Hi Guys,
Since the value of the power choke for my favourite radio
was painfully omitted in it's schematic, it occurs to me I'd
better measure it before it fails! It has no markings at all.

This is my cheap DMM from Jaycar http://www.jaycar.com.au/productView.asp?ID=QM1548.

Can this be trusted to accurately measure inductance with any accuracy?
I don't want to have to disconnect the old part more than I have to.
Cheers, Art.

It is specified at +/- 3% and will read up to 20H, so should be fine for a regular choke. When measuring, make sure that at least one end is isolated, as the power supply capacitors will upset your reading if left in circuit.

The critical factor is it's DC resistance as that vs current draw is what governs the correct voltage of the B+ rail.

Thanks for the replies, so if I measure inductance and DC resistance
should I expect to find an EVATCO (Hammond) choke
to closely match both?
or favour DC resistance and current rating over inductance value?

Coming to think of it.. It should be possible to use a different choke (in the event of the original one failing) to do away with a resistor pack that is replacing a field coil at the moment.

Art, I'm a little confused over your last post as you say a resistor is replacing the field coil and the radio also has a choke. Has the speaker been replaced with a permag type? If so, the field coil filtering function could be replaced with a resistor or a choke, but it is unusual to see both.
Can you post a copy of the circuit to clear up my confusion.
What is the make / model of the radio, as I may have its schematic available here.

Hi Gandhn,
Yes it has a choke, and also a field coil speaker replaced with
a permanent magnet type.
It's Airzone 602.
I have been given the schematic, but it's detail would be lost through resizing here.

I was wondering perhaps if a different value chake should replace both the original choke and the field coil resistor pack
(should the original choke ever fail).
There is a capacitor to earth where the choke connected to the field coil though.

I don't have a circuit of the Airzone 602 chassis, but I have looked at a 604 circuit on Kevin Chants site and it may be similar. Both use a 42 output valve and the 604 does indeed use a choke in the power supply and a 2000 ohm field coil speaker.
As the speaker is now a permag type, a 2K resistor makes sense replacing the field coil to keep the voltages correct.
I would not consider using a different choke to replace the resistor and the current choke, as the present configuration provides for 2 stages of filtering with 2 series elements,choke and field coil or resistor, with 3 capacitors from HT to ground.
It is not a common configuration and should be quite effective in reducing hum level.

Apparently it is.. I hear no hum, even on dead air.

I will check out the 604, just curious to see if the choke value is given there.
I'm nervous pulling it apart,
so rather, I have a list of things to do with the chassis all at once
the next time I do pull it out.

Model 603 lists a Magnavox 152D speaker with 2000 Ohm field and a 43 as output. It is an AC/DC model.

I did get the 602 schematic, it has a 2000 Ohm field coil, but also an independent choke.
42 output valve and 80 rectifier.

I was looking in an old ham book, and it appears the power supply
part of the circuits are pretty standard according to the rectifier valves used,
until it gets into multiple voltage outputs.

This one is identical to mine:
Image Link

It uses the 8μF caps too, and in the same place,
and my power tranny secondary is also 800 Volts.
The 30H choke is where my speaker field coil was.

The circuit in your last post does not have a capacitor from the rectifier filament to ground, making it a "choke input filter". If a capacitor is connected at this point, it is a "capacitor input filter". I will have to read my old texts, but I think the difference will make a difference in voltage regulation on load.
What I am saying is that they are two different configurations and not just saving the cost of a capacitor. The Airzone 604 is a capacitor input and I would have thought the 602 would be also.
If you cannot post the 602 circuit, could you email it to me?

For an 80 / 5Y3 The higher the applied voltage on the plates, the lower is the input cap in value to the point that at 500V and highest current 20H choke only, no input cap.

I stand corrected, it's not the same!

This one should be ok, not downsized much, just the supply section:
Image Link

Is it overly cautious of me not wanting to desolder the choke?
I haven't really dealt with many, but have seen a few open chokes.

I removed one at the weekend for the purpose of being able to access terminations for condensers I wanted to replace and it's been a while since I have done any major surgery to a radio.

Two things to remember:-

1. Be careful with any coil. The wire that makes up the windings are delicate and often thinner than human hair. This includes current limiting/smoothing chokes. I made the mistake of not disconnecting mine before removing it from the chassis and almost paid a price. The connecting wiring tore the connections away from the choke causing the binding materials to fall apart. Fortunately I was able to resurrect this mess and bind it all with silicone sealant without damaging the choke but to assist with this I had to change the connecting wires that hooked the choke up to the chassis to a smaller gauge so there was less pressure on the choke.

It all came down to the fact that this was the only order it all could have been done in anyway as the choke's connections were facing away from the bottom of the chassis but it could have been handled a little better.

2. The radio will either perform poorly or not work at all without the choke in the circuit. Circuits with chokes fitted are designed to work with them, not without them. Don't be tempted to get by without the choke in place as the rest of the circuit will protest either straight away or at a later stage, usually with an unhappy ending.