Marconi table top radiogram - 30s?

[RojDW48]

Hello - can anyone help me identify this dear old thing - the little type plate is missing. It's a Marconi 78rpm tabletop radiogram. The radio works really well but I'm struggling with the record deck.

[yestertech]

That looks like an RG22A
The service data has some info about motor and pickup maintenance, auto brake adjustments etc .
It's dated June 1951 so the t/table suddenly looks old fashioned !!

Andy

[peter_sol]

What seems to be the problem with the record deck?

[RojDW48]

Hi - thanks - Yes, if it's 50s it would be well behind the times! Problem with deck is the motor doesn't run - but I haven't yet established whether or not the levers are actuating the switch. I should be able to take a look tomorrow.

[paulsherwin]

Microgroove LPs only arrived in 1948 and took at least 5 years to become mainstream. This radiogram was made towards the end of the 78 -only era but isn't all that unusual.

What is unusual is that it's survived with its original deck. Most 78 decks were replaced with 4 speed autochangers in the late 50s / early 60s.

[Techman]

Yes, it is a bit behind the times when you consider that GEC were fitting the Garrard RC72 which had all three speeds and a light weight pickup in their radiograms of 1950 - I have one.

Much more to the point, though - you say that the radio works fine. Well, I wouldn't run it for too long unless a certain capacitor has been changed, as you may not be able to switch it off quick enough before the smoke and possible flames - and that will be the end of one or both transformers, output valve and rectifier, which will make it not really worth repairing. This, by the way, was exactly what happened (with its previous owner) to the 1950 GEC gram that I mentioned above, which basically made it scrap.

Do a forum 'search' for "that capacitor" if you want to know more. The actual part is the grid coupling capacitor to the output valve. You may already know all about this capacitor, but just in case

[RojDW48]

Many thanks - I shall take a look. I have very rudimentary circuit reading skills but I assume it is directly connected to the output valve? I guess I should change the smoothing cap as well.

[Techman]

This may now sound like a complete contradiction after what I said about 'that' other capacitor, but why would you want to replace the smoothing capacitor? Is there a lot of 'mains hum' from the speaker regardless of whether the volume control is turned right down to minimum? There seems to be a little bit of an obsession from some folk for the random (or complete) changing of parts for no good reason. If there's mains hum as mentioned above, or the metal case starts to become warm after a short time then it needs replacing, otherwise leave it alone. If it's lasted over 60 years and is still working, then it may well last another 60 years. How do we know how long a replacement electrolytic smoothing capacitor will last? Many of these high working voltage 'double cap', ie, two-in-one type capacitors may not have been manufactured for many years, so the 'new' one that you buy may be 'old stock' that's been kicking around in storage for many years and be not much better (or worse) than the one you've just chucked out - this is just my opinion, though!

As I said before, the capacitor that MUST always be replaced is the one that couples the anode circuit of the first AF amp to the control grid 1 of the AF output valve. The modern version of this particular capacitor IS BETTER than the original version that are ALWAYS electrically leaky, and it only takes a very small leakage to put a small positive voltage on that grid that can cause so much damage.

Getting back to your question about the deck, it will be very easy to find out why it's not working, but may be harder to actually put right depending on what's wrong. With the unit disconnected from the mains, check for continuity with a ohm meter through the motor. Check the same across the deck switch when moving the 'start' lever to start. If all is good, check for voltage at the motor when the unit is connected back to the mains and the deck switch is 'on' - BUT TAKE CARE - high voltage can kill!

The magnetic pick-up head will probably want servicing. The armature that holds the needle is mounted in a rubber suspension. This is likely to have gone rock hard or disintegrated. Don't even think about trying to play any 78s on it in this state, it will shred them - use a scrap one that doesn't matter for testing. You'll need clock/watch makers skills to work on the pick-up head and there are very fine wires inside.

I had a look the other day, and the service info is available from the link at the top of this page - three versions to choose from, I think. The manufacturers version should be good for £1.99.

Good luck with it

[RojDW48]

Many thanks - I have been distracted by an old Bush/Garrard record player recently but I did check the switch which was OK. The motor has 2 big 1mF capacitors attached (suppressors?) I'll let you know when I've got a bit further. Thanks again for all the info.

[Heatercathodeshort]

The two 1uf capacitors suggests that it is a Hysteresis motor. The capacitors will be useless and will require replacing with non electrolitic components. I have scanned the EMI Service manual 'Electric Gramophone Motors' section 5 for July 1939. The principle is the same but yours may be a light weight version. Hope this helps. John

[G4XWDJim]

Someone has asked me to repair what sounds like one of these Motors. All I've been told about it so far is that it has a large rotating disc in it.

I assume that apart from the capacitors being leaky and the possibility of shorted turns or open circuit windings there's not much to go wrong.

A question please though for those with experience of these motors - when properly cleaned and lubricated how much torque should they have?

Jim

[RojDW48]

Thanks John - that looks very helpful. The motor drive appears to 'lean' on the outside edge of the turntable - I dread to think of what we would now call 'rumble' which might result!

[peter_sol]

I think your motor has 2 caps. one is a dropper cap and the other is a starter cap
Neither is critical in value.
You need to dig them out of the wax. If the values are 1.1uf and 1.3uf I have replaced those with simply 1uf at 1000vdc and the motor ran faultlessly.

[RojDW48]

Hi - thanks - I have found this via RS - Ducati Energia 1μF ±5% 500 V ac Polypropylene Film Capacitor 28mm - under motor capacitors, is that suitable? It's not a DC motor is it?

Yes there are 2 caps

[peter_sol]

Those should be OK No it is not a DC motor.

[Heatercathodeshort]

Originally Posted by G4XWDJim

When properly cleaned and lubricated how much torque should they have?

Hello Jim, Sounds like it is the massive pre war hysteresis motor that drove the turntable and mechanism on the EMI record changer K3. Plenty of torque and very silent in operation. A bit like an 'electric light meter'! John.

[RojDW48]

The valves all appear to be original and comprise X142, WD142 X 2, N142, U142. I know the U142 is a rectifier - are the 2 WD142s push/pull output or something to do with the receiver side?

[AC/HL]

X142 = UCH42 is the frequency changer
WD142 = UAF42 is the IF amplifier and AGC rectifier
WD142 = UAF42 is the AF amplifier and detector
N142 = UL41 is the AF output.

http://www.service-data.com/product....62/4655/m13462

[RojDW48]

Many thanks - that's very helpful (and thanks for the equivalents) - I had almost worked it out by looking at - http://www.******************/page_m.htm - which is free (I expect you all know it!) and came up with the circuit for the 'Companion' T18DA, which appears to be the same radio/audio unit minus the gram section. I assume it is the AF coupling capacitor I need to replace?

[RojDW48]

OOPs! did I break a rule? I tried pasting a link and some of it is blank and it doesn't work.