Portadyne Princess coil puzzler - any thoughts please?

[AlanBeckett]

What Reaction coil?
It looks to me like a coupling coil and a Long and Medium wave coil in series. The LW coil gets shorted out when the set is switched to MW.
Repanco did it like that - see attached.
Alan


  Repanco_octal.pdf (Size: 844.73 KB / Downloads: 87)

[Yorkie]

What Reaction coil?
It looks to me like a coupling coil and a Long and Medium wave coil in series. The LW coil gets shorted out when the set is switched to MW.
Repanco did it like that - see attached.
Alan

Oh well, the older I get, the less I know, but the more I think I know. (Delusions of adequacy?). At this rate, it will end up on the pile!

Thanks for the Repanco circuit Alan, which states 'reaction winding not used'. Happy days - 'Radio Experimental Projects Co' as I recall, who used to have an adresss such as 'Much Park' or whatever. I don't know if that was the address on the circuit, (which looks like you left it folded up in your back pocket when the trousers went in the washing machine, though is perfectly readable)!

I'm not sure how the Princess arrangement works because the 'volume' control also doubles as the reaction control.

I've never experienced one of these sets working, so don't know what to expect, but hope I soon will, though I'm not holding my breath.

Right, I'm off into the workshop to have a poke around with it.

[AlanBeckett]

Can I have another Errrr?
It looks to me as though the pot simply varies the bias on the 6BA6, like my MM does. The 68R is there to stop the bias going down to zero.
Why it's returned via the Aerial coupling winding I couldn't say, but as it's decoupled at the junction of the 68R and the pot there won't be much feedback, not that you'd want much or it would oscillate.
Alan

[Skywave]

It looks to me as though the pot simply varies the bias on the 6BA6 . . .
Why it's returned via the Aerial coupling winding I couldn't say.
Alan

That pot. is the circuit designer's idea of an R.F. gain control. With strong signals arriving at the aerial, the R.F. amp. needs a larger -ve bias that when weak signals are received. So with strong signals, the wiper of the pot is towards chassis / ground. In this condition, the pot. also attenuates the level of the aerial signal reaching the R.F. amp. For weak signals, the inverse: less bias; less attenuation. Hence, the pot. does double-duty as an R.F. gain control.

Al.

[Aerodyne]

David
I haven't read through this thread so apologies if I'm repeating things - but I'd start by assuming that the smallest coil - at the top - was medium wave, the biggest coil (middle) was long wave and the bottom small coil was the coupling from the previous stage (12BA6 anode).
-Tony

[Yorkie]

Thanks for reading the post Tony, and for your comments, which seem entirely logical. However, what troubles me is that the middle coil on the damaged one is much larger and has a resistance far higher than on the undamaged aerial coil, which I've now checked and which may point me in the right direction, or at least I hope so! I've attached pics of the two coils for comparison, annotated with the resistances of the windings of each coil.

Referring to the circuit, the resistance of the windings on the undamaged (aerial) coil are:

Winding that goes to the aerial via the 0.002uF cap/7pF cap/lowerend of the pot at the top end of the widing and to chassis at the bottom end = 17R
Upper winding on the circuit for MW which goes to the grid of V1, the other side of the 7pF cap, the trimmer and one gang of the tuning cap = 3R
Lower winding in series with the MW winding when on LW, but shorted to chassis is when set switched to MW = 21R

Thus, for LW the total resistance of the two right hand windings on the circuit = 3+21 = 24R.

The winding which is the coupling coil to the aerial is the middle of the three.

The resistance of the three windings on the damaged coupling coil, the connections of which are presently unknown, are 4R, 24R and 70R.

Thus, the second coil has two windings which are close to the first coil, but one which is far higher.

I’m therefore speculating that the 4R winding equates to the role of the 3R winding on the aerial coil, and the 24R equates to the 21R winding. Thus on LW the total resistance of those two windings on the damaged coil in series = 24+4 = 28R. The 70R winding is so much higher, that I’m assuming it is most likely the coupling winding that goes from the anode at and 7pF cap at the top end, and the HT line/top grid of V1 (12BA6), and that it has a higher inductance to give stronger coupling.

I’m open to other suggestions, but does that sound logical?

The obvious thing to do now is to wire it that way, and give it a try.

It’s the most complicated simple set I’ve had in my hands, with minimal data available, and no info on how to line it up when/if it works.

I've attached the circuit again for convenience.

Thanks for reading - I hope it makes sense!

Confused, of Cottingham

[pwdrive]

Thanks for reading the post Tony, and for your comments, which seem entirely logical. However, what troubles me is that the middle coil on the damaged one is much larger and has a resistance far higher than on the undamaged aerial coil, which I've now checked and which may point me in the right direction, or at least I hope so! I've attached pics of the two coils for comparison, annotated with the resistances of the windings of each coil.

