mfj 989c question

[K4HXC2]

Got a mfj 989c tuner today and hooked it up to the IC-756proIII. selected dummy load on the tuner and keyed the transmitter.
Cranked up rf to about 7 watts indicated on the Icom and it auto-tuned ok.
What didn't seem right though, the swr meter on the tuner indicated almost all the power was reflected and almost none forward giving a swr way up there around 6 to 1. Further, none of the tuner adjustments had any effect at all.
I measured the resistance of the dummy load through the antenna connector on the tuner at 57 ohms so the mismatch wasn't that bad. The Icom manual say it won't tune into that kind of swr but it tuned fine. So I can't figure this out.
Why does the Icom say the swr is fine but the panel meter on the tuner says it's not? And why do none of the adjustments make any difference? Am I having a duh! moment?

[W5FYI]

In checking the schematic for the '989D, when the dummy load is switched in, the transmitter feeds it directly and the tuning network is bypassed. But I'm not sure why the meter indicates the way it does.

[K4HXC2]

You're right, I see that too, There's that Duh! moment I was talking about. But it looks like the meter circuit stays in so maybe we're 50% there.

[NO2A]

Why are you using the autotuner on the radio? The dummy load on the `989 presents a perfect load if it`s operating properly. The tuner is not inline when using the dummy load. If you`re using the radio`s tuner with the dummy load it will give you a false reading on the `989.

[KB3KCJ]

Just a thought; but, did you get the tuner new? Did you gently shake to see if anything rattled around?  Did you take off the cover and retighten everything that had a screw?  When I got mine new, it had seven things wrong which would make it work inappropriately.  You might want to get a magnifying glass and check the solder joints inside.

[W8JI]

How much negative does the meter go?

[K4HXC2]

As I suspected, the tuner meter circuits turned out to be bad, actually indicating backwards!
We put a pre-MFJ Ameritron ATH-30 on there, tuned it up to the same dummy load (external this time, as a control) and everything indicated correctly with both tuners in coax direct into a 50 ohm dummy load. Because the MFJ was indicating almost all reflected power under the same conditions I wondered if something is wired incorrectly and escaped MFJ's QC.
In any case, we'll troubleshoot this, ought to be easy to find.

Later

73

Dennis K4DAZ

[WB6BYU]

It could just be the adjustment pots for forward and reflected power.  There are 4 of them:
forward and reflected for each power range.

A quick check is to put the tuner direct into a dummy load and adjust the forward power pots
for a proper reading.  Then reverse the tuner (leaving it in direct/bypass mode) and adjust
the reflected power pots to read the same value.

If you can't get the meter to read high enough, it may be a blown detector diode.  But check
the trim pots first.

Or, if you really do have forward and reflected power reversed, just swap the wires to the two
meter movements.

[W5FYI]

Could be that the meters are wired up wrong. Check that the connection from R120 goes to the "forward" meter and that from R119 goes to the "reverse" meter.

Alternatively, maybe the leads from the sensor to the switch are reversed. Switching them wouldn't hurt anything--either run the lead that goes to the B side of the switch to the A side and vice-versa (these are the wires in the shielded line coming from R203 and R204).

Either way, you'll probably have to recalibrate the meters because whoever was suppose to at the factory missed it. GL

[W8JI]

Reversing the input and output cables on the tuner would have the same effect as reversing the wires from the directional coupler system to the meter.

So will reversing the wires from the transformer inside the directional coupler.

DO NOT leap into adjustments or wiring until you are sure what is wrong. It could either anything from the external cable wiring to the direction coupler transformer wires flipped to meter leads flipped from side to side.

[KF7UNX]

I purchased a second hand MFJ-989C and had the same problem with the meters reading "backwards".
There was way more reflected power than there was forward power.

I called MFJ and they recommended replacing the 2 diodes on the rear board where the receiver plugs in.

I did that and it straightened the meters right up.. $2.50 in parts and it’s as good as new..

So, if you run into someone with the same problem, have them replace the 2 1N34A diodes (D101 & D102) with 1N270’s.

73 - William, KF7UNX

[K8AXW]

W8JI: Welcome back! 

[N4TTS]

The thread got bumped. JI's reply was from August of 2011...

Don N4TTS

W8JI: Welcome back! 

[K4HXC2]

Having the benefit of William's information it seems that if D101 were to short out, the symptoms would be exactly as described.
I'm surprised that neither myself or anybody else who read the thread didn't see that.
I wonder if we're all so gun shy about MFJ that nobody actually thought it through.....

73 and thanks to William, KF7UNX and all who contributed...