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Old 5th Jan 2021, 10:21 pm   #41
peter_scott
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Default Re: My newly found Marconi 707

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Originally Posted by em536716 View Post
We now have a Nice EHT!

The spot is quite bright!
It does look very good.

You can always move the unfocused spot around the screen with a magnet to discover the goodness of the phosphor.

Peter
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Old 6th Jan 2021, 12:13 am   #42
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Default Re: My newly found Marconi 707

Hi Ed,
That's a great tube.
Regarding tube burn protection, I've found that if you are in TV mode and you use the on/off volume control to switch off the set, all is OK. If you simply switch straight back to radio from TV mode, that is when there is a problem, so best to stick to switching off the set before you go back to radio mode.

I'll dig out some pictures of the cap restuffing later. I've not written a thread on the set in question as the owner does not want to advertise exactly what type of set it is.
When it is finished I might write a joint BVWS article together with the set's owner.

Cheers
Andy

Last edited by beery; 6th Jan 2021 at 12:14 am. Reason: Too many OKs!
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Old 6th Jan 2021, 10:07 am   #43
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Default Re: My newly found Marconi 707

Great news with the CRT, it certainly looks nice & bright
I would agree with the comments of using a microwave diode in place of the EHT rectifier, it would take the strain off the heater winding.


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Old 6th Jan 2021, 10:56 pm   #44
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Default Re: My newly found Marconi 707

Thanks for the kind comments everyone and suggestions.

As I mentioned, I wanted to move my attention to the radio and power supply to ensure this was in order before I progressed. I also wanted to have a go at trying to give some life back to the original finish and food grain on the cabinet top and front and sides. For this I used linseed oil.

Thankfully this has pretty much brought back the shine to the cabinet, but unfortunately has not revitalised the failing parts of the woodgrain which appears to be lifted and near to flaking off on the top of the cab. At least it doesn’t appear dry everywhere else anymore! I have applied it one more time tonight to see if this can help with improving it. I will post photos of this tomorrow after inspection.

Onto the chassis...
I could see a couple failed solder joints. Both were to RF bypass capacitors which were leaky so left these out of circuit to test the power supply.

After a long soak on the variac, I slowly brought it up throughout the day. The power supply came up, to the correct HT voltage, around 295 on the “broadcast” mode which is fantastic. For this test I replaced the grid cap to the control grid of the audio output valve. Plugged into the gram input and all appeared singing and dancing.

When time comes to work on the TV portion of the set, I will be installing solid state diode on the underside, along with some Russian high voltage smoothing caps for the EHT.

I’m away for most of the day tomorrow but will revisit this with trying to free up the tuning control. I could rotate this using some pliers I have but it was extremely hard.

After I achieve this, I will move onto working on the radio portion of the set.

Any suggestions for freeing up the tuning control would be well appreciated!

Regards,
Ed

Last edited by em536716; 6th Jan 2021 at 10:59 pm. Reason: Needed to add a comment regarding using EHT diodes!
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Old 7th Jan 2021, 12:10 am   #45
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Default Re: My newly found Marconi 707

I would not have used Linseed oil or oil of any type. This will have soaked into the cabinet, staining the wood, making it almost impossible to strip and repolish at some later date.

The old varnish has probably broken down but to be honest didn't look too bad from your pictures. Maybe just a good clean and polish using a good quality beeswax furniture polish. There are some very good ones on the market in both light and dark shades suitable for antique furniture. Just my thoughts. John.

With these rare and vintage receivers it is good policy to never do anything that can't be reversed particularly when the cabinet is concerned. John.
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Old 7th Jan 2021, 8:38 am   #46
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Default Re: My newly found Marconi 707

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With these rare and vintage receivers it is good policy to never do anything that can't be reversed particularly when the cabinet is concerned
I have to agree with John, I am surprised that you have done that after reading your previous post.

Quote:
Unfortunately a fellow collector friend was going to come up here to Guilford next week and collect the cabinet to restore it for me. Obviously ensuring he preserves all of the original finish as possible, but since we are in lockdown now, I canít see this happening as it isnít safe - Iím too nervous to attempt any cab repair on my own, souly because of its rarity.
I have refinished many set's over the years, but with such a rare & valuable set, I would have had this refinished by a experienced professional, regardless of how long it took to complete.


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Old 7th Jan 2021, 11:56 am   #47
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Default Re: My newly found Marconi 707

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Unfortunately a fellow collector friend was going to come up here to Guilford next week and collect the cabinet to restore it for me. Obviously ensuring he preserves all of the original finish as possible, but since we are in lockdown now, I canít see this happening as it isnít safe - Iím too nervous to attempt any cab repair on my own, souly because of its rarity.
Your nervousness at attempting a cabinet repair on a set as historically significant as this is well founded. I think it's important to be clear in advance with your cabinet restorer about what techniques s/he will be using. In my opinion, there are only two paths to take.