Referring to the circuit, the resistance of the windings on the undamaged (aerial) coil are:

Winding that goes to the aerial via the 0.002uF cap/7pF cap/lowerend of the pot at the top end of the widing and to chassis at the bottom end = 17R
Upper winding on the circuit for MW which goes to the grid of V1, the other side of the 7pF cap, the trimmer and one gang of the tuning cap = 3R
Lower winding in series with the MW winding when on LW, but shorted to chassis is when set switched to MW = 21R

Thus, for LW the total resistance of the two right hand windings on the circuit = 3+21 = 24R.

The winding which is the coupling coil to the aerial is the middle of the three.

The resistance of the three windings on the damaged coupling coil, the connections of which are presently unknown, are 4R, 24R and 70R.

Thus, the second coil has two windings which are close to the first coil, but one which is far higher.

I’m therefore speculating that the 4R winding equates to the role of the 3R winding on the aerial coil, and the 24R equates to the 21R winding. Thus on LW the total resistance of those two windings on the damaged coil in series = 24+4 = 28R. The 70R winding is so much higher, that I’m assuming it is most likely the coupling winding that goes from the anode at and 7pF cap at the top end, and the HT line/top grid of V1 (12BA6), and that it has a higher inductance to give stronger coupling.

I’m open to other suggestions, but does that sound logical?

The obvious thing to do now is to wire it that way, and give it a try.

It’s the most complicated simple set I’ve had in my hands, with minimal data available, and no info on how to line it up when/if it works.

I've attached the circuit again for convenience.

Thanks for reading - I hope it makes sense!

Confused, of Cottingham

Hello David, the high inductance of the coupling coil is generally refered to as a high impedance or self resonant primary ( you might already know this) it is designed along with it's self capacitance and anode stray and inter electrode capacitance to resonate at a particulat frequency, this frequency is usually below the low end of the band that is being tuned ie MW to compensate for the decreasing L/C ratio at that end of the band, in the case of your receiver it could well be resonating on the LW band .
Looking at the size of that coil i would say it is several milli henry and would tune down to 50 kcs if you were to stick a standard tuning capacitor across it.
In most radio receivers high impedance coupling coils are usually closely coupled to the secondary to which it was designed to work with.

Lawrence.

[Yorkie]

Thanks for reading my post and for your erudite comments Lawrence!

The other thing that's making me feel that the middle winding in each of the two coils is the coupling coil (The aerial one certainly is), is that the circuit shows two of the windings in each coil tuned by a slug, and one not. Given that there is a slug at each end on the coils, that figures, unless of course the cores were screwed well into the coil where the centre winding is site.

I do have an inductance meter so can measure the inductance of the windings, then work out the tuning range when I've measured the capacitance of the tuning cap fully open and fully enmeshed. Fortunately, there are resonant frequency calculators on t'internet to do the sums! From long ago, I recall a rhyme to help remember the formula: 'one over two pi root LC, gives you the resonant frequency'. (Or words to that effect!).

As to the eventual alignment, (says he, more in hope than expectation!), data sheets for similar TRF sets such as Champion ones may provide some useful info.

[pwdrive]

While on the subject of high impedance primary coupling RF coils here is one out of an old comms receiver, there are two coils on the coil former a single layer type and a larger wavewound type.
The single layer coil tunes from 1.6 Mhz to 3 Mhz, although the wave wound coil looks more like LW coil it is in fact the high impedance self resonant primary, the anode connection to this primary has a capacitor of just a few pf going from the anode of the RF amp to the signal end of the secondary (the single layer coil) this coil and its associated tuning capacitor feeds the grid of the mixer. the coils are slug tuned. Note the wave wound coil is shown approx three quaters of its original size...I nicked some of its Litz wire for anothe project....

Lawrence

[pwdrive]

Thanks for reading my post and for your erudite comments Lawrence!

The other thing that's making me feel that the middle winding in each of the two coils is the coupling coil (The aerial one certainly is), is that the circuit shows two of the windings in each coil tuned by a slug, and one not. Given that there is a slug at each end on the coils, that figures, unless of course the cores were screwed well into the coil where the centre winding is site.

I do have an inductance meter so can measure the inductance of the windings, then work out the tuning range when I've measured the capacitance of the tuning cap fully open and fully enmeshed. Fortunately, there are resonant frequency calculators on t'internet to do the sums! From long ago, I recall a rhyme to help remember the formula: 'one over two pi root LC, gives you the resonant frequency'. (Or words to that effect!).

As to the eventual alignment, (says he, more in hope than expectation!), data sheets for similar TRF sets such as Champion ones may provide some useful info.

Wot's erudite.

They allowed us to take in log tables to the C&G exams to work out reactances resonances etc. The on line calculators certainly beat that.
I need a univeral bridge, every time I bid for one on Ebay I lose......

Godd luck.

Lawrence.