Portadyne Princess coil puzzler - any thoughts please?

[Yorkie]

As it's been 'persisting down' with rain all day untl now, it put paid to my other plans, so I've managed to find some time to get on with restoring this little TRF set, having fitted two new droppers on a heat sink which seem to be working well and stabilise at 80C. I've renewed all the waxy caps, tested the valves, which I'm pleased to say as as good as new, but at switch on and the 'MOT' - Moment of Truth, I was greeted by slience

The audio section seems to be working OK, but not the RF stages.

Have investigated further I discovered that some wag, with the intellectual capacity of a mollusc, had been at it before me and had pulled the little tagboard off the coil beneath the chassis breaking the delicate wire connections on the coil. Unfortunately, the wires are just hanging loose so there isn't a clue as to what wire goes where. (The aerial coil above the chassis appears to be intact, but is different, so I can't make a direct comparison).

There are three separate windings - a smaller one at the top, a large one in the middle, and a medium sized one at the bottom, close to the large one. Before I can begin to consider which of the six wires go where, I need to know which coil is which as compared to the diagram. (Ignore the fact that I've connected some wires to the tags - I made some assumptions about what wire goes where and almosrt lost the will to live!).

I've attached the circuit, and I'm wondering if the large coil is that which goes from the anode of V1and the 7pF cap (which is absent on the set so has probably been removed by the phantom fiddler), and the other end to the HT line/pin 6 - the second grid of V1. On the diagram, of the other two coils, one is depicted as having more turns than the other, so I'm wondering if that is the medium sized coil. Fortunately, the continuity of all the coils is good and the resistances are: 70R, 24R, and 4R. I assume that the ends also need to be wired up in the correct phase.

It might help to ascribe numbers to the ends of each coil, so let's call the larger coil L1, the medium coil L2, and the smallest coil L3. Any thoughts on which connections go where anyone please? I'd suggest that one end of L2 goes to chassis, and the other end I'd suggest goes to the start of L3, the other end iof L3 going to the trimmer, second gang of the tuning cap and the 500pf cap in parallel with the 2M2 resistor. Then as I said above, L1 to the anode/7pF cap and second grid of V1. I'd rather try to get it right first time than to have to keep fitting and removing the coil to chage windings based on the time honoured 'SWAG' technique. (Stupid Wild Assed Guess).

I've attached a not very good pic of the coil, and the circuit.

Any thoughts would be much appreciated - as the bard wisely wrote: 'In the kingdom of the blind, the one-eyed man is king'!

[Mark R]

David,
just a guess here, but the circuit shows a slug for two of the coils, can you remove the slug and see which coils they relate too?
That would narrow it down by one third, and from memory the larger coil is usually MW, but I may be totally wrong.

cheers Mark

[Yorkie]

Thanks for reading the post Mark, and for your comments.

There's a slug at the top, and one at the bottom of the coil so in fact all the coils are subject to slug tuning. (No doubt they've been twiddled with). If you look at the circuit, as shown you'll see that the twin gang wavechange switch is open circuit, so both the upper and lower coils are in series to give maximum inductance to tune the LW band, but when the wavechange switch is closed, it shorts out the lower coil, thus reducing the inductance to tune the MW band. The other coil is the coupling coil, but the puzzle is: which coil is which?

I think I may check the resistance of the three windings on the aerial coil, because logically they'll have similar resistances to the oscillator coil. I can then wire the oscillator coil in the same configuration. I got so annoyed that some numpty had butchered the coil that the red mist came up and I reached my frustration threshold, so I shut the workshop door on it before I started hyperventilating

I've warned this radio in no uncertain Basil Fawlty terms that unless it bursts into life and starts behaving, I'm not going to make a back for it, but I'm not convinced that it's listening.

Oh well, I'll have my happy face on tomorrow, and on Saturday I'll be hooking up with Colin at Rufforth Autojumble at York to have a good rummage around.

[Joe]

Would there be anything to gain by comparing it to the Nobel you got at the NVCF?

- Joe

[Mark R]

I would guess the slug at the bottom is the MW coil then, the top coil LW.

good luck.

cheers Mark

[pwdrive]

Hello David, looking at the circuit I would have thought that the two tuning coils on your mystery connections coil would have a similar number of turns and a similar resistance to the two tuned aerial coils.
The large coil could be the coupling coil and a self resonator.
What does the oscillator do?

Lawrence.

[Steve_P]

First, source or obtain a 7pF capacitor.

Now, one coil is separate, this is the one that goes between the anode of the 12BE6 and the HT

The others are linked together, and one of them has a trimmer and a switch. This coil is probably the larger one, as it is shorted out for MW. The other coil starts off with the 7pF capacitor, the other end of which goes to the HT end of the separate coil mentioned above.

The Antenna coil is similar, so copy it.

Meter out the coils and you should be able to find out which is which by the ohms reading.

Cheers,

Steve P.

[pwdrive]

Looking at the photo's it looks as though the aerial coupling coil is designed to resonate with it's throw out 15 ft wire aerial on MW, the 7pf capacitor is top coupled from the aerial to the tuned circuit to give a more uniform gain, the anode circuit of the RF amp is the same, the coupling coil is probably designed to resonate near the old droitwich frequency of 200 Khz ( High side) resonating near this frequency would also partly compensate for the reduction in gain at the low end of the MW band.
As to physically which way round the coupling coil is connected then try it and see, one way will give max gain the other way might cause the RF amp to oscillate.

Lawrence.

[Yorkie]

Hello David, looking at the circuit I would have thought that the two tuning coils on your mystery connections coil would have a similar number of turns and a similar resistance to the two tuned aerial coils.
The large coil could be the coupling coil and a self resonator.
What does the oscillator do?

Lawrence.

Thanks Lawrence - I'll check the aerial coil for comparison.

As to what the oscillator does, of course it doesn't! I must have been asleep at the wheel when I wrote that - it's a TRF not a superhet, so it's the reaction coil. Except that at present, it's a corpse.

[Yorkie]

Thanks for the helpful advice and encouragement guys.

I do have a 7pF cap, so that isn't a problem.

As to the 'Noble' set Joe, that has quite different much larger air-spaced coils, but I guess that the relationship of the turns will be similar, if not the actual resistances, as of course it's the inductance that matters - not the resistance of the windings, and a slug-tuned coil will probably have lower resistances due to the inducatance introduced by the slugs, but I won't know till I've checked. At least the Noble hasn't been got at, so I'm hoping that a new dropper and changing the waxies might be all that's needed, plus a back panel. It uses all octal valves (12K7, 12J7, 35L6, 35Z4), which all test like new.

I'll have a little furtle around with the coil later on today. At least the windings aren't open circuit - just torn from their moorings.