Replacing resistive line cord on Pilot Little Maestro

[valve-fan]

Greetings....

I'd be grateful for advice on replacing the resistive line cord on a 1940's era Pilot Little Maestro (wooden case) with a modern three core mains cable.

Studying the circuit diagram and Forum posts, it appears the biggest voltage drop required is for the valve heater and pilot light chain and that this can be achieved using a series capacitor. ( Approx 68v needed?)

Would a series resistor be the best way of supplying the HT rectifier circuits? (Given the voltage drop required would be fairly low and that heat disipation in a crowded chassis would not be a problem?)

If the above is not too far off the mark, what capacitor and resistor values/types will be required?

The circuit diagram indicates the existing (resistive) mains lead has taps for HT and pilot lamp/heaters.

I've never worked on an AC/DC circuit or encountered a resistive cord means of mains supply - with the added complication that I've inherited this receiver as a family heirloom and have yet to take delivery.

I'm told that while "complete", it has spent the last 60 plus years stored ed in a very dusty attic. I have replaced caps and out-of-value resistors on domestic receivers from the fifties and sixties, but am a bit out-of-my-depth re replacing a resistive cord with a modern mains lead.

Pics of the chassis suggest severe lack of space might be challenge re finding space for an extra cap and resistor and I'm having difficulty working out how to fit them (and a new mains lead) - and where the connections should be made.

(I've never found translating cilcuit diagrams to practical component lay-outs easy when building even simple valve receivers.)

Could a Forum member who has done a similar job or has greater experience/expertise than myself, kindly supply a sketch showing how and where a suitable value cap and/or resistor can be installed? (With the component values..)

As a newcomeer to the Forum, I'm not sure if this is an appropriate request. Apologies if it's not.

Might it be better for me to ask for a Forum member willing to help to PM me -and perhaps we could arrange an e-mail exchange for me to see a sketch diagram?

The Forum is a great resource .... and I'm very glad to have found it.

Many thanks.

Jack (located in west London suburbs)

[Radio_Dave]

There were several models using the same name. You'll have be certain which model you have but if you look a post #8 in this thread http://vintage-radio.net/forum/showthread.php?t=22573 you see photos of how I did my pre war Little Maestro.

Do not use 3 core mains cable on an AC/DC radio and in case you are not aware, that old line cord is lined with asbestos, so be careful how you deal with it.

David

[valve-fan]

Many thanks, David (Radio Dave).

Yes - just realised (and unfortunately after posting) that 3-core mains cable must never be used with an AC/DC circuit.

The link is useful - thank you for it.

Will report back when radio reaches me and I can ID precise model number.

Can I PM you for additional advice after I have a model ID?

Best wishes,

Jack

[Radio_Dave]

Hi Jack,

It's nothing personal but it would be best to use this thread for help and advice. Using PM's negates the purpose of a discussion forum and breaches forum rules too.

You will also benefit from the combined wealth of knowledge from all the forum's members and not just me.

Regards
David

[valve-fan]

My mistake - apologies to all.

I am grateful for advice from as many Forum members as possible and will use public postings in future.

Best wishes,

Jack

[julie_m]

I'm sure forum member Ed Dinning could wind you up a suitable transformer to supply your set. It would run cool, cost less to run, and -- if it's a proper transformer, not an autotransformer -- you could even earth the chassis.

[Mike. Watterson]

If keeping it "original" then you need a suitable sized earthed inline metal box with a suitable wattage and value resistor. Do use earthed cord to box and captive double insulated two core from box. In use the dropper housing case should only be warm to the touch, so probably a lunch box sized eddystone style aluminium box is needed. A plastic one will not let the heat out, which could be 25 to 40W?

A suitable wattage regular 220v-240V mains halogen lamp can be used as a dropper, but as the resistance varies with power (good for this application!) it needs a lower wattage than the resistance suggests. The resistance cold of Halogen can be 1/2 the hot resistance.
The radio case usually can't take the heat of an internal dropper.

A capacitive dropper is a design change.

A 110V isolating transformer is less original but safest. Usually these are USA models adapted to UK by addition of a line dropper.

[valve-fan]

My thanks to AJS and Mike W - I'll have to wait until the radio reaches my work-shop, but I suspect there won't be room to add a transfomer: I'm worried about finding space for a couple or resistors and a cap.

I'm not concerned about preserving originality (altho' I respect the reasons for it) - so introducing a cap and resistor(s) for voltage dropping isn't an issue. Any further advice on values/types for these components and a replacement pilot lamp (likely to be needed) wd be appreciated once I've identified the precise model.

Also, any tips for the ID process wd be useful - I don't yet know whether any ID "plate" or other markings remain.

