My newly found Marconi 707

[em536716]

Originally Posted by mark pirate

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.


Mark

Cheers Mark.

I’ve actually bought most of the RF Mica caps from justradios. Brilliant quality stuff.

[Al (astral highway)]

Originally Posted by em536716

..., 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.

Hey Ed, this sounds like the wood veneers getting unstuck, very common with age, heat and dessication.

Originally Posted by em536716

When time comes to work on the TV portion of the set, I will be installing solid state diode on the underside,

Good move. I have HER608's here yours FOC.



Pop a few in series for good measure They're 6A 1000 PIV rated IIRC. Please acknowledge here and then PM me if interested.

Originally Posted by em536716

.


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

I'd just go softly softly and get some Servisol in there. Sometimes it can help to soften up solidified grease/gunk with topical application of heat with a hair dryer. This isn't a specific to your set, just in general.

[Heatercathodeshort]

Use the original U52/5U4G H.T. rectifier. J.

[em536716]

Originally Posted by Heatercathodeshort

Use the original U52/5U4G H.T. rectifier. J.

I’ll be keeping original HT rectifier. Contemplating using solid state diode for EHT.

[rambo1152]

Originally Posted by em536716

I’ve actually bought most of the RF Mica caps from justradios. Brilliant quality stuff.

How many of these were you intending to replace Ed?

[em536716]

Originally Posted by rambo1152

How many of these were you intending to replace Ed?

I’ve ordered a few different values but only the two Domino type ones which are leaky. I’ve carefully checked the others and they actually appear OK. So for now I will leave them be.

[peter_scott]

Mica capacitors are very rarely faulty.

Peter

[HamishBoxer]

I know this has probably been asked before but is there any import duty when ordering from Just Radio?

[mark pirate]

is there any import duty when ordering from Just Radio?

I have had many orders & have never paid any duty.


Mark

[HamishBoxer]

Thank you Mark.

[murphyv310]

Originally Posted by Heatercathodeshort

Use the original U52/5U4G H.T. rectifier. J.

The 5U4/U52 are directly heated valves with a filament, although the surge is less than using a silicon or selenium rectifier, the voltage across the HT Line rises very quickly and doesn't stabilise to its running voltage until the load increases with the other valves warmed up.

On a set that is only used rarely I'd advocate using the likes of a GZ33 or GZ37 , being an indirectly heated valve the warm up time is much longer so its kinder to smoothing capacitors and other components, the peak surge voltage will never reach what it was with the 5U4 or silicon etc.

[Al (astral highway)]

Originally Posted by em536716

Contemplating using solid state diode for EHT.

Great, so would you like me to pop some in the post to you?

[em536716]

Originally Posted by astral highway

Great, so would you like me to pop some in the post to you?

Thanks for the Offer, Astral Highway, but I’ve got some here I can make use of

[em536716]

Hi everyone.

All replacement capacitors are now finally here, so will get started and post some updates soon. I’ll see if I can link some YouTube videos as well, as a visual aid.

Thanks,
Ed

[draenog]

BTW, I notice from your pictures that you have the tuning dial glass in place. This is pretty much irreplaceable and can easily be accidently broken shifting the heavy chassis around. When working on my pre-war TVs for any length of time I always remove it, putting it in a safe place until the work is complete.

[em536716]

Does anyone know where I could find a C clip suitable for one of the dual pot controls?

The little C clip holding the outer shaft in place was already loose on the control, a failed attempt at putting it back on sent it flying across the room...

[merlinmaxwell]

he little C clip holding the outer shaft in place was already loose on the control, a failed attempt at putting it back on sent it flying across the room...

A quick repair is use some wire wrapped round and twisting the ends together snipped a couple of turns of the twist away. This will let the pot work and be so good you may forget you have done it!

[em536716]

Thanks for the tip.

I’ve now thankfully found the C clip but having trouble re attaching. Originally the control was actually bent when I got the set, but thought nothing of it until a slight touch pinged the C clip off.

I’ve attached some photos of it. Is the shaft broken? It moves back and forth freely but is firmly screwed in place. I’m a little stuck here as id like to sort this before I continue.

I’d hate to remove the pot for a closer inspection as it’s hidden under a spiderweb of wires, and the EHT bleed chain - but I’m sure I’ll have to!

[beery]

Hi Ed,
have a look here:-
https://www.vintage-radio.net/forum/...=hmv905&page=5
Look at post 97 to see what is on the inside of these pots.
If the C clip is removed and the shaft is allowed to slip backwards, the track wiper washer (for want of a better name) will fall off, which results in more play in the shaft and scoring of the track if the control shaft is rotated.

The key to putting them back together is to remove the retaining nut at the front of the pot (you have to remove the pot from the chassis to get to this).

I do have some spare morganite pots for spare parts. I might have a spare timebase control for your set from the scrap 709, but it would need some sort of repair I expect as the chassis was very rusty. I'll have to have a look in my spares box.

Cheers
Andy

[em536716]

Originally Posted by beery

Hi Ed,
have a look here:-
https://www.vintage-radio.net/forum/...=hmv905&page=5
Look at post 97 to see what is on the inside of these pots.
If the C clip is removed and the shaft is allowed to slip backwards, the track wiper washer (for want of a better name) will fall off, which results in more play in the shaft and scoring of the track if the control shaft is rotated.

The key to putting them back together is to remove the retaining nut at the front of the pot (you have to remove the pot from the chassis to get to this).

I do have some spare morganite pots for spare parts. I might have a spare timebase control for your set from the scrap 709, but it would need some sort of repair I expect as the chassis was very rusty. I'll have to have a look in my spares box.

Cheers
Andy

Thanks Andy. I appreciate the help.

Please let me know if you find something suitable spare!

In the meantime I’ll dismantle this one and try to repair it. Wish me luck!