01-06-2012, 08:27 PM
(01-06-2012, 07:29 PM)Yorkie Wrote: 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.
