Icom IC-7300 & IC-7610 “Built-In” Tuning Pulser

Overview

A tuning pulser is a device which generates a series of short-duty cycle tone bursts which are applied to the audio input of a transmitter. Instead of tuning up a tube linear amplifier (such as the Ameritron AL-80B in my case) using a constant envelope signal such as CW or FM for longer than several seconds can stress the tube(s) in the amplifier, creating excess anode heat. This can shorten the life of the tube(s). This article applies only to tuning up vacuum tube RF amplifiers. It does not apply to any other type of amplifier (solid state, for example), nor does it apply to antenna “tuners“.

Since nothing I make looks nice I had considered buying one of these tuning pulser devices to give my AL-80B a break during tuning. I’m not a slouch when it comes to tuning up an amplifier, but doing it with short duration pulses sounded much better. Doing so reduces the average current considerably (see below), and the peak-reading power meter will still show and hold accurate readings. After a short conversation with W8JI (the designer of the AL-80B and many other amplifiers) on the Ameritron Amplifier Users Group on Facebook, I used a tone generator utility which he linked me to generate a signal similar to an amplifier tuning pulser.

The Importance of Duty Cycle

Courtesy of Wikipedia: The term duty cycle describes the proportion of ‘on’ time to the regular interval or ‘period’ of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on. When a digital signal is on half of the time and off the other half of the time, the digital signal has a duty cycle of 50% and resembles a “square” wave. When a digital signal spends more time in the on state than the off state, it has a duty cycle of >50%. When a digital signal spends more time in the off state than the on state, it has a duty cycle of <50%. Here is a pictorial that illustrates these three scenarios:

Working out the percent duty cycle is straightforward:

100*\frac{(TimeOn)}{(TimeOn+TimeOff)}

So, 100*(15)/(15+75) which rounded up is a 16.7% duty cycle. The amplifier is only amplifying for around 17% of the time while you tune up using this method. This reduces the stresses on the amplifier considerably while tuning up, allowing extended periods for tuning up without overheating the tube(s) in your amplifier.

Implementation

The idea was to grab the audio generated by the tone generator app, save it as a WAV file, then copy it to the SD card I have in my Icom IC-7610 as a voicetx file. The main pain in the backside was figuring out that the Icom radios won’t play just any type of WAV file. The WAV bitrate for IC-7610 must be 256kbps, with a sample rate of 16kHz. The IC-7300 requires a 8kHz bit rate (overall WAV bit rate of 128kbps) so there is a separate file for each radio available for download below.

For your convenience, and for my convenience (I don’t have to write a long post about how to record the audio which you hear on your PC, then convert it, set the metadata to make sure it has a name in the radio, blah blah blah) I created a WAV file in the correct format for IC-7610 and IC-7300. These files are one minute long, which should be plenty of time for even the slowest tuning process! Download these files below. Make sure you download the correct file for your radio. KI8D also kindly provided his tuning WAV file from his IC-7300. You can download that below also.

Once you have downloaded the file, copy it to your Icom’s SD card. Don’t have an SD card? RTFM and get one. They’re very useful for screenshots, saving audio, and THIS!
The location in which to place the file is IC-7610/VoiceTx. I am assuming that this is more or less the same for the IC-7300/VoiceTx in the IC-7300 radio.

You may also change the number in the file name from 1 to 8. For example, the default name is voicetx8 which means it will appear in slot T8, like this:

If you rename the file to voicetx7 it will appear in slot T7. Guess what happens if you rename it to voicetx6? Right, you get the gist. I set the WAV file metadata so that the file would show up as “Amp Tune” within the radio. If you want to change this, you’ll need to change the file’s metadata.

The Result

Here is a video of the radio, file, and amplifier tune-up in operation. I hope this proves useful for you!

A thank you…

Thank you for reading. It’s been a while since I posted anything on this website, and I can only apologize. I have recently moved across the country and started a new job, so things have been a bit crazy. Thank you for your patience and thank you also to the approx. 200 daily visitors for keeping this website alive and giving it some meaning. I have recently set my shack back up in my new location, so if you hear me on the air (likely 20 or 40 meters) please give me a shout!!

Cheers and 73 de AD5GG.