[AMRadio] Need some help here

[Dan Hall]

Thanks Bill:
I will comment on your comments with some snippage.

> Well, Dan, I am of the firm belief that these transmitters are
> single-frequency beasts only.

I have gathered that, and yes I will go back to a single frequency scheme.


  I have
> tried to stay with the existing circuitry as much as possible, even to the
> extent where in most cases to the transmitter can be retuned back to the
> broadcast band.

Me too.

(exciter coil)
> I have found that some coils can't be tapped down without excessive loss.
> The unused turns should be shorted out with a strap.

I'll take another look at shorting things before I take out the original
coil and wind a new one. The bias comes to the PA tubes from a resistor
network to ground via the center of this coil.


Can you look at this
> signal with a 'scope?  If it isn't a clean sine-wave, then you may have to
> use other sets of taps on the coil or replace the coil.  If you see a
> sine-wave with a big dip where the peak belongs, you indeed have problems
> with the unused coil turns.

Bingo! Very ugly sine at the grids of the 833As and it changes drastically
when tuning the exciter through resonance. The purer the waveform the lower
the output it seems.

>> 2000 Volts?  That sounds low, but perhaps that is correct for 833's   At
any rate, they should put out well over
> 1000 watts at full output.

The main power transformer has two HV taps one at 2230VAC and the other at
3300VAC. I'm using the lower voltage tap to reduce the power output.


  I favor a PI-L arrangement if you have a choice,
> and can help you configure one if you need.

It's a PI-L, I'm using 145pf fixed C1,  around 9mmh L1, 750pf C2 and around
4.5mmh L2. The coils are variable with shorting of the unused portion and C2
is a 80-1300pf  variable in parallel with a 300PF fixed. I can make this
fixed with a 3000pf and a 1000pf in series.


.  You may
> have to also short intermediate turns so the coil can't adsorb energy in
the
> unused portion.  Use a scope to verify the output waveshape is a good
> sine-wave.

After I get the drive in shape I'll look at this



> Another problem may be the RF choke in the B+ line.  Make sure it isn't
> resonant.  I have had to replace the RF choke in several transmitters.

I'll check
>

> Not sure why the PA would ever act as a doubler, but sounds like tank

This when waaaay out of tune, experimenting


>Regarding the
> tank circuit, how are you tuning it?  I calculate the tube impedance from
> specified voltage and current, multiplied by a factor, think it is 0.8.  I
> then take a film resistor of somewhere around that value (surprisingly it
> has turned out to be 3300 ohms in many cases) and, with power disconnected
> to the transmitter, attach it between one of the PA plates and ground.  I
> then connect an Autek RF-Analyst to the antenna terminals, and adjust the
> plate tank circuitry for an SWR of 1:1.  It must be 1:1, anything higher
is
> not acceptable.   MFJ units don't seem to work in this application.  I
then
> remove the resistor (important!), turn on the power, and the trasmitter is
> tuned and ready to go!

Sounds like a plan, do you use a guessed at current based on .7 efficiency
and desired output?
I am using an AADE L/C meter to set values based on the charts in the 1990
handbook. At that, C1 is way out of wack but I attributed it to stray
capacitances.

Lets say for sake of arguement that I want to shoot for 2000VDC at 400mils

Thanks Bill, and I think this stuff is of great use for most on the list.

Dan

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