug which is located on the chassis.
Loosen the two terminal screws on the sound output transformer and slide off the two speaker connections.

Remove the control knobs. This can be tricky. The timebase controls use normal grub screws, note that there is a scalloped portion of the outer concentric controls to allow better access to the grub screws on the inner knobs. The outer concenric knobs just slide off. Other controls, such as the wavechange use a self tapping screw that passes through a slot in the control shaft; these screws need to be removed completely.

Chassis screws are located underneath, but note that they attach to four spikey legs that dig in to the cabinet, you will need to lift the chassis slightly, one side at a time to walk it out of the cabinet.
Make sure that you have somewhere ready where you can place the whole chassis including its four feet. This is because some of the tag boards exrend below the chassis slightly and they will break if say the chassis if half hanging off the bench. Tip the chassis un to the end where the transformers are. Make a couple of wooden skids that run the width of the chassis and counterbore or countersink holes so that you can screw the skids into the 2BA tapped holes in the spikey feet. This really helps protect those tag boards. I think mine were made of soft wood 1 1/2" deep by 1/2" high x the width of the chassis.
Obviously cleaning and dusting the chassis is the first thing to do. When removing IF valves (KTZ41), don't mix them up otherwise you will mess up the alignment, this is a good general rule for any TV that does not use miniature valves in the reveiver section.
Check the condition of the wiring that goes from the mains input panel at the rear. This is cotton sleeved rubber, which if undisturbed may well be ok. That part of the wiring in my set was ok, but all of the normal un-sleeved rubber wiring required replacing.

Transformer tests
Obvously observe the usual precautions that you would when applying power to a pre-war radio for the first time. The chassis must be earthed.
When you are happy that the mains side of things is sufficiently safe, you can prepare to power up the radio and HT transformer.
First unsolder the lead that goes to the top cap of the EHT rectifier. Unsolder it from the tag on the transformer of course!. The EHT transformer is the one on the underside of the chassis.
Make sure the unsoldered wire is well out of the way. Remove the EHT rectifier valve. I actually recommend that you disconnect the mains feed to the EHT transformer for now anyway.
Remove the HT rectifier valve.
Switch the wavechange to one of the radio wavebands (ie, not Television).
Check again that the chassis is earthed.
Make sure you are nowhere near the EHT section.
Before mains is connected, make sure that the on-off volume is set to on and that you have a remote way of switching the set on and off such as a safeblock.
Apply power and check for smoke as you would when testing a radio. If all is ok then run for a few minutes. The valves should warm up, but nothing will happen as there is no HT rectifier. Don't run it for hours like this as it will lead to cathode poisoning of the valves. After safely disconnecting the power check to see how hot the HT transformer is (on the top of the deck).

Don't worry about testing the EHT transformer to start with. It will fail! It can be a great help and much safer to have external 4V and 6.3V heater supplies to test the TV section without the EHT transformer (eg. if it has been removed for rewinding). I wound a heater transformer for testing my set, if I still have it then I can lend it to you.

That's about all you can do before you replace the HT smoothing caps. I would recomend restuffing the original caps to keep the set looking original.

Take lots and lots of photos with your phone, so that every wire you move and every capacitor you restuff can go back as it was.

Get the radio working first because this shares alot of circuitry with the Television side of things.

In the same way that you should not run a radio with no speaker load on the output transformer, running the television section with the scan coils unplugged can cause the line output transformer to fail. Remove the line output valve if you need to run the TV section without the scan coils. I did a simple mod to my set which is to use a couple of spare pins on the scan coil plug to provide a link for the HT to the line ouput valve, so no scan coils means no anode or screen feed to the line output valve.

Be very, very wary of the mains EHT, you only get to touch it once and it is game over.

Don't worry about testing the CRT if you do not know how to do it. Better to leave it alone than to risk damaging it. Best to leave it safely in the cabinet until the rest of the set is working.

Please see my HMV905 thread here:-
https://www.vintage-radio.net/forum/...ghlight=hmv905

Cheers
and Happy New Year pre-war set tinkering.
Andy

Thanks for your comments and advice on how to proceed Andy.

I’ve actually read your thread on the restoration you made on your 905 extensively. Very well done!

I’m in no rush to get power to it, and actually was thinking of taking the same approach as you’ve suggested. Begin with the radio section. I’ve built a homemade jig for radio chassis to manoeuvre around, but I think this might be too big.

Will have a better idea on condition of cab and chassis once it’s in my possession. As far as testing the Emiscope tube, whilst I could do it on my CRT tester, dc resistance of the heater is fine for now. We know it won’t give perfect emission anyway!

Cheers again
Ed

[em536716]

Originally Posted by beery

Hi again,
you can get the manual here:-
https://www.thevalvepage.com/resourc...ls/manuals.php

You need to enable unsecured content for this site in your browser for the site to work properly. If it is working you should see it default to service info beginning with the letter 'A'.
Type in HMV and select HMV904 from the list.

Do check out the rest of Jon's site here:-
https://www.thevalvepage.com

Cheers
Andy

Thanks Andy. I’ve actually got the service data all here and ready!