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Old 5th Sep 2017, 12:43 pm   #1
1100 man
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Default Marconi model 557

Good afternoon,
It's not often I post in this section of the forum as my focus is usually '50's & '60's television. However, I accidentally bought this rather handsome radio whilst at Beaulieu autojumble at the weekend! It had belonged to the seller's Grandfather and he remembered it as a boy. Sadly he was not allowed to keep it as it didn't fit with the new decor!
Before I knew it I had parted with £15 and struggled off with it- it sure is heavy!!

It looks to be complete and unmolested- the only obvious replacements are two 'Radiospares' caps done a very long time ago. It has a lovely TCC 8mfd electrolytic in a paper sleeve- I wonder if that will reform? It still has a 2A 2pin plug so hasn't had any use for a long time.
All valves are present although the U12 rectifier looks a little white so may not be too well!
The other valves are a W42, X42, DH42, another X42 and a KT42.
Apart from the mains cable, there doesn't seem to be any rubber wiring so no crumbling insulation to deal with.
It has a very large energised speaker which looks to be in good order.

My plan will be to replace various critical capacitors, check the insulation resistance of the mains transformer, try to reform the electrolytics and then go from there.
I just wondered if others had thoughts , comments or experience of this model. It's very solidly built and with that large speaker should hopefully sound very nice. I would guess it's age to be 1946- 1948 ish? although I may be miles out!
All the best
Nick
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Old 5th Sep 2017, 12:53 pm   #2
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Default Re: Marconi model 557

Hello Nick,
What a great purchase for £15! - it would be Pre-war 1937 or so
Steve.
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Old 5th Sep 2017, 12:54 pm   #3
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Default Re: Marconi model 557

It's a bit older than that- Trader sheet gives 1937 as release date.


Should be quite a good set with its RF stage up front. Underchassis view looks to be quite uncluttered too, always helpful.


Service sheet available "up top"- use the Trader 559 one not the 879. The Trader one seems to be more useful than the Broadcaster one.
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Old 5th Sep 2017, 1:03 pm   #4
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Default Re: Marconi model 557

A gift at that price and no doubt will perform well with the usual bits changed.
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Old 6th Sep 2017, 9:57 am   #5
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Default Re: Marconi model 557

Many thanks for the replies and the pointer to the correct 'trader' sheet. I didn't think it was as old as 1937. Strange to think that my father was a 10 year old lad when it was made- he will be 90 years old very soon!
Should be fun to restore, although it will be a 'minimal intervention' exercise. I will only change essential components.
Come to think of it, it does have a bit of an 'art deco' look to it which is consistent with it being a '30's radio.
I will update when I've had a proper chance to have a good look at it.
All the best
Nick
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Old 6th Sep 2017, 10:20 am   #6
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Default Re: Marconi model 557

I must try that on swimbo (accidentally bought) These are very nice radio's the dial gives a lovely warm red glow , good luck with repair . Mick.
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Old 6th Sep 2017, 10:36 am   #7
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Default Re: Marconi model 557

These are a nice family of sets from a time when the superhet "wireless" had really come of age and a set like this with RF stage and SW would have been a source of pride and interest. It's odds on that Mr. Chamberlain's weary voice would have been heard through that very speaker one September morning.

The original reservoir/smoother was an assembly of several units in a block where the two holes at the rear RHS are and were often replaced by individual components dotted around the chassis in an era when folk were less interested in "originality" and more interested in keeping it going pragmatically. Some characterise a non-original set as "got at"- I don't, I actually like this sort of thing as it implies that a set was good enough to still be used 20 or 30 years on- another universe in electronic progress and consumer fashion terms!- and it was felt worth getting back into action quickly and reliably. Rather than being all-original because it was felt to be so-so and got put in the attic after a few years. Personally (and opinion will be very divided here....) I would fit new, suitably rated electrolytics (your choice whether to restuff/otherwise conceal) as the ones present aren't original anyway. It'll be cheap insurance for the difficult/expensive to replace mains transformer. Bear in mind that any set with field-coil smoothing and directly-heated rectifier will have off-load HT far higher than running HT for several seconds before the other valves warm up (frequently in the vicinity of 500V), so replacement capacitors should have generous ratings.

