RadioBanter - View Single Post - Ameco AC-1 problem

April 7th 04, 09:33 PM

in article , N2EY at
wrote on 4/6/04 12:29:

Uwe wrote in message
...
in article , N2EY at
PAMNO wrote on 4/5/04 18:59:

In article , Uwe

writes:

I will just have to fiddle a bit more
with the pi network (since at the B+ voltages suggested here my plate
current would be way high) and I will have to live with the chirp.

Are you getting a "dip" in plate current? If not, the coil is probably too
large or too small. Unless you get a real dip, the output network isn;t
right.

I have used a very similar transmitter with 350 volts on the plate, and the
dip
is clean and pronounced.

Jim, the original docs I got for this tx call, at 40 m, for a 15 turn coil
on the coil form provided with the kit, which I hear was 1.25" diameter.
If I use the formula for air coils this turns out to be roughly a 22
microhenry coil.

22 microhenries? I get more like 8 microhenries using the formula

L = (a * a * n * n)/([9 * a] + [10 * b])

where
a = radius of coil in inches
b = length of winding in inches
n = number of turns

Well, the way I use the formula is 1.25*1.25*15*15/((9*1.25)+(10*0.45)) =
351.5/15.75 = 22.3

O.45 is the length of the 15 windings.

Do I not use the formula properly??

My meter is built into my bench power supply (thats why it reads up to
250mA), so I am measuring plate and screen current.

I put in a second meter which would only measure the plate current but its
reading is practicly identical to the first one, as if there was no grid
current.

My antenna is a dipole of about 75ft. length each side, connected with a 50
ohm coax, no balun or such things.

I will need a few days to try out some of the things you and also Paul
suggested and it might really be a good idea to get an SWR meter and a
tuner.

All in due time and I will surely get back to you.

Thanks for the help

Uwe

The coil which works best with my tx is 8 turns on a 1,125" ceramic core.

But do you get a dip?

Be aware that the AC-1 went through some changes in its lifetime. Some
models used a filter choke, others did not. Some used a 730 uuf
loading capacitor, others just a single-section 365 uuf one. Coils
changed too.

To get guess work out of it I just bought and built a L/C meter and
measured my coil to have 2.7 microhenry. So I am way off, but it works, sort
of.

The air caps are 36 to 420pf at the plate and 15 to 728pf at the antenna, so
that seem right.

All this happens with B+200V and 35 mA plate current.

LC = 25,330/(f * f)

so for 7 MHz, the LC constant is 516. Your 2.7 uH coil should resonate
with 191 uuf.

Older ARRL handbooks give typical values for pi network for 50 Ohm antenna
loads and my values are in range for the caps but my coil is too small.

The ouput voltage on my antenna measured with a scope is up to 75 volts peak
to peak, with a 50 Ohm load that would mean I get more out of the tx than I
put into it and I am not of the sort who says this might happen.

So my conclusion is, and tell me if this sounds right, that I have an
antenna which is far from 50 ohm resistive at 40m and that that makes
everything weird.

That's defintitely part of the problem. What antenna are you using?
Have you tried a resistor or lamp load?

The dips in plate current are nearly imperceptible and they are not aided by
my 250mA full scale meter. They may be 2 or 3 mA.
I tune with the help of my scope.

The meter tells more. You can use a pilot light (#47, 150 mA) instead
of a meter.

Sudden thought: Where is the meter connected? Are you reading plate
current, or plate-and-screen current combined?

Here's something else to try:

Often trouble of this sort is due to the RF choke used. What RFCs are
you suing, particularly in the plate circuit? Although the LC meter
may say they are a certain L, in real life they may have all sorts of
unwanted resonances.

To test this idea out, do the following:

- Remove the plate RFC
- Connect the antenna end of the plate coil to the B+ where the RFC
used to be connected. This point should already be bypassed to ground
through a disk capacitor of about .01 uF
- Disconnect the "loading" capacitor
- Remove the plate coupling capacitor.

What you will then have is the 200 volts being fed to the plate
through the coil, with one end of the coil going to the plate supply
and the other end connected directly to the plate of the 6V6. The
plate tuning capacitor is connected between the plate of the 6V6 and
ground.

End result is no plate RFC and a parallel resonant circuit. There's no
connection for an antenna yet, but that's not important right now.

Test out the rig and look for the plate current dip. It should be very
obvious because there is no load connected.

This is just a temporary setup to see if the RFC is OK.

73 es GL de Jim, N2EY