MARCONI 851 :- my best effort so far

[jmedley]

This was the second pre-war Marconi that I have worked on in the last year.
The first being a 346 that had B7 valve bases.

This later model 851 has an extremely similar circuit but uses Octal valves.
Both of these Marconi's use an RF amplifier valve prior to the mixer.
They are from the period 1935 to 1939. when they were extremely expensive and then the state of the art for domestic sets.

The last time that this 851 appeared to have received attention, was around 1945 to1950 when the top of chassis mounted box of electrolytics, was removed and discarded.
The replacements had been hurriedly attached beneath the chassis.
It was just a botched up job where the very large sized electrolytics could swing about like a bunch of grapes, one was connected to the wrong point in the circuit.
So much for the misty eyed memories of those early "engineers"
The cabinet had all its veneer intact but cosmetically poor, so I decided to strip it right back and re-finish it.
The large oval speaker worked, but the cone was flapping about due to the breaks at the outer edges.
I used lots of copydex and kitchen roll to repair it.
The inside of the cone is now not pretty, but when mounted it is out of sight
and performs very well. Using a small box mounted in the original position,
I fitted modern electrolytics and wired them into the circuit. The valves were all good, the waxies were all replaced, together with 2 out of tolerance resistors.
The wiring on the top side of the chassis was crumbly especially around the output transformer, so I dismounted it and replaced all of its external wires with silicon cable.
The dial was very dirty but had all the station names intact and needed cleaning.
The gold station pointer was repainted
I spent many hours scraping and cleaning everywhere and then some painting of the top surface of the chassis.
All of the cans were removed and all of the mica caps checked to be OK
The wiring to the valve top-caps were replaced bar one.
A full re-alignment check found that only small amount of tweaking was needed.
Thats enough for now.
Maybe I could continue later about the cabinet

Jim

[vinrads]

That's a very clean chassis Jim. I have one of these they work very well, as you say a tuned rf stage is a bonus. The dials stand the test of time unlike some others. Well done with the restoration, regards Mick.

[HamishBoxer]

A very nice job to say the least.I believe I have one of these in storage but we dont get the keys to the new house before the end of April.

Afraid it will be a while after that before I restore anymore of mine.

[jmedley]

Thanks, Vinrads and Hamish for the kind words
Jim

[turretslug]

That cabinet has come out beautifully, and that chassis is almost too good to be safely hidden away! Yes, that back-lit scale has held up well, without the stewed-tea darkening of some in that series.

I really like these sets- that chassis seemed to last for a few years in a time of rapid change and whilst the woodwork detailing changed, obviously to mark yearly models and tastes, they're all solid and nicely put together with some lovely veneer. The chassis went into lots of models and, whilst it changed from 4 volt B7 to 6.3 volt IO valves over the run, there was very little other component change other than transformer heater winding, implying that the valves were essentially re-based, re-heatered and tweaked and that the chassis likewise was pretty much right to start with. Some got magic eyes and/or a second SW band but the core was established. With the RF stage, dual-rate co-axial tuning, potted-down AGC for the IF amp and simple, sturdy chassis, they surely represent a point of goodness in simple-to-use mainstream AM radios in the way that the Hacker Mayflower represents that point in mainstream FM radios. I reckon that the IFTs with their sturdy ceramic biscuit support frame and shaped iron-dust cups would cost about as much to make now as a whole DAB radio innards!

Well done, sir- a piece of furniture to cherish.

[jmedley]

Thanks turretslug

[Phil G4SPZ]

Lovely radio, Jim, well done! This series of big pre-war Marconi/HMV sets are among my favourites too. Some years ago I restored a model 559 which is now in Griptons showroom.

Quick question - is the dial lamp in your set a single-ended tubular SBC 240 volt 15 or 25 watt type, about 3" long? I have a couple of spares if you ever need one.

See you at the NVCF this year?

