Yaesu FT-857D: The curious case of… the PWR OUT bars

Recently, also to test a new 30 Ah LiFePo4 battery (but that’s another story), I dusted off the rig that is normally kept in the backpack for portable activities, ready to use (and I emphasize that it is ready, not that it is used).

After turning it on, I connected the trusty Bird wattmeter with the dummy load and tested the various output powers on all bands.

The powers were all within specifications, but I noticed that on the HF frequencies from 14MHz and above, the bars indicating (more or less…) the power emitted by the radio only showed a couple of notches even though the radio was actually set to 100W.

The radio was actually emitting 100W, so let’s say the problem was only “cosmetic”.

So, I started investigating to understand what the heck was happening…

I entered the service menu (A+B+C + PWR ON) and went through all the options in search of any settings for the power meter bars. There was nothing about it.

The only explicit setting related to the bar indicator on the display is for the S-Meter, but it has nothing to do with what I’m interested in…

So, I began analyzing the circuit diagram to understand how the RF signal sampling circuitry was designed: essentially, there are 2 lines for FWD and REFL that originate from the antenna connector and, through diodes, resistors, voltage amplifiers, lead to the processor in the form of DC voltage (mV).

Using a multimeter, I measured the DC millivolt values at various points of the sampling circuit with the radio set to 14MHz and 5W. Obviously, they were consistently the same for all the HF frequencies, from 1.8MHz up to 50MHz… so at this point, I feel inclined to exclude a hardware problem (I admit, I even disassembled the mainboard and tested various resistors along the path… all fine; well, I took the opportunity to apply some “fresh” thermal paste).

I slept on it.

In the following days, without a specific plan, I tried reviewing all the settings in the service menu and also found a small software that can backup/restore them, exporting to a specific file for safety. So, I saved the “original” values of the radio.

At this point, there are two possibilities: either something got messed up in the firmware and I’m out of luck (I’ll keep it that way) or there’s some setting in the service menu that influences (obviously not documented) how the CPU displays the PWR OUT bars on the screen.

Naturally, I pursue the second path (also because the first one would have arleady been a “game over”).

By the way, did I mention that I got the radio “used”? 🙂

Without much hope, I proceed to realign the output power on the different bands (service manual handy, RF generator, wattmeter, and lots of patience…).

I glance at the display (I was at 21MHz at this point) and I realize that the bars are “magically” fixed! (???).

At first, I couldn’t explain that, mainly because the actual output powers were already fine (the wattmeter always showed them within specifications).

However, I noticed that, on a couple of bands, the menu values were very high even though the power was indeed 100W.

Let me try to explain with an example: the values adjustable in the menu for each item range from 0 to 255. For the powers, 0 represents 0 watts, and as the value increases, the watts emitted by the radio also increase.

However, there is a limit on some bands where, beyond a certain point, increasing menu value does not influence the output power: on 50MHz, the value of 160 is the one that brings the radio to 100W, but I found it set at 240 (unnecessarily high). Something similar happened on 28MHz.

So, probably these numerically high values compared to the full power value cause the processor’s scaling calculations to go haywire, resulting in only 2 bars indicating the actual full power.

I would say that someone had tampered with the values in the past without really knowing what they were doing… 🙂 but fortunately, it wasn’t one of those guys who “boosted” the watts…

P.S. Is it clear now why I used quotation marks in the phrase “…original values of the radio” a few paragraphs above? 🙂


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