Yaesu FT-7900r problem - low audio, limited range...

[crawler#976]

Well shucks...

BajaXporer and I worked the Prescott Rally last weekend, and it became apparent that my radio was not working properly. I couldn't get out to either the start or finish of the stage - roughly 2 miles west and 5 miles southeast of our position. BX mentioned my audio was very low when we weren routete, and after some testing on Saturday, it showed limited range as well. That sounds like maybe two different issues.

1. No power output

2. Perhaps a microphone problem

I put a new antenna cable on to eliminate that - the old one had a home solder PL-259 connector. Unfortunately that didn't help.

So, instead of just throwing parts at it, I purchased a MFJ-822 SWR/Wattmeter, a Dummy load, and connecting cable. I'll check it out tonight and report my findings. Anything under an SWR of 2 should be OK - not optimal, but OK.

As I recall, I've always had reports of low audio with this rig, but it was clear and would get out fine.

Mark

 

[crawler#976]

Hooked up the SWR/Wattmeter and got an initial reading of...

25W forward, 20W reflected, SWR of...

...off the scale.

That pointed at the antenna or internals of the radio. I did a very close inspection of the antenna and NMO mount. The whip was solid, the mount was solid and grounded, the antenna mount/coil looked OK on a cursory look. It wasn't cracked, but the threads looked weird.

The mount is attached to the drivers side fender. After 2 years of trail rides and numerous whacks by brush and tree limbs, the threads had pulled out of the coil body and apparently lost connection, or we causing a ground to the mount. I tapped the thread insert back into the coil, and ran a test.

Forward 44W, Reflected 3, SWR of 1.5 - problem solved. I got a good radio check, and got a full signal return from the repeater in Flagstaff - a 50 mile line of sight distance from home.

I now have an inexpensive SWR/Wattmeter and dummy load for 2M that is available for others to use if needed.

 

[Rallyroo]

Glad you figured it out.

I had a similar situation a couple months ago. My rig kept beeping at me warning of high SWR. Mine was a bad coax to PL-259 connection. Snip and soldered a new connector and the rig was back up and running.

 

[1leglance]

Mark I would like to come up and borrow your setup and do a little radio testing after I get my overhead console built and the radio installed.
Right now I am using the HT and the roof antenna and it seems ok.
Thanks for the offer for others to make use of the equipment that really helps.

 

[crawler#976]

When you're ready for it Lance, let me know!

 

[taugust]

Good find on the poor contact, Mark. I have begun checking my antennas often due to similar experiences. One thing I have found that is common with those Larsen NMO antennas is the spring contact will work away from the mount contact. I check and rebend downward on the spring contact every few months, just to make sure. If it moves away, and you don't realize it, you can burn up the finals on the radio with high SWR. Also give it a shot of WD-40 to keep the moisture out. I have also found corrosion inside there.

 

[1911]

I check and rebend downward on the spring contact every few months, just to make sure.

I have always done that also and my Larsen is still working well after several years.

 

[ScoutII]

Well shucks...

BajaXporer and I worked the Prescott Rally last weekend, and it became apparent that my radio was not working properly. I couldn't get out to either the start or finish of the stage - roughly 2 miles west and 5 miles southeast of our position. BX mentioned my audio was very low when we weren routete, and after some testing on Saturday, it showed limited range as well. That sounds like maybe two different issues.

1. No power output

2. Perhaps a microphone problem

I put a new antenna cable on to eliminate that - the old one had a home solder PL-259 connector. Unfortunately that didn't help.

So, instead of just throwing parts at it, I purchased a MFJ-822 SWR/Wattmeter, a Dummy load, and connecting cable. I'll check it out tonight and report my findings. Anything under an SWR of 2 should be OK - not optimal, but OK.

As I recall, I've always had reports of low audio with this rig, but it was clear and would get out fine.

Mark

Cool deal on the SWR meter and finding the issue. I do know on the 7900 you have to hold the mic up close. Its designed to that way to help with background noise. Some people modify the mic for that reason too.

 

[crawler#976]

Definately need a new coil - the problem reoccured after a 300 mile trip. It's kinda funny (not...), but the coil sold by itself is $50.00, a complete antenna is $55.00.

Pretty much have decided to ditch the fender mount and poke a hole in the roof.

 

[ScoutII]

Bummer on the coil, is it getting whacked by limbs?

I drilled a hole in my roof for both applications. I found these nice stout NMO connectors on e-bay That's a link:sombrero:
They take the smaller FME connectors which is nice when fishing in the cable.
I've tried those pre made cable assembles with NMO connectors and part that tightens to the bottom of the roof is just to small and comes loose on the first small branch that hits the antenna. That being said, mount it closer to the rear of the cab to give it more strength or add a backing plate to the roof. Or run a shorter antenna.

 

[crawler#976]

Yup, it takes a lot of abuse.

I may leave the fender mount in place with a Kulducky stubby for tight/low trips, and use the roof mount for everything else.

 

[crawler#976]

Replaced the antenna yesterday with a new Larsen 2/70.

SWR on 146.46 was excellent - 1.1 to 1. The reflected power needle barely even moved. Checked the dummy load for output - 48W on high, 20W on med 1, 11W on med 2, 7W on low. Factory specs are 50, 20, 10 and 5W respectively.

Still seem to have audio problems - I'm getting varied reports of poor audio from multiple repeaters. From what I can tell the position of the truck makes a big difference in what is recieved - if I turn the truck with the antenna to the east, the reports from the repeaters in that direction improves.

I need to do the roof mount and see if that helps...

 

[crawler#976]

Well, the initial testing of the permanent NMO roof mount seems to be encouraging. SWR is better than before - well under 1.1 to 1. I got two repeater audio/signal checks that were excellent, one from Flagstaff (45 miles), the other from Mingus Mt (20 miles). I'd had a lot of ignition noise with the other mount, and the majority of it is gone as well. I'll try to do some simplex testing in the next day or two.