On Feb 20, 9:39�am, (Scott Dorsey) wrote:
Paul P wrote:
Does anyone have a guess what value choke L112 found here
http://www.ppinyot.com/hammarlund.ht...Supply%20Choke
might be? *I am getting a 190 ac volt drop across this puppy. *It has a cold
DC resistance of 97 ohms. *How I got there is also explained at the
hyperlink page above.

I doubt very much that L112 is faulty. The bias supply
capacitors, rectifier and resistors are much more likely
to be bad.

The short of it is, the negative bias voltages are down across all
associated tubes that share that supply. *

Even a guess at this point would be nice. *I have never calculated a choke
in this configuration before.

The choke has AC going through it, since it's on the input side of the
rectifier. *

No, it doesn't.

It has pulsating rectified DC going through it.

There are two reasons why it is in the center tap lead:

1) All the power supplies can share it

2) Insulation requirements are less.

My guess is that it's not just a normal choke but is a swinging
choke with a controlled saturation characteristic.

Agreed.

*It's acting as a
current regulator; as current rises the magnetic flux in the core rises
and at some point the core saturates and the impedance of the winding shoots
way up, reducing the current flow.

Not exactly.

In a choke-input filter, the inductance must be above a certain
critical value or the filter isn't really a choke-input filter.

This critical value (called critical inductance) is directly related
to the total load resistance. The lower the load resistance, the
less inductance is needed. The load resistance is just the output
voltage divided by the total current delivered by the rectifiers.

In a transmitter, the load resistance and current vary
all over the place with keying, modulation, loading, etc.

The inductance of real-world iron core filter chokes depends to a
certain extent on the direct current through the choke, which
magnetically saturates the iron. One way to reduce
this effect is to include an air gap in the iron core. The wider the
gap, the less the inductance variation. But such
a gap reduces the overall inductance.

If a choke with constant inductance were used, it would
need to have enough inductance for the lowest-current/highest load
resistance condition, yet enough current
capability for the highest current condition. That means a wide air
gap. Such a choke would be large and expensive.

Instead, a choke with a narrow air gap is used, Its inductance
varies with the current - more inductance with less curren,
less inductance with more current. This is exactly what is
needed with a choke input filter. Such chokes are called
"swinging chokes".

The power supply for my 150 watt homebrew rig uses a swinging choke in
the high voltage supply. This choke has 25 henries inductance at 30 mA
DC but only 5 henries inductance at 300 mA DC.

A 190V drop across it might be normal, or it might be a sign you are trying
to pull way too much current through the thing. *What voltages do you measure
across C162 sections A and B? *They should be fairly close to the maximum
capacitor ratings, I suspect. *If they are within a reasonable range, I'd
say the swinging choke is fine, otherwise I'd suspect something is on the
secondary side is pulling too much current and it's hit the wall.

All of the rectifiers, filter caps and resistors in the power supply
section are suspect. This is particularly true if any
selenium rectifiers or carbon composition resistors were
used.

73 es GL de Jim, N2EY