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Author Topic: FT-101 output tuning  (Read 2479 times)
G7MRV
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« on: December 20, 2008, 03:29:26 PM »

Hi, not sure if this should be in boatanchors!

A colleague of mine, M1BBV, has just aquired a Yaesu FT-101 as his first HF rig.

Ive been reviewing the PA adjustment procedure, and run into a slight problem. He has an ATU and G5RV antenna, but no high power dummy load. The procedure seems to require setting up the finals at full power into a 50ohm dummy load.

Is there a way, perhaps by setting an arbitrary values of the controls, and setting a very low power output, to perhaps adjust the ATU for lowest SWR, before finally adjusting the finals at high power?

Also, how do you adjust the unit for lower tx power operation? is it just a case of lowering the mic gain?

Im afraid im not sufficiently familiar with this type of rig.

Regards

Martin G7MRV
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WB2WIK
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« Reply #1 on: December 20, 2008, 03:39:32 PM »

The FT-101 output power on CW, which would be used for "tuning," is adjustable all the way down to just about "nothing" using the CARRIER control on the front panel.

You can easily adjust that down to 5W or less.

It has no effect at all on SSB, however.

This rig runs full power on SSB all the time, there is no way to reduce it unless you modify the rig.  Turning down the mike gain to reduce power on SSB is a very bad idea.

WB2WIK/6
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G7MRV
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« Reply #2 on: December 20, 2008, 03:50:15 PM »

Thanks,

so, if he can turn the carrier setting down real low to adjust the PA, i can supply him  a load that will take that, until he can afford a high power load.

I understand what you say about it being a bad idea to lower the mic gain too much. But how hard is it to adjust the SSB output power? Im sure he would like to be able to do so. Do you know of a mod to do this?

One thing i noticed myself, the manual only gives the input power, as 280W on SSB. Im presuming this equates to an RF output power into 50ohm of 100W? The manual doesnt seem to explicitly state the RF output ( i should be able to work this in my head, but its been a few years now since i moved out of the industry!)

thanks for your help!
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N6AJR
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« Reply #3 on: December 20, 2008, 07:37:05 PM »

One trick for tuning, I always tuned all the knobs for maximum hiss  on the space between stations.  this will get you in the ball park on tx.  I also use a dual cross needle swr meter as then you don't have to push in and set, pull out and check.
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K2DC
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« Reply #4 on: December 20, 2008, 10:08:26 PM »

Martin,

The FT-101 has sweep tube finals (6SJ6's) which I think don't take kindly to being mistuned into a poorly matched load. If you can supply him with a matched load that will take 10 watts, that should be plenty to drive and calibrate an SWR meter. I would recommend that he tune up as best he can into the load at 10W, switch to the antenna to set the tuner for best match on the G5RV, and then to complete the tuning process. A bit cumbersome, but you gotta do what you gotta do.

The 280W referred to in the manual is the plate input power, not the output power.  At 60% PA efficiency, that equates to 168W tube output without accounting for any tuning circuit losses.  With new tubes, I think they used to run on the order of 120W output or so.  And there is no way to control output power on SSB other than lowering the mike gain which, as Steve said, is not a good idea.

GL & 73,

Don, K2DC
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K9FON
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« Reply #5 on: December 21, 2008, 11:37:14 AM »

Just get a bigger dummy load. Dont try to hack up a wonderful old rig to lower the SSB output.
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G7MRV
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« Reply #6 on: December 21, 2008, 12:04:09 PM »

Thanks for the advice guys, i will pass all this onto him, im sure he will master this rig in no time!

K6FON - the reason i ask about a mod for lowering the output power, is to allow him to use lower power when working locally, not for tuning, which we've established can be done using the carrier control.


Cheers guys

Martin G7MRV
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WB2WIK
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« Reply #7 on: December 21, 2008, 04:23:54 PM »

The easiest way to reduce the SSB output power of an FT-101 is to introduce a source of negative DC voltage into the ALC input pin on the accessory connector (rear panel).

Using a 1mA voltage divider across a 9V "transistor radio" battery makes this pretty easy to do: A 10K ohm potentiometer wired directly across the battery with the variable tap point connected to the ALC IN of the FT-101 (remember polarity! -- the "+" terminal of the battery goes to chassis ground of the rig, and the center point of the potentiometer connection goes to the ALC input pin, to make sure it's negative polarity feeding into the rig) allows you to control SSB output all the way down to "zero" if you wish.

I've done this lots of times, it works fine and does not require modifying the rig itself or even removing the cover.

WB2WIK/6
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VK1OD
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« Reply #8 on: December 22, 2008, 01:40:07 PM »


I am guessing the need to reduce power is to comply with a Foundation Licence restriction of what, 10WpX?

This post is about complying with such a limit

The reality for ALL PAs is that they are limited either by saturation or some effective automatic power limiting control system.

Reducing the power in an FT101, whether by reducing mic gain, or reducing IF gain (by modification, or external ALC drive does not limit the peak output of the PA other than starving it of drive. Occasional voice peaks may still drive it to 150W+pX.

The FT101 does not contain instrumentation to measure PEP output. Most ham external inline power meters do not properly measure PEP, or anything that is of use in determining the PEP of SSB telephony.

In the absence of an accurate PEP responding inline power meter, the Foundation operator has no idea of their PEP, except that it would be less than saturation (some 150W+pX).

Lets digress for a moment and explore the difference between reducing mic gain and reducing IF gain to reduce power output.

Reducing mic gain reduces power output wrt carrier leakage, so reducing mic gain by 12dB will reduce carrier suppression by 12dB. That is no big deal on modern radios with excellent carrier suppression, and probably not a big deal on a well adjusted stable older radio. IF gain reduction does not degrade carrier suppression.

Neither method does anything to limit the peak excursions.

A Foundation operator using a 200W radio at 10W faces the same challenges of an operator with a 400W limit using a transmitter capable of grossly more than 400W... just Foundation operators do it with less knowledge and experience.

Owen


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G7MRV
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« Reply #9 on: December 22, 2008, 02:09:23 PM »

Actually, in this case, no, it isnt to comply with foundation license power limits. My friends callsign was one of the last series of the old 'class B' licenses, and as such he now hold full status. His desire to control the output level is just to be able to use just the power necessary for the contact, for instance when ragchewing with me, only 16miles away!

Owen's points are valid however and important for Foundation and Intermediate license operators here in the UK, who may not be able to afford modern, new equipment that has easy control over the output power.
It is important for them, and probably more so their mentors/elmers, to know just what is happening in the rig! It would be very easy, it seems, for these classes of licensee to unwittingly breach their conditions if using older gear
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VK1OD
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« Reply #10 on: December 22, 2008, 04:38:50 PM »


Well, in that case, if you must, turn the mic gain down.

It is not uncommon that the quality of the transmitted signal is poorer at reduced power ouput.

One wonders whether it isn't better to be hear with a strong but cleaner signal.

This flies in the face of the common practice of operating a modern solid state transceiver at 10% power output to drive a 2kW amplifier, and the myth that it always produces a cleaner signal than the barefoot radio at rated output power.

If there isn't a regulatory or interference reason to reduce power, I would run it at rated power.

Owen
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