Family folk-lore includes use of the words "Pilot" and "Little Maestro" in connection with the radio, which stood for many years on a shelf in my grand-parent's kitchen. But - apart from it having a wood cabinet - that's all I know at present.

Jack

[David G4EBT]

If the wooden cabinet has square edges at the front, rather than rounded, it will most likely be the post-war model and will have a heater transformer and be AC only, so you won't have the hassle of converting it, but you'll have other work to do to bring it up to scratch - replacing waxy paper caps, maybe out of spec resistors, and perished wiring etc.

The pre-war rounded corner model had a line cord dropper.

I had a square-cornered model with no mains lead and before I dismantled the set, had assumed that because it didn't have the BBC Third Programme marked on the dial, it was a pre-war line-cord model. However, when I took the set apart, I saw the heater transformer, so it was the post-war AC only model. (It was made in 1946, so pre-dated the Third programme. Hence, no dramas about the need to replace a line cord).

If indeed yours is a pre-war line cord model 'Radio Dave' has outlined how to overcome that by using a capacitive dropper, for which he has shown the circuit changes needed, and te componenet values. (There is space inside the cabinet).

The term 'Little Maestro' was used extensively over a period of many years for widely differing models - the later T105 for example, which had a brown Bakelite cabinet, though many were factory sprayed in bright colours such as sky blue as Bakelite was by then falling out of favour.

Good luck with the restoraton. When you know exactly which model it is, if you do a search on the forum, and post a few pics, you'll almost certainly find that one or another member has 'trodden the path' before you, so you'll have lots of pointers.

[Herald1360]

If you haven't already found it, www.radiomuseum.org is a good resource for identifying old sets from pictures.

[valve-fan]

Brilliant - thanks G4EBT (David) and Herald1360: you and other Forum members have made my first day on here very worth-while.

I now how enough info. and sources to make a start on assessing my particular Pilot Little Maesto, whatever the variant - something I'm looking forward to discovering when it (finally...) reaches me.

I hadn't realised there were so many variants - assuming from a quick Internet search (not initially covering this Forum) that all were AC/DC using resistive line cords. I now know better....

Having learned so much about replacing such cords, I'm now almost hoping mine has one - almost, but not quite. I'd be much more confident dealing with an AC-only transformer model- and replacing waxy caps and out-of-value resistors.

Will report what I find in due course....

In the meantime, again many thanks.

Jack

[julie_m]

Note that in many of the AC-only versions of these sets, the transformer is only supplying the heaters; the HT is coming straight from the mains, and the chassis is live.

[Brigham]

The line dropper is one of the features which makes these sets more interesting than other run-of the mill types. For that reason, I had mine professionally restored with a new line dropper.

[valve-fan]

Impatience to know if the Little Maestro had a resistive mains cord meant a long cross-country drive to collect it - but I can now reveal it's an AC only model with a mains t/f showing no heat or other damage (still need to be tested, of course).

The wood cabinet is battered with the rear card-board cover broken, but the wiring looks complete with no signs of having being "got at". Waxy/paper caps will be replaced along with out-of-value resistors and stage-by-stage tests carried out.

Would replacing the existing two-core mains lead with three core just require connecting the earth wire to chassis?

Jack

[Sideband]

Originally Posted by valve-fan

Would replacing the existing two-core mains lead with three core just require connecting the earth wire to chassis?

Before you attempt this, you MUST ascertain whether the transformer only supplies the heaters or if it is a full mains isolating type. If the former, then the chassis will be connected to one side of the mains and may be live and you MUST NOT connect an earth lead to it. If the latter type then in theory you can connect an earth. Whether this is necessary is really a matter of personal preference.

There were a great number of Pilot radio's that just had a heater transformer and could not be earthed.

Originally Posted by ajs_derby

Note that in many of the AC-only versions of these sets, the transformer is only supplying the heaters; the HT is coming straight from the mains, and the chassis is live.

As stated above. More correctly, the chassis is connected to one side of the mains and may be live. If this is the case, you only need to make sure that the chassis is connected to the neutral side of the mains but must still not be earthed.

[Refugee]

I am intending to convert a Marconi T11DA to 110V and be done with the line dropper and I bet it was designed to be a potential export model for sale over the Atlantic in its day.

There is no space for a transformer inside the tiny cabinet.

The chassis works well running from the outputs of a dual channel bench power supply that gives me 60V 0.5 amp plus 30V 1A in series after restoration. The valves are in good order to get this with the entire set slightly under run but with conversion to 110V AC It should run well.

It belonged to my great aunt so it also has family connections as your set does.

I intend to fit a connector that only fits one or other of a number of 110V transformers I have and this will most likely be the two flat pin type hard wired to the set.