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Old 21st May 2017, 5:07 pm   #1
electronicskip
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Default Marconi T21A Restoration. (The beginning)

I recently collected a 1949 Marconi T21A from a kind forum member free of charge, which had been sat in an old barn for many years, and was consequently in extremely poor condition.
Covered in pigeon droppings and layers of mud n grime, and with possible woodworm.

I've treated the woodworm holes several times, but on reflection I think even the woodworm has moved out to a new home before I acquired it.
The Pigeon droppings have been scraped/washed off to reveal a poor but restorable cabinet.

I've opened the back up and found probably 60 years of dust and cobwebs.
This one will be a total challenge to me but I think it's worth restoring and I love a challenge.

This is the cabinet AFTER I cleaned it off
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Old 21st May 2017, 5:39 pm   #2
JacKam_
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Default Re: Marconi T21a Restoration.(The beginning)

Well done ! Now you can repair it, but do not touch the spider nets. They contain good anibiotics for wounds.

That reminds me my first Kolster-Brandes 840 I repaired, than joined this forum...

Good luck!
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Old 21st May 2017, 6:09 pm   #3
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Default Re: Marconi T21a Restoration.(The beginning)

I got a set in exactly the same state as that one a couple of years or so ago.

Just give it a good 'blow out' with an air blower at the bottom of your garden and after making the necessary static checks for obvious short circuits, give it the mains and see what happens.

I don't go in for all this capacitor changing nonsense before powering up business, never have and never will. Power up in short stages monitoring the HT with a DVM and feeling for any slight warming of the smoothing can. Once you know the state of play of the set you can go ahead and replace the output stage grid coupling 'that' capacitor.

I think I replaced three capacitors and one resistor in the set I did that was in as bad a state as yours and it's been in virtually constant use in the kitchen ever since and works perfectly, other than the volume control pot track that now needs some attention.

You probably know all the above anyway.
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Old 21st May 2017, 6:11 pm   #4
David G4EBT
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Default Re: Marconi T21a Restoration.(The beginning)

Well at least you know that no-one has been in there tinkering with it!

On the plus side, the dial is intact, the knobs are original and it has its back.

Just had a look at Trader Sheet 938 (excellent scan by the way, with a very good under-chassis drawing of the main component placements).

The set was released in 1949, so is now 68 years old, so a few comments might not come amiss, which will be self-evident when you study the circuit:

Most importantly from a safety perspective, is that though the set is AC only and has a mains transformer - it isn't an isolating transformer - it's an auto-transformer, so basically, it's a 'live chassis' set and that should be borne in mind if doing any live testing. It does have an Earth socket but that isn't connected to the chassis - it has an Earth isolating cap (C2), which for safety's sake should be replaced with an X or Y class cap. If using earthed test equipment, on no account should it be connected directly to the chassis, or in effect, you'll be taking one side of the mains input to earth.

Two other caps that really must be changed before you're temped to plug in the set to 'see if it works', are:

A) The Audio Coupling cap - C28.
B: The RF Bypass cap which is across the main input and hence, must be an 'X Class' cap.

I'd also consider replacing the DC isolating caps for the pick-up socket (C22 & 23).

Rather unusually, the 32uF reservoir cap (C32), is in its own can separate from the smoothing cap (C31), which is in a twin 16 uF can with a second HT smoothing cap C24. I imagine they separated the reservoir cap from the others as it has to work the hardest, and will perhaps stay cooler than if it were wrapped around the smoothing cap, as was the norm. They're not mounted in an upright can above chassis, but are horizontal beneath the chassis. You may be lucky and the caps may reform, but if there is any hum that's a good sign that they're on their way out.

The set has three push buttons for pre-set stations - one on LW, the other two on SW, and also manual turning.

It has an internal 'plate aerial' so should work on LW/MW without an external aerial, but will need one for SW.

I wouldn't worry about woodworm - the holes aren't where the worms get in - they're 'flight holes' where they leave the cabinet when they've pupated from being worms and have become beetles.

Hope these observations might help a bit - apologies if you know all this stuff anyway, in which case, please file this under 'burn before reading'!

Good luck with the restoration.

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Old 21st May 2017, 6:17 pm   #5
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Default Re: Marconi T21a Restoration.(The beginning)

Quote:
Originally Posted by David G4EBT View Post
I'd also consider replacing the DC isolating caps for the pick-up socket (C22 & 23).
If you do, use Class Y capacitors.

Unless you really want to use the PU input though, I'd be inclined simply to disconnect it. In any case, it won't work unless the input to it is floating (as a pick-up normally would be).
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Old 21st May 2017, 6:29 pm   #6
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Default Re: Marconi T21a Restoration.(The beginning)

Yes, if there's a filter type cap across the mains, just snip it out and chuck it.

It's also worth doing a static resistance check of the mains on/off switch as they often develop high resistance if left in the 'off' position for decades.

Don't use the on/off switch to switch the set on when first powering up as if something like a filter capacitor goes bang it can sometimes weld the switch contacts together, so only switch on and off at the power socket on your bench for starters until you have the set in a working state.
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Old 21st May 2017, 7:43 pm   #7
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Default Re: Marconi T21a Restoration.(The beginning)

The Volume On/Off can very often be successfully sorted with a good squirt of a decent switch cleaner. Saves dismantling or replacement as they do go high resistance or read open circuit.
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Old 21st May 2017, 8:29 pm   #8
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Default Re: Marconi T21a Restoration.(The beginning)

From the outside it looks identical to my T19a, which I'm guessing is an earlier model.

I look forward to reading about the restoration of this radio.

Good luck.
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Old 21st May 2017, 10:13 pm   #9
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Default Re: Marconi T21A Restoration. (The beginning)

Hi Ken,
Your set looks like another T21a to me personally? - I thought the T19a had a metal grille/ slightly different dial (same looking case) - I dug this one out of the heap for a picture!
Doesn't really matter anyway these both work well I think, and are easy to work on.
Regards,
Steve.
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Old 28th May 2017, 7:29 pm   #10
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Default Re: Marconi T21A Restoration. (The beginning)

I'm wondering how best to restore the wooden cabinet, as its quite far gone, but I'm determined to keep it.
So far ive stripped it down and started a deep clean of the innards just for a start.
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Old 29th May 2017, 9:44 am   #11
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Default Re: Marconi T21A Restoration. (The beginning)

Yep - the up-side is that it's all there .... back/valves/knobs/licence plates and .... valve diagram! Marconi products are generally quite pleasant to work on ....... once you've treated them to gentle treatment with an airline!
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