[Amps] LCR mteter.

Jim Thomson Jim.thom at telus.net
Fri Apr 2 00:23:16 PDT 2010


Date: Thu, 01 Apr 2010 07:43:20 -0700
From: "Bill, W6WRT" <dezrat1242 at yahoo.com>
Subject: Re: [Amps] lcr meter


On Thu, 1 Apr 2010 06:56:18 -0700, "Jim Thomson" <Jim.thom at 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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