[jmedley]

Thanks Phil
The original lamp holder is SBC but there was no lamp fitted. I dismounted the SBC lampholder and made an sbc "bijou" adapter from and old lamp I made a small convertor cable with the sbc bijou one end and a B.C lamp socket at the other, which I cable tied to the reflector / heatshield. I had in the drawer an orange 15 watt B.C. pigmy lamp which worked OK.
I can now easily reverse the situation.
Trader sheet 350 quotes 15watt 230v

Thanks for your kind offer of an original lamp, if you are going to NVC next month we could meet up again

Jim

[Phil G4SPZ]

Hi Jim,

The original bulb rating was indeed 15 watts. That rating isn't available any longer but I got a 25 watt version, same cap and envelope size and shape. In my Marconi the lamp is run from a tapping on the mains transformer at about 200 volts, and it performs very well. Your chassis looks so good that it deserves an original lamp.

I'll try to dig one out and bring it to the NVCF. It'll cost you the price of lunch!

[McMurdo]

very nice job, my kind of set. I'd like to hear about your efforts on that lovely figured cabinet

[jmedley]

Hello McMurdo. During the last few years I have tried several methods and products some OK and some not so good.

This is how I completed the Marconi 850 cabinet

If I attempt a complete re-finish, then I must be very patient and have a lot of free time available
“Preparation is absolutely everything” hackneyed phrase but very true
Vacuum everything using an old paint brush to poke out the stubborn bits of dirt
Strip every thing off the cabinet, chassis, speaker, speaker frets, dial frames etc. etc.
Clean everywhere inside and out with soap and water make sure to clean out the control spindle holes.
Materials I use :-
1/ Home strip Paint and Varnish Remover, usually from Tool-station, this has no fumes is safe and not volatile
2/ 600grit, 1000 and 1200 grit. wet/and dry paper
3/ Water soluble Polyurethane
4/ home-made cotton wiping pad to apply polyurethane (usually a bit of an old cotton shirt)
5/ T. Cut, the original type

Slather on the stripper and after about 20 minutes scrape the muck off with a scraper then repeat until its mostly gone. Following up with more stripper then I use one of those “one-sided” razor blades and scrape with a trailing edge .
I try to remove 95% this way. When all dry sand down the cabinet starting with 600 grit and then 1000 and finish with 1200 trying to get the veneer as smooth as possible being VERY careful
near the edges
Then with a smallish amount the polyurethane in a saucer, I thin it with about 10% water
Wipe
Next apply the non thinned polyurethane with the cotton pad, along the grain
When dry check for ridges and blobs, care fully & lightly sand down these blemishes
Apply one or more coats , when dry sand down any imperfections
Finish with a of coat 10% diluted

Again when all dry burnish with the Tcut and polish off when the Tcut is dry
Wax polish

[Phil G4SPZ]

That's a really interesting process Jim. Basically you are using water-based polyurethane varnish, but with the same 'rubber' application technique as used in French polishing. Do you use gloss finish polyurethane? The end result looks lovely.

[jmedley]

Hi Phil
I have tried both mat and gloss. This time it was gloss but of course the T Cut
provides the finished effect whichever one is used.
I was pleased with the pad application method rather than brush, so long as you ensure that the surface of the pad has no folds or wrinkles.

Jim

[MurphyNut]

When re-varnishing my wooden radios I tend to use a brush and wet and dry back the finish between coats to remove any brush marks, then a final T-cut. This is time consuming but I like the "depth" to the finish and it doesn't look too new.

[jmedley]

Yes the T cut is a good final finish answer:
however I have not tried "Burnishing Cream" as supplied by Liberon etc.
I use the cotton pad not as a rubber, but just to wipe the polyurethane onto the cabinet, I find that it is smoother than a brush finish and does not drip.
We all use our own preferred methods which is fine, these are just how I do it

I am about to try replacing the Marconi logo using a water slide transfer, this will be my first attempt!!

Jim