Airlomba.net

MFJ-941E BOX OPENING

UPDATE (2012.07.30): Increase Meter's light bulb life. (link).
UPDATE (2012.07.30): Added C-L-C characteristics at the bottom of this page. (link).

I just received a brand new MFJ-941E Versa Tuner II...
And after a little while, I started to get disappointed. Why? Well, as I moved the unit in my hands, I could hear something loose inside it. So I had to open it and look for what was unsecured. I must confess that in the days before the acquisition, I read some reviews on the Internet and found out that it is "normal" in MFJ products to find some bolts unsecured...

One great thing (in my opinion) about MFJ products is their Limited Warranty (at least for this unit). In fact and I quote "This warranty is NOT void for owners who attempt to repair defective units". Great! Because I prefer to open my unit instead of sending it back to the factory and wait for an unknown amount of time.

To open it, you just have to remove 2 screws on the top cover and 6 screws from the sides (3 on each side). Removing the cover, you get what you see on the pictures below (click on the pictures to open a full size picture on a new window):

MFJ-941E open heart MFJ-941E open heart MFJ-941E open heart

The first "bug" was the loose bolt on the 1:4 balun. This was the cause the sound I heard when moving the unit around:

Detail of the 4:1 balun

The second "bug" and incredibly serious was that the plates of the ANTENNA capacitor could touch one of the INDUCTOR wires as you can see on the pictures below. I don't understand how this could have happened or passed the Quality Check.

Detail of the ANTENNA capacitor Detail of the ANTENNA capacitor Detail of the ANTENNA capacitor

A final note on the hygene of the unit...
At last, I also was disappointed with the lack of hygene inside the unit. In fact, lots of parts present flux residues, indicating that some components were soldered inside the unit's box (this manufacturing process is comprehensible, but some care must be taken in order to avoid troubles). If you look closely to the above pictures you'll see small yellow drops of solder flux, as you can see on the picture below:

Detail of the TRANSMITTER capacitor presenting fluz drops


C-L-C T-network characteristics

In order to know what kind of values I am dealing with, when a use this unit, I had to measure the inductance value for each INDUCTOR rotary switch position. I also did the same for the capacitors and built the tables below.

Some day, as long as I get more experience with this unit and working the bands, I'll make a table that relates the different bands I can work on my Sirio 827 with the approximate C-L-C values, so it get quicker to "tune" the network.

The values were measured using an R-L-C bridge, model RLC-100 Programmable RLC Meter, from Digimess (Grundig).

TRANSMITTER CAPACITOR
POSITION
C (pF)
0
339
1
305
2
274
3
242
4
213
5
182
6
153
7
121
8
91
9
57
10
31
INDUCTOR
POSITION
L (uH)
A
24.0
B
17.0
C
14.3
D
11.3
E
8.6
F
6.1
G
3.9
H
2.9
I
2.1
J
1.5
K
1.0
L
0.7
ANTENNA CAPACITOR
POSITION
C (pF)
0
343
1
312
2
279
3
246
4
215
5
187
6
159
7
128
8
100
9
68
10
39


The variable capacitors (TRANSMITTER and ANTENNA) are of same the model (0-300pF).


INCREASING THE METER'S LIGHT BULB'S LIFE EXPECTATIONS

I don't know the exact voltage type-rating of the light bulb inside the unit's meter, but I imagine it is a standard 12V miniature light bulb. Since the unit is powered by my shack's power supply and this one delivers 13.8V, I had to place a resistor in series with the buld to reduce its voltage a little bit.

So I replaced the red wire existing between the "MP" point in the printed circuit board and the meter, by a common 100 Ohm - 1/2W resistor. NOTE: I don't know if all MFJ-941E units share the same color for this wire...

Once the resistor in place and the unit's powered with 13.8V, you get about 11V across the light bulb. Off course, the brightness of the light bulb has decreased a little bit, but the final result is perfectly acceptable and the bulb will last longer.



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