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Author Topic: Low Xmit Power on Yaesu ft-840.  (Read 1453 times)
KC3YYZ
Member

Posts: 15




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« on: April 28, 2017, 08:09:31 PM »

Hi, I recently picked up a used FT840 and an MFJ-945E antenna tuner at a  garage sale a few weeks ago. The guy who sold them to me said he had no idea if they worked or not and priced them accordingly, so I decide to take the risk.

Soooooooooo...........

If finally got around to testing them out the other day. The mfj-945e seems to work ok as the readings are what I would expect when checked with an analyzer.

But the 840 seems a little weird. The output power is just a little more than half of what I expected using the 840's built in power meter, and even less when using the power meter in the 945E.

Using a CW carrier keyed on the mic into a dummy load here is what I got:

                   840's Power Meter(W)           945E's Power Meter(W)
160                       40                                 30

80                         75                                 40

40                         55                                 40

30                         50                                 30

20                         75                                 40

17                         55                                 35

15                         50                                 35

12                         50                                 45

10                         55                                 30

I get similar modulation on SSB when I whistle into the mic. The MFJ power meter is slower than the 840's meter for sure!!!

Obviously at least one of meters is bad.. Smiley Probably the MFJ........... Grin

 I didn't notice anything blatantly wrong on the 840 when I opened it up for inspection..
I did notice there is a YF-112C CW filter installed.


Any thoughts on the low output?

« Last Edit: April 28, 2017, 08:32:05 PM by KC3YYZ » Logged

Mark
KC3YYZ

"What you own is your own kingdom
What you do is your own glory
What you love is your own power
What you live is your own story"
                                        N. Peart
KC3YYZ
Member

Posts: 15




Ignore
« Reply #1 on: April 29, 2017, 05:03:25 AM »

Oh One more thing, when in AM mode both meters read 25 watts on the money on all bands 160 thru 10....


Logged

Mark
KC3YYZ

"What you own is your own kingdom
What you do is your own glory
What you love is your own power
What you live is your own story"
                                        N. Peart
N5VTU
Member

Posts: 388




Ignore
« Reply #2 on: April 29, 2017, 10:52:54 AM »

I've had that same radio for about 25 years and found it to be quite a robust little rig.  The fact that it's putting out 25w AM carrier is good, as that's what it's rated for.

When you say you used a CW carrier keyed on the mic, what does that mean?  Are you not using a key connected to the rear 1/4" phono plug?

Simple things to check:

Is the rf power control (concentric knob shared with mic gain control) turned fully clockwise?

Is your power supply capable of providing the required 13.8v at 20A for full power operation?

I'm not familiar with the MFJ-945E, but make sure if it has a bypass mode, you're using that so that the tuning features are removed from the circuit.  It almost sounds like the tuner is causing the radio to see a mismatch instead of the 50 ohm dummy load and is folding back power to prevent damage.  I'd try to connect the radio to the dummy load directly (no 945E) and see what you get.

Stephen
N5VTU


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KC3YYZ
Member

Posts: 15




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« Reply #3 on: April 29, 2017, 08:51:25 PM »

I've had that same radio for about 25 years and found it to be quite a robust little rig.  The fact that it's putting out 25w AM carrier is good, as that's what it's rated for.

When you say you used a CW carrier keyed on the mic, what does that mean?  Are you not using a key connected to the rear 1/4" phono plug?

Simple things to check:

Is the rf power control (concentric knob shared with mic gain control) turned fully clockwise?

Is your power supply capable of providing the required 13.8v at 20A for full power operation?

I'm not familiar with the MFJ-945E, but make sure if it has a bypass mode, you're using that so that the tuning features are removed from the circuit.  It almost sounds like the tuner is causing the radio to see a mismatch instead of the 50 ohm dummy load and is folding back power to prevent damage.  I'd try to connect the radio to the dummy load directly (no 945E) and see what you get.

Stephen
N5VTU





Sorry for the confusion. No I did not have a key connected to the 1/4" plug on the back. I just put the radio in CW mode and keyed the mic. Maybe that's what I did wrong?? I don't have access to a key at this moment.

The RF Power control is maxed out.

I just noticed that my power supply meter is staying at 13.8 volts when I key up and the Amp meter on it shows about 10 Amps draw with the RF power maxed, but my inline meter (between the radio and P/S) shows voltage dropping to 11.7v / 10A at the same time.

I tried with both the 949E in bypass mode and totally removed and got a little better readings, but the difference was minuscule.
I also tried different cables....

