MFJ-969 Deluxe Vera Tuner II

2023-08-28 386 words 2 minutes

This past weekend, while playing around with my antenna setup and my new FTDX10, I needed to do some SWR measurements. I have a few SWR meters laying around, but I also had this MFJ tuner sitting on the shelf doing nothing. It had a nice cross needle meter as well as an integrated coax switch which would be useful for switching between my antenna and a dummy load. I don’t necessarily need the tuner parts, but this unit is about the same size as the FTDX10 and they stack nicely atop one another, so I figured I’d use this.

After I set everything up, the meter readings were all over the place. I opened it up and tried calibrating it with the trimpots, but nothing worked.

After a little research, I read that the germanium diodes can go bad if they are hit with high power or a lightning strike. I didn’t know the history of the unit, so I decided to just swap them out to see if that fixed it.

For some reason, I took a bunch of pictures of me taking out the original diodes, but none of the replacements.

I replaced the original diodes with 1N34A diodes in the glass package. They were a bit larger than the blue diodes that I removed, but the board hole spacing was large enough to accomodate them.

While I was in there, I also replaced the capacitors in the peak detector circuits since they were about 25 years old. I also replaced the incandescent bulb in the panel meter with a white led.

I put everything back together and calibrated the unit. I don’t have a lab grade power amplifier, so I just calibrated it to the power settings of the FTDX10. For the SWR meter, I set the needle to 1:1 on my 50 ohm dummy load. I then added a second dummy load in parallel and calibrated to meter to read a 2:1 SWR.

When tweaking the output power and antenna load, the meter reads accurately, or at least accurately enough for my purposes and it matches up with measurements from my other SWR and power meters. What is it that they say? A man with one SWR meter knows his SWR. A man with multiple SWR meters is never quite sure.