Good luck- a "proper" radio.
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Old 14th Sep 2017, 11:59 pm   #8
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Default Re: Marconi model 557

Good evening,
Well I've had a chance to have a much more detailed look and start doing a few checks.
The correct 'Trader' sheet seems to be No 605 which covers this family of models. The model 557 (which I have) seems to be the most basic of the group being a table radio with no tuning indicator. Released in 1937 priced at £12/12s.
It does have 5 valves plus rectifier and a thumping great mains energised speaker and is very substantially made.
With these sort of projects, I enjoy tracing out the circuit and correlating it with the actual diagram. I find it most instructive and it identifies where components are and what is connected to what. I also make measurements of transformers and resistors and start to make a list of bits to replace or investigate further.
So far, everything is pretty good. The mains transformer tests out OK as does the speaker speech & field coils which is a relief! The mains switch and all the valve heaters are also all good.
5 of the valves are 'Marconi' ones but the DH42 is a rather attractive 'Dario' TBC14. I've not come across this name before but research suggests that it might be something to do with Philips. Does anyone have any more information?
The speaker cone is in excellent condition and moves freely which is good news. The only wiring I can see which needs attention is the ends of the mains lead where the rubber has been exposed to the air and gone hard.
The main capacitor block must have been removed many, many years ago as there is no mark where it was. One of the TCC replacements is a thing of beauty and I will try and reform it if it will behave. In fact, all the caps look very well made being in metal cylinders with an outer cardboard tube rather than the dreaded 'waxies' found in later sets. I will test them for leakage & capacity and see how good they are.
Interestingly, there are no fuses of any sort in the design- not sure what is supposed to happen in the event of a fault
Hopefully more progress to report soon,
All the best
Nick
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Old 15th Sep 2017, 6:58 am   #9
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Default Re: Marconi model 557

Nice set! Good luck with it, Nick. I restored a model 559 some years ago and was very impressed with its quality of construction.

I believe Dario was originally a French make. The TBC14 seems to be equivalent to a Mullard TDD4.
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Old 15th Sep 2017, 8:14 am   #10
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Default Re: Marconi model 557

Nice set, and an early elliptical speaker.
Love the hexagonal/octagonal knobs of this period.
The rectifier look OK, the getter is still shiny.
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Old 15th Mar 2018, 1:02 am   #11
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Default Re: Marconi model 557

It's been a while, but I've finally got round to doing something with this set. Now, I don't listen to broadcast radio, so I couldn't see what use it would be to me when it was working, so there was no incentive to get on with it!

However, amongst other things, I am partial to a bit of traditional jazz- Chris Barber, Kenny ball etc, so I thought this would be a fine use for this radio. I plan to use it with an AM modulator and with its huge speaker and large, solid case, I imagine it will sound great. So that has spurred me on to fix it!

Rather than a total rebuild, I decided on a policy of component replacement only where necessary so as to keep as much original as possible. It still has it's original cotton covered rubber mains lead, complete with 2A 2 pin plug. Sadly this is very crumbly, so will have to be replaced, but I am using it for the time being.

The main capacitor box had been removed many moons ago and some very vintage caps used to replace it. See post No 8 above for pictures. I wondered, out of sheer interest, if these would reform, so I replaced the U13 rectifier with two silicon diodes and powered up on a variac. The HT current was monitored on the AVO and the voltage gradually increased over two evenings. All the other valves were removed.

The grey 'Radiospares' cap was the first to succumb and started to get hot. The red RS cap and the lovely TCC cap both measured open circuit, so a failed mission.
I tacked in some temporary replacements, along with the output valve's grid coupling cap which was unsurprisingly leaky. Having measured and obtained the voltages I was expecting on the various valves, I decided that all was safe to plug the valves back in excepting the rectifier. Switching on, with a short length of wire as an aerial, I was greeted with stations on MW which was encouraging. The speaker also sounded good which was even more so!

The poor old U13 rectifier is past its sell by date which is sad as it's a Marconi branded one and may be original. I removed my temporary diodes, plugged the valve back in, and wound up the voltage on the variac. Suddenly, a mass of purple arcing erupted between the heater and the anodes, so I guess there's air in there I will need to try and source a replacement, but I will go back to the two diodes for testing.
That's as far as I've got at the moment, but it's an encouraging start.
All the best
Nick
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Old 15th Mar 2018, 4:09 am   #12
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Default Re: Marconi model 557

Nick, I make the rectifier to be U12 or a U12/14. Am I wrong?
U14 is the direct and better replacement, a UU5 will do too.
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Old 15th Mar 2018, 9:56 am   #13
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Default Re: Marconi model 557

Hi Sam,
No, you are quite correct: it's a U12! No idea why I put U13.
Thanks for that
Cheers
Nick
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Old 15th Mar 2018, 10:52 pm   #14
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Default Re: Marconi model 557

Hi, I agree with Phil that the TBC14 is equivalent to a Mullard TDD4. The max anode current of TDD4 is 4mA and grid bias -7v whereas the DH42 is supposed to be 1.1mA and -3v respectively. I doubt if this will cause any problems but if you do experience audio distortion (having replaced "that capacitor" C25 in Trader Sheet 262) you might want to adjust the grid biasing R14. I restored one of these sets a few years ago and in the end it worked well. The issue that had me puzzled was that it was very "deaf" on SW 49m Band. In the end I changed C12, the oscillator anode bypass capacitor, from 0.005uF to 0.01uF which improved things. If you are only using the set with a modulator (presumably on MW) then that is not worth messing with. Good luck.
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Old 15th Mar 2018, 11:06 pm   #15
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Default Re: Marconi model 557

I restored one of these back in 2007. Success thread here
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Old 19th Mar 2018, 10:14 am   #16
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Default Re: Marconi model 557

These are great sets. EMI used the chassis in several differently numbers sets and radiograms with small modifications. I have a 557 and a 559 which have the same chassis in different cases. The 559 has a tuning indicator but otherwise they are the same. I have to restrain myself from buying more when they become available. I think there was also a version with p/p output.