Logged

Mark
KC3YYZ

"What you own is your own kingdom
What you do is your own glory
What you love is your own power
What you live is your own story"
                                        N. Peart
N7EKU
Member

Posts: 719




Ignore
« Reply #4 on: April 30, 2017, 07:22:06 AM »

Hi,

That magnitude of voltage drop shows that there is either something wrong with your power supply, or the DC power cable (or both).  Are there inline fuses in the DC supply cable?  Make sure they have good connections and maybe replace the fuses.  What gauge is the cable?  It should be something like 14AWG for lengths a a few feet, and bigger for longer lengths.

What kind of power supply do you have?  Make sure it is rated for 20A or higher.

73,


Mark.
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Mark -- N7EKU/VE3
KC3YYZ
Member

Posts: 15




Ignore
« Reply #5 on: April 30, 2017, 08:23:51 AM »

My power supply is a QJE QJ-PS30SW I.

I'm using #10 AWG wire.
There are inline fuses that are only about a year old or so...

As long as it is not a problem with the radio, then all is fine.


I was thinking about getting a new power supply anyway. The QJE fan is louder than I like... Undecided
Logged

Mark
KC3YYZ

"What you own is your own kingdom
What you do is your own glory
What you love is your own power
What you live is your own story"
                                        N. Peart
N5VTU
Member

Posts: 388




Ignore
« Reply #6 on: April 30, 2017, 08:26:58 AM »

Putting the radio in CW mode and keying the microphone is incorrect.  You need to connect an actual CW key, or at least rig a pigtail from the 1/4" key jack on the rear of the radio and short the two connections together while in CW mode to send a steady CW carrier.  That CW carrier should vary directly with the position of the rf power output control, from about 5w to~100W if you have a good 50 ohm load.   

Based on the fact that the rig will put out the specified 25W AM carrier, I think the radio is operating normally.

I also agree with N7EKU that 11.7 volts is an excessive drop. You need to address that first if any test of power output is to be meaningful.

Download a manual if you haven't done so yet.  It has a good troubleshooting section.

Stephen
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KC3YYZ
Member

Posts: 15




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« Reply #7 on: April 30, 2017, 09:05:10 AM »

Thank you both! As per your advice, I hooked up  the radio directly to the power supply and I am getting nearly 100w. I had a rigrunner, generic powergate type thing, and an inline v/A meter between prior..

I also found this : http://www.eham.net/ehamforum/smf/index.php/topic,114604.0.html

Now I just need to figure out where the problem, but that should be simple!!! (I hope!!)

I will let you know what I find... I am leaning towards the powergate knockoff....

Thanks again!!!!
Logged

Mark
KC3YYZ

"What you own is your own kingdom
What you do is your own glory
What you love is your own power
What you live is your own story"
                                        N. Peart
N5VTU
Member

Posts: 388




Ignore
« Reply #8 on: April 30, 2017, 10:07:17 AM »

Great!  As long as you keep the radio supplied with 13.8vdc +/-10% @ 20A per the spec, it should be happy.  Every connection (even fuses) between the power source and the radio will result in I^2R loss (some negligible, some quite noticeable), as will undersized and/or lengthy power wires.

The FT-840 was a great little entry level rig (my first HF rig ever) in its day and there's no reason that with a little TLC you can't enjoy it for years to come.

Good luck
Stephen
N5VTU

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N7EKU
Member

Posts: 719




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« Reply #9 on: April 30, 2017, 08:46:34 PM »

Hi,

Well.  If you put your "bad" power connections back again, you can just use a DMM and check the voltage at different points along the way while transmitting into a dummy load.  At some point (hopefully just one) you should see the big voltage drop.  Probably it is a bad connection, crimping/soldering job, or something on the power distribution board that can't handle the current.  Also, is the in-line current meter rated to handle a full 20A or so?

If it was my rig though, would not transmit too long under these conditions -- I'm not sure how healthy it is for the rig.

73,


Mark.

Thank you both! As per your advice, I hooked up  the radio directly to the power supply and I am getting nearly 100w. I had a rigrunner, generic powergate type thing, and an inline v/A meter between prior..

I also found this : http://www.eham.net/ehamforum/smf/index.php/topic,114604.0.html

Now I just need to figure out where the problem, but that should be simple!!! (I hope!!)

I will let you know what I find... I am leaning towards the powergate knockoff....

Thanks again!!!!

« Last Edit: April 30, 2017, 08:48:37 PM by N7EKU » Logged

Mark -- N7EKU/VE3
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