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Old 04-30-2019, 04:50 PM   #1
llonen
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Default MFJ 994BRT (remote Antenna tuner)

I have this coming into the workshop over the next day or so, does anyone have a schematic for this tuner. The fault description I have is that it will no longer tune on at least one band, specifically 7mhz. I will post pictures in due course of disassembly once I have the tuner on the bench.
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Old 05-08-2019, 06:08 AM   #2
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Default Re: MFJ 994BRT (remote Antenna tuner)

Haven't come across "a one band not working" on big automatic antenna tuners like that. I am guessing a cold solder joint on an inductor, bad relay or a smoked HV tuning capacitor in the tuning section.

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Old 05-08-2019, 06:16 AM   #3
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Default Re: MFJ 994BRT (remote Antenna tuner)

schematic for it
Attached Files
File Type: zip MFJ-991B_993B_994B.zip (1.62 MB, 1 views)
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Old 05-08-2019, 06:49 PM   #4
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Default Re: MFJ 994BRT (remote Antenna tuner)

Yeah that is what I am thinking, thanks for the schematic. The guy still has to remove the tuner from a 30 foot pole that its installed on. Been rather windy these last few days, will put up an update when I get the unit in the workshop.
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Old 05-16-2019, 07:41 PM   #5
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Default Re: MFJ 994BRT (remote Antenna tuner)

Well finally got the tuner, nothing much to see with this one, couple of relays tested bad. Replacements ordered and fitted, the real test will be when the unit is back in service since although it tests out good on the antenna system I have here I doubt the system has the same tuning profile and or characteristics that the clients install has.

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Old 05-17-2019, 12:26 PM   #6
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Default Re: MFJ 994BRT (remote Antenna tuner)

usually they are easy to repair if the damage is in the tuning section. There isn't much there and that's a good thing.
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Old 05-17-2019, 01:18 PM   #7
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Default Re: MFJ 994BRT (remote Antenna tuner)

i thought antenna tuners went on the desk next to the radio???
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Old 05-17-2019, 05:38 PM   #8
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Default Re: MFJ 994BRT (remote Antenna tuner)

not these ones. They go outside and are automatic tuning. Look at this MFJ or SGC-230. These are more efficient, since you are tune there where it matters, at the antenna. Think of it as an electronic self adjusting tuning box or balun. The only downside is, you need another cable (power) to it.
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Old 05-17-2019, 07:33 PM   #9
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Default Re: MFJ 994BRT (remote Antenna tuner)

This one has the option of sending the supply through the coaxial cable, the most efficient place for an antenna tuner is at the feed point of the antenna since it avoids all the losses from the reflected RF back down the cable which ends up as losses to heat. The downside is the cheaper ones have no easy management, if you need to work on them IE make adjustments to how they tune or carry out a reset you need to access the internal controller. That said the 600 watt and upwards tuners are not cheap, and they don't really tune at 600 watts. Tuning has to be carried out at low power before ramping up the RF.
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Old 05-18-2019, 06:41 PM   #10
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Default Re: MFJ 994BRT (remote Antenna tuner)

I had forgotten about sending the power in the coax feature. On the SGC-230 etc you need to put a separate power and control cable.
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