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yaesu ft-7900

Sat Jul 04, 2015 11:49 am

remote mounted Yaesu ft-7900, amateur band TX, with aircraft, marine, amateur and PMR receive coverage.

1/4 whip for 2 meters on the centre of roof and co-ax duct-taped to the inside of the roof
to avoid the sunroof catching,

boot mount, linked to front, remote Mic lead to centre dash,
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Re: yaesu ft-7900

Sat Jul 04, 2015 8:00 pm

Takes me back to the olde days of CB radio

Re: yaesu ft-7900

Wed Feb 08, 2017 5:13 pm

Perhaps a bit cheeky, bumping this old thread - but the topic is bang-on!

(Also, first post - hello everyone!)

I've just installed my FT-7900 - the main unit in the luggage area, remote head in the top cubbyhole above the sat nav, hands-free mic on the driver's door pillar.

Aerial is currently on a mag-mount (I'll improve that later).

Now, to the substance: The general recommendation, I'm well aware, is to run a power cable direct from the battery to the radio. I did this on my last car - Qashqai - and it was a real pain to do. I looked at doing it for the X-trail, and it looked, again, to be very difficult to find or create a way from the engine compartment into the cab. In the end I cheated, and took the power from the rear accessory socket (I've since discovered that there's a big in-car power feed in the passenger footwell - ho-hum!). The set worked fine (or so I thought): I was testing it out with another radio ham, and all was great for a good few minutes - then the set turned off. It refused to switch back on - holding the power button on the radio produced a series of faint "clicks" from the speaker - almost as though the power was being cut off as soon as the load came on.

I went back into the boot with the multimeter - checked for voltage, fiddled around for a minute or so and then all was well (but not sure why!).
Back into the hot seat and the set came back on. Resumed testing - maybe OK for two or three minutes and it went off again. Went through the same carry-on, and again it worked for a short while then went off again.

Any suggestions as to what might be happening? The socket is supposed to be good for 10A. The first time it went off, I was transmitting (full power - 50W) - for which the spec says the radio should draw about 8.5A. The second time, it was on receive, so should only have been drawing an amp or so. Is this some sort of "feature" with the CANBUS system (which is just a name to me - I have no real idea what it IS)?

Ultimately, I might need to use a better power supply (although the rating for the socket is more than adequate, even with the set at max power) - but I'd like to understand what's going on.

Suggestions welcome!


Tony M

Re: yaesu ft-7900

Wed Feb 08, 2017 5:29 pm

Volts drop.

Check the voltage at the rear socket

Engine off with no load on socket you should see around 12 vdc
Engine running with no load on socket you should see around 13 to 13.5 vdc

Then do the same tests with the radio tx at 50w

Best regards

Milton :biggrin:

Re: yaesu ft-7900

Wed Feb 08, 2017 5:53 pm

milton wrote:Volts drop.

Fair comment - I'll have a look at that tomorrow. I did note the rather small size of the wire feeding the power socket!

Whilst the radio could well see reduced volts during transmit, it transmitted fine for a good 2 to 3 minutes, with no evidence of adverse effect on the transmitted signal. It was the way the thing switched off completely and wouldn't come back on that made me think that it might be the car interfering with the 12V feed. The second and third times it went off, the set was on receive, so voltage drop should have been minimal. (Also, the engine was running during the tests).

Thanks again,

Tony M

Re: yaesu ft-7900

Wed Feb 08, 2017 5:57 pm

mine runs on a 10mm that feeds the invertor in the boot,

just teed into that,

agree that the thin wire will struggle, look at the wire on the radio,

using just a 1/4 on 144, ( 19 and half inches ) and the swr on 70cm is 1,3 to 1 ( 3/4 wave )

Re: yaesu ft-7900

Wed Feb 08, 2017 6:57 pm

Thanks for that.

Is the inverter fed from the big wire in the passenger footwell, or elsewhere? (I presume its not original equipment?).

I've seen reference to a permanent live feed to a connector up behind the rear light cluster, but haven't investigated that one yet - again, it may be a fairly small cross section (most things are, compared to the radio lead!).

My aerial is 5/8 wave on 2m, with a coil mid-length to deal with 70cm - a little bit of gain without too much length. SWR is unity across 2m (haven't checked 70 cm, since I seldom use it). It also has a tilt mechanism - you can pull the aerial up out of its bottom bit (against a spring) and tilt it down along the roof - but on the last car I discovered that it could vibrate itself up against the spring and fall over whilst travelling up the motorway. Not good to find it swinging in the breeze (wouldn't do much for the SWR, either, I guess). Now held firmly in place with self-amalgamating tape.


Tony M

Re: yaesu ft-7900

Wed Feb 08, 2017 9:33 pm

10mm direct from battery to boot, new install,
along with a few spares from

boot to dash, 4 core
dash to battery, 2 x 3 core
and battery to boot, 2 x 3 core

so can connect circuits round the truck,

Re: yaesu ft-7900

Thu Feb 09, 2017 6:27 pm

Thanks for the info - sounds like a very professional job!

I think I've sorted the problem with the radio:

I was convinced it was the car doing something odd - but this turned out not to be the case.
I checked the voltage drop using the rear power socket as feed for the radio:
Engine off, radio off, 12.03v. radio on receive, 11.99v. Tx on low power (5 watts) 11.72v, Mid1 11.63v, Mid2 11.49V, high (50 watts) 11.04v.
Engine on, radio off, 14.6v. radio on receive, 14.6v. Tx on low power 14.34v, Mid1 14.28v, Mid2 14.14V, high 13.8v.

So, only dropping about a volt with a 50 watt transmission - I could have lived with that, but I decided to go to a better supply point to try and sort the intermittent radio problem.

I took the feed from the big red cable in the passsenger footwell, and created a new earth connection down in that corner for convenience, good quality heavy twin feedline, fused in both 12v and earth. Got that all sorted, powered up the set and it was fine - for about 5 minutes. It again powered itself off, and refused to switch back on. (I was not a happy bunny at this point!)

Nothing I tried brought the radio back to life - I was still convinced the problem was something to do with the car's electronics, till I tried supplying the radio from a separate 12v gel-cell - and still had the problem. (As you held the power button down, there was a sort of "ticking" noise from the speaker).

A bit of digging on the internet revealed that others had seen the same issue with the FT-7900 - the investigations got rather involved, but stem from the internal circuitry not latching a main 13.8v supply line in the radio. There was a suggestion that it could have something to do with the cable connecting the radio's head unit back to the main box. I dug out the short lead that comes as standard with the set, fitted the head directly to the body - and the problem went away.

I've done a full reset on the radio, followed by re-loading the memories from the PC - its working fine in the house.

I suspect that the little end connectors (RJ-45s, or something like that) on the extension cable may have got pulled about a bit as I ran the cables - they are not very robust.

I've ordered a new cable - at under £10, it's just not worth fiddling on any more.

Hopefully, all will be finally sorted next week.

Thanks to all for the help.


Tony M

Re: yaesu ft-7900

Thu Feb 09, 2017 6:37 pm

I just got a 6 pin double ended from Ebay 5 meter,
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