The first is to do only the absolute minimum to the cabinet beyond cleaning and polishing. No 'Danish Oil' (which will look completely wrong), no 'spirit pens'. A soft cloth lightly charged with wood stain and buffed off again can work wonders, though never let it get into bare veneer or timber sections or it will soak into scratches, emphasising them.

The second is a proper, high grade, strip, preparation and re-lacquer using cellulose and (importantly) including toner, sprayed on - as originally. Some might propose 'french polishing' but again, though giving a nice result, this would be non-original here and incorrect. The task should only be tackled by an experienced person. This is not the set to 'learn on'!

As for the seized tuner slow motion, personally I would remove this, soak it in thinners, helped along with an old toothbrush. When it's free, lightly re-lubricate, though remember the ball bearings need to 'grip' the internal and external ring tracks.

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Old 7th Jan 2021, 12:03 pm   #48
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Default Re: My newly found Marconi 707

Perhaps youíre correct Mark, John, but please donít be concerned.

Iím not actually looking to strip off the old finish, itís mostly fine but appeared quite dull. My intention here was to just hydrate and protect it, which I believe it has.

I would like to keep as much of it as original as possible, I felt like actually, this has improved the look to the cabinet.

Moving forward, I wonít be touching it any further. As mentioned previously a friend has offered to look at the cab for me, but this wonít be for sometime. Let my curiosity get the better of me!
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Old 7th Jan 2021, 12:12 pm   #49
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Default Re: My newly found Marconi 707

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Quote:
Unfortunately a fellow collector friend was going to come up here to Guilford next week and collect the cabinet to restore it for me. Obviously ensuring he preserves all of the original finish as possible, but since we are in lockdown now, I canít see this happening as it isnít safe - Iím too nervous to attempt any cab repair on my own, souly because of its rarity.
Your nervousness at attempting a cabinet repair on a set as historically significant as this is well founded. I think it's important to be clear in advance with your cabinet restorer about what techniques s/he will be using. In my opinion, there are only two paths to take.

The first is to do only the absolute minimum to the cabinet beyond cleaning and polishing. No 'Danish Oil' (which will look completely wrong), no 'spirit pens'. A soft cloth lightly charged with wood stain and buffed off again can work wonders, though never let it get into bare veneer or timber sections or it will soak into scratches, emphasising them.

The second is a proper, high grade, strip, preparation and re-lacquer using cellulose and (importantly) including toner, sprayed on - as originally. Some might propose 'french polishing' but again, though giving a nice result, this would be non-original here and incorrect. The task should only be tackled by an experienced person. This is not the set to 'learn on'!

As for the seized tuner slow motion, personally I would remove this, soak it in thinners, helped along with an old toothbrush. When it's free, lightly re-lubricate, though remember the ball bearings need to 'grip' the internal and external ring tracks.

Steve

Thanks for your suggestions Steve.

I as mentioned above I did use a Linseed oil. As my previous comment explains, I did this just to give some life to the original wood grain and protect it. Iím not looking to refinish the cabinet.

I think it would be too much of a risk to make something so old look brand new again, in my opinion. As a collector, most of my pieces are fully restored, but for such a rarity like this one, my main objective is to protect it, and keep its originality. I do understand however, that perhaps using the oil was not the correct, safe way of doing this.


I will pass these comments onto the person whoís going to eventually touch parts of the cabinet up.

I wonít continue to try this myself as mentioned.
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Old 7th Jan 2021, 12:16 pm   #50
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Default Re: My newly found Marconi 707

Steve - I’ll have to try again at removing this. Last night I didn’t have much success. Will try again, to give it a good clean

Last edited by em536716; 7th Jan 2021 at 12:17 pm. Reason: Incorrect spellings
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Old 7th Jan 2021, 12:54 pm   #51
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Default Re: My newly found Marconi 707

It always helps to decipher how things are put together and what needs undoing by ensuring they're spotlessly clean first and the lighting is good. Also I take copious close-up digital photographs as I dismantle things, part by part. It's amazing how useful these can later prove! Good luck!

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Old 7th Jan 2021, 4:41 pm   #52
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Default Re: My newly found Marconi 707

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It always helps to decipher how things are put together and what needs undoing by ensuring they're spotlessly clean first and the lighting is good. Also I take copious close-up digital photographs as I dismantle things, part by part. It's amazing how useful these can later prove! Good luck!

