Date: Thu, 01 Apr 2010 07:43:20 -0700
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
Subject: Re: [Amps] lcr meter
On Thu, 1 Apr 2010 06:56:18 -0700, "Jim Thomson" <Jim.thom@telus.net>
wrote:
>
>## exactly what it sez... the mfj 259 is a glorified swr meter. Don't even
>think abt using it to set taps on a tank coil,
REPLY:
I have to disagree on not using it to set taps on a tank coil. When
connected in reverse, i.e. connecting it to the output of a pi-net,
with a non-inductive resistor of the appropriate value from plate to
ground, it does an excellent job of helping you find the right tap.
## Not a chance Bill. Sure Myself, and everybody else does the
same trick u do,,,, put a non inductive Resistor, between anode and chassis
and the put the MFJ on the output, and activate the T/R relays.
## Tune and load caps are then tweaked for flat swr on the MFJ. This
procedure alone, doesn't help FINDING correct coil taps one bit !
EG: I could easily have almost double the uh required for say 40m....
and simply REDUCE the tune + load cap values.. till swr on mfj reads 1:1.
## I could also use 50% less uh than required... and simply increase the
tune + load cap values for 1:1 swr. You need a real lcr meter to find
the taps
for each band. You can also use the real lcr meter to 'pre-adjust' both
the
vac tune and vac load caps.... per the spread sheet. Then with mfj hooked up
in rvs.... it's just a very slight tweak of the tune + load cap values to
get
flat swr on the mfj.
## So in one sense, u are correct... the mfj is an invaluable tool, when used
in
reverse... to watch the swr go flat, after tweaking both caps.
## OK, here's something else invaluable u can do with the mfj rvs
connected.
After tweaking the tune+ load caps for flat swr on say 1850.... leave the
tune + load
caps alone.... and now tune the FREQ on the mfj up/down, till u find the 2:1
swr points
and note em down. Then repeat the process on each band. What u should get
IF the
loaded Q is the same on each band is that the 2:1 swr points will DOUBLE in
BW..
every time u move up a band. IE: on my 3x3 amp, I get 67 khz on 160m...
and 145
khz on 80m, and 300 khz on 40m.. and 600 khz on 20m... and u should get 1200
khz on
10m. 15m should be around 900 khz. That amp is only a 160-15m amp
anyway
[no 10 +12m]. Every thing fell into place except the 20-17-15m bands. 20m
needed a tiny bit more uh. On 15m, 2:1 bw was way less than what it
should be
[ loaded Q was too high for my taste] . And that was with the max amount
of tubing coil that I could use.. such that the tune cap was 99% un meshed on
21.450.
The fix was to wind a small .66uh coil from 3/8" tubing [ 7 x turns on
a 1.5" ID]
and insert it between plate block caps, and vac tune cap. [ which also put it
at right
angles to the main 20-17-15m tubing coil]. OK, now the main 15m tap was
reduced
a tiny bit... and now the 2:1 swr BW is a helluva lot wider. I purposely
did it this way,
[ loaded Q =8] since the 15m tap is also used on 17m band. On 17m, the
loaded Q
rises to 12. In this one case, the .66 uh coil was derived from gm3seks
spread sheet,
which will allow inputting stray L between anode and tune cap. An alternative
method
[that I also tested.. and works] is to simply tap into the main tank coil,
abt 1 turn.. with
the tune cap. On 6 or10m monoband amps, the entire tank coil was made from
flat wide,
cu strap, with the tune cap tapped dead center in the middle of the coil.
## this above trick, of course, just uses a tiny bit of uh b4 the main PI
net.. + the stray
anode to chassis C of the tube... to form a step down L network....
something the PI
net can more easily be designed around.
## without the small ,.66 uh coil b4 the C1 cap.... my loaded Q on 15m was
like 15....
and rose to a loaded Q = 20 on 17m.
## utilizing the 2:1 BW on the rvs connected mfj is a great asset. You
can tell when stuff
is outa whack for a given band right away.
## another thing I learned [ the hard way] way to disconnect both vac
tune and vac load
caps, and install the B+k 875 across each one, and note down the exact
pf, for every turn
of the turns counter, from MIn to MAX.
## that alone save a lot of time trbl shooting stuff. OK, the amp resonates
on the mfj... but BW
is too narrow, too wide, won't resonate at all [ one or both caps ran outa C
on 160m, or tune
cap doesn't have a low enough min C on 10m ,bw is too narrow on 20m etc] . The
1st thing
u ask urself, how much tune and load cap is dialled in ? It makes life
real easy when u can
look up the numbers on the turns counter, and cross ref to ur cheat sheets, and
say... ' it equates to
256 pf.... and load is 753 pf.... which is outa whack with the Pi spread
sheet. '
## Ok, at this point I tweak the tune and load caps [ via cheat sheet] per
the PI spread sheet,
THEN re-tap the main tank coil for resonance, using the rvs connected
mfj... or in the case
of tubing coil... I can only get so close,since the ideal tap position on the
coil, might be on the back
side of it, or some other awkward spot, which would require a real long lead.
later... Jim VE7RF
For all the other purposes that MJF claims (grid-dipper, etc, etc) I
agree it's junk.
73, Bill W6WRT
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