[Antipodal]

Hi everyone,

Some of you may have read between the lines on my recent post about the Air King Model 222 and felt a vague disquiet.I mentioned that it seemed not to be very powerful and I suspect I now know why.

On first checking out this radio on the bench , as I mentioned. I had no schematic and only a scruffy bit of what we in N.Z. call "mains" cord .And there was this barretter in the mains line.And I am unfamiliar with a.c./d.c. sets.Furthermore I was ,  and am ,  very aware of the danger of fizzing the filaments of the two 25 volt tubes as these are rare in N.Z. and hard to replace .So the plan was to start from zero volts on the variac until I had the right voltage across the filaments of the valves.

 So I reasoned that if I wired the variac one lead  (the black )to the switch and thence to the chassis and the other (the red ) to pin 6 of the 25Z5 (the start of the filament chain) and also to  the anode of the 25Z4  and turned up the variac till I had 6.3 volts across the 6D6 I should have 25 volts across the 25Z5 and 25 volts across the 25L6 filaments and no more .Which I did!

Does all this make sense?

At that point I measured the voltage from the variac across the two "mains "leads ( the red and the black) and found it to be 65 volts. And the radio played at that voltage.From this I falsely concluded that the right voltage for this set was 65 volts and it was the job of the line cord to reduce the 117 voltage to 65 to make the radio run. I can see you all shaking your heads and saying "Take another look at the schematic , David".

 Yes, there has already been an "Aha! " moment. The "hot"117 volt lead should go to the anode of the 25Z5 and THEN to the line cord and then to pin 6 of the 25Z5 and the rest of the filament chain.This would give a higher H.T. voltage and probably a higher output.And the correct voltage across all the filaments.

The blushes are fading now and I am working out how to get this set to go on our 240 volt mains. I have several 240 / 110 volt transformers and,as mentioned, one 240 /65 volt transformer so I think I have the problem solved with the transformers hidden in a box and four wires from the box to the set we will be in business.
It pays to take your time in this radio fixing game. 
Cheers everyone.

[BrendaAnnD]

There is a simple fix for the heater string. Someone here will have the formula, unfortunately, I am not one of them.

It involves the replacement of the resistance part of the line cord with a capacitor that will drop the mains from 240 down to 65V@50Hz.

As far has the HT (B+), you will have to computer the total current draw from the rectifier tube and add a resistor of the proper ohm value and power. This would have to go on the anode side of the rectifier, since those are not designed to operate from a 250V mains.

[Radioroslyn]

Here's the heater dropping info  UK Vintage Radio Repair and Restoration - Dropper Calculations (vintage-radio.com) I think you'll find the spreadsheet at the bottom most helpful.
The hv w/240v mains you'll need to get rid of 130v or so @ 30ma which is abt 4.3k @ 5w or better. It maybe be a little less than 30ma.
YMMV

[morzh]

It is a formula from the grade 9 physics of the soviet school program.

The Z=SQRT [(1/(6.28*f*C)^2)+R^2).
C is your cap in Farads, R is your total filament resistance (your voltage, say 25V over your current, say 150mA; given as an example.
Then you calculate the C such that the total U (say 220V) over Z gives you your nominal current (the mentioned 150mA).
For 120V it is about 6.8 to 8uF in most cases.

[Antipodal]

Big thank yous to BrendaAnnD,Radioroslyn and Morzh for your helpful replies . I have checked out the U.K. Vintage Radio Repair Dropper Calculations site. It is a model of elegant and accessible  explanation  and very helpful.At this stage though , I will probably stick with the transformer(s) option.
At the moment I'm pleased to report that the 1939  Chrysler/Philco C-1608 is now working again.I replaced most of the capacitors and repaired the speaker  cone and the set has proved both sensitive and powerful.The push button station selector responded very well to contact cleaner and now works well.I had to fabricate a new shaft for the tuning condenser as this had broken off right where the shaft comes out of its cover tube.
Next up is the Packard / Philco P4535. I have yet to figure out how to repair the manual tuning shaft  assembly which is used when you want to tune to a station other than those selected by the push buttons.This shaft,1/8 inch in diameter is the one turned by the big chrome knob in the centre. The thread on this shaft has been identified as left hand 1/8 inch American buttress thread.The thread on the rod is sound but unfortunately the nut in which it turns is stripped and that nut is the one which pushes and pulls the ferrite slugs in the permeability tuner.So, yes, I can still run the radio using the push buttons but not do manual tuning.If anyone has a nut of that specification I'd love to hear from you.
But a big shout out to the Philco Phorum and all the members.It is a great place to air one's concerns about the radio being worked on and I'm always so pleased and delighted to get such rapid and helpful replies.Cheers everyone.