Steve
Donít worry Steve! Have got loads of photos taken of it. The securing screw is what is stuck at the mo. Should hopefully have it off by the end of the day

Thanks again for your advice
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Old 12th Jan 2021, 12:31 am   #53
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Default Re: My newly found Marconi 707

Hi all,

Whilst waiting on more parts, I took the time to partially clean the chassis, and unstick the tuning control, using a variation of heat and partial disassembly to remove dried grease, and redo.

Here are some photos of the cleaned chassis. Valves will be tested in due course. Apart from 3 Osman valves, the rest are Marconi originals!

Picture 1 to 3 shows development of cleaning.
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Old 12th Jan 2021, 12:54 am   #54
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Default Re: My newly found Marconi 707

With regards to new capacitors, for .05s Iíve bought a massive job lot of RS .047s to use in place. I wouldíve liked to use the original value but I canít seem to find any reliable sources. All other values were found however. Iíll be using reliable NOS British and Russian types which I believe are top quality.
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Old 12th Jan 2021, 4:10 pm   #55
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Default Re: My newly found Marconi 707

I would steer well clear of NOS capacitors, in my experience they are often leaky and have shifted in values.

With a set of this rarity & value, I would only use new high quality components.

I always use high quality caps from these guys: https://www.justradios.com/
They stock a lot of values not obtainable elsewhere, and the postage is quick too. I have no connection, other than being a very satisfied customer.


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Old 12th Jan 2021, 4:23 pm   #56
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Default Re: My newly found Marconi 707

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Hi all,

Here are some photos of the cleaned chassis. Valves will be tested in due course. Apart from 3 Osman valves, the rest are Marconi originals!

Picture 1 to 3 shows development of cleaning.
OSRAM?

Cheers

Mike T
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Old 13th Jan 2021, 1:35 am   #57
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Default Re: My newly found Marconi 707

Hi Ed,
I gave up on using nos capacitors a long time ago as they were too unreliable.
The only ones I would use would be Mullard mustards and any type of mica capacitors. Mind you I would bother buying old Mullard mustards these days as they fetch silly money!
Resistors of the carbon compound type, I recover from scrap chassis, if I don't have the right value of prewar resistor in stock then I will repaint a 50s one.

Regarding values, 0.047 will do perfectly for 0.05uF, 0.022uF will do for 0.02 and 0.025uF. It's all a question of tolerance and also the circuit function. If you are talking coupling and decoupling of signals then the value is not critical. For timebases it is a little more important, but still not that critical. For RF and IF tuning it's a whole different kettle of fish with mica, ceramic or air spaced being the order of the day.
Have a look at the parts list in the manual, the stated tolerance of each original capacitor will give you a clue as to how critical its exact value is.

Cheers
Andy
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Old 13th Jan 2021, 6:41 pm   #58
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Default Re: My newly found Marconi 707

Hello everyone,

Just to clear a few things up with the capacitors.

I wonít be using NOS paper and wax type, as obviously by now, they will all be bad. Instead I have found NOS Russian K75-10 type (paper in oil) capacitors, RS (Yellow) NOS Polyprop Film, and a couple NOS Domino mica type, and Mullard Mustard caps.
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Old 13th Jan 2021, 6:43 pm   #59
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Default Re: My newly found Marconi 707

Quote:
Originally Posted by Cobaltblue View Post
Quote:
Originally Posted by em536716 View Post
Hi all,

Here are some photos of the cleaned chassis. Valves will be tested in due course. Apart from 3 Osman valves, the rest are Marconi originals!

Picture 1 to 3 shows development of cleaning.
OSRAM?

Cheers

Mike T

Lol cheers Mike. I didnít catch that. Osman sounds like the name of the chap who runs the local kebab shop!
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Old 13th Jan 2021, 6:44 pm   #60
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Default Re: My newly found Marconi 707

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Hi Ed,
I gave up on using nos capacitors a long time ago as they were too unreliable.
The only ones I would use would be Mullard mustards and any type of mica capacitors. Mind you I would bother buying old Mullard mustards these days as they fetch silly money!
Resistors of the carbon compound type, I recover from scrap chassis, if I don't have the right value of prewar resistor in stock then I will repaint a 50s one.

Regarding values, 0.047 will do perfectly for 0.05uF, 0.022uF will do for 0.02 and 0.025uF. It's all a question of tolerance and also the circuit function. If you are talking coupling and decoupling of signals then the value is not critical. For timebases it is a little more important, but still not that critical. For RF and IF tuning it's a whole different kettle of fish with mica, ceramic or air spaced being the order of the day.
Have a look at the parts list in the manual, the stated tolerance of each original capacitor will give you a clue as to how critical its exact value is.

Cheers
Andy

Cheers Andy for the advice.

Everything is ordered now. Covid has unfortunately slowed down my post so things are taking longer than expected to arrive!
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