Fortunately the capacitor boxes on both my sets had been retained so I was able to have the doubtful pleasure of re-stuffing them. I use them regularly to receive transmissions from the various 1930s internet radio sites via my pantry transmitter.

Here is a link to my restoration https://www.vintage-radio.net/forum/...ad.php?t=74987
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Old 20th Mar 2018, 12:17 am   #17
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Default Re: Marconi model 557

Thanks Paul & SB for the links: I have now read them both. There seems to have been two ranges of valves fitted to these chassis. Mine has the earlier type W42, X42, W42, DH42 (TBC14 in mine), KT42 & U12/14 rectifier.
Looking through my valve data book, there are any number of equivalents for the U12/14. 4V/2.5A heater, 350 or 500V anode. My meagre collection of radio valves yielded a brand new MU14, IW4/350 & IW4/500. They all give more HT than the U12 should have done, but the IW4/350 with a 150 ohm series resistor seems the best combination.

My capacitor box must have been replaced when the radio was fairly new as the replacements are very vintage themselves. The TCC is a thing of beauty in itself (see post No 8). My plan is to leave them in place but disconnected & connect modern replacements next to them.
All the paper caps seem to be a metal cylinder covered in a cardboard tube rather than 'waxies'. So far, they are not causing any problems and are all still in circuit. The only wax type was the grid coupling one to the output valve which was very leaky and was replaced before switch on.
The chassis itself is in very good condition with no rust and thankfully the energised speaker is present and in excellent order. The cone looks like it came out of the factory yesterday!
So far, the only valve issue is with the U12 rectifier which produced a firework display inside due to air I imagine.
MW & LW work but are not very sensitive. In fact, connecting the aerial to the top cap of V2 produces a much louder signal so it would seem that V1 is not doing much. I need to do some more accurate fault diagnosis with a signal generator.
I still find it amazing that this is 80 years old and that most components including the valves are perfectly functional!
All the best
Nick
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Old 20th Mar 2018, 12:46 am   #18
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Default Re: Marconi model 557

Nick, before diving in with a sig gen, just check out the waveband switching and also the aerial input coils, where faults are more likely. A few resistance checks with an ohmmeter should pinpoint or rule out any problems. A low emission V1 is also possible of course!
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Old 20th Mar 2018, 1:04 am   #19
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Default Re: Marconi model 557

Hi Phil,
My next move is to remove the can covering the coils & caps related to V1 and to investigate. Before I go much further however, I want to tidy up the components already substituted/ replaced. At the moment, these are just tacked in on a ' get it going' basis and need to be installed properly! I'm waiting for some 4.7 & 10 mfd 450V axial caps from RS at the moment.
I did swap V1 with V3 just to prove a point, but there was no improvement.
Many thanks
Nick
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Old 20th Mar 2018, 3:56 pm   #20
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Default Re: Marconi model 557

Quote:
Originally Posted by 1100 man View Post
Thanks Paul & SB for the links: I have now read them both. There seems to have been two ranges of valves fitted to these chassis. Mine has the earlier type W42, X42, W42, DH42 (TBC14 in mine), KT42 & U12/14 rectifier.
Looking through my valve data book, there are any number of equivalents for the U12/14. 4V/2.5A heater, 350 or 500V anode. My meagre collection of radio valves yielded a brand new MU14, IW4/350 & IW4/500. They all give more HT than the U12 should have done, but the IW4/350 with a 150 ohm series resistor seems the best combination.
The MU14 and IW4/ are indirectly-heated, these types generally seem to have a lower forward drop than directly-heated types, perhaps due to the more consistent spacing of anode/cathode possible over a wider area, or simply having a bigger cathode surface. It does at least give the opportunity to make the equalising resistor a small power film fusible type. I'm not keen on directly-heated types anyway, the near-instant heating results in marked HT overshoot before the other valves warm up, and the tape filament can break and flap against an anode, potentially destroying the mains transformer.

The also-equivalent late-manufacture Brimar R2 in the 559 here has an electrode structure that looks identical to Brimar 5Z4Gs- I wouldn't be surprised if that's what it has, just fitted with 4V heater and B4 base for legacy support purposes.
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