SEA 222 “Tune” switch modification

I had the pleasure of a visit from an old friend recently – the venerable and capable Stephens Engineering Associates SEA222 Marine HF radio. We’ve been “friends” for several decades now . I’ve repaired well over a hundred units over the years, and have spent quite literally  d o z e n s  of hours  carefully studying it’s magnificent design.  I can say confidently from a position of experience: It’s an exceptional rig, and holds up well in comparison to modern radios.

A customer sent his in for repair, and along the way – quite literally “out of the blue” I had an idea for a fairly simple solution that resolves an operational weakness of the rig: The lack of a “TUNE” button.

Most HF radio’s expect do regular day to day business with an automatic antenna tuner, and sport a “TUNE” button, that – when pressed – causes the rig to transmit a low power signal allowing the automatic antenna tuner to do it’s thing and set up for operation on the current frequency. For reasons I’m not privy to, the very smart guys at SEA declined to add one. Unfortunately, this decision essentially forces the rig’s operator into having to “modulate” – meaning – making some kind of sound into the microphone, sometimes for many seconds – in order to produce RF power the automatic antenna tuner needs in order to find a tuning solution. Keying the mic by itself wasn’t enough. SSB transmitters don’t make RF power without some kind of modulation. Yelling  “Fiiiiiiiiiiiive”  (or something similar)  was required to get things done.

This has an undesired side effect: When you modulate, it’s all but impossible to control the transmit power output, therefore the tuner is exposed to the full transmit power of the rig while it’s trying to find a tuning solution. This can really stress the diodes in the detector circuit, and will likely shorten their lifespan considerably. The power amp circuit won’t like it much either, since it will be exposed to high SWR as the tuner goes about finding a tuning solution.

For a variety of reasons – best practice is to tune up with relatively low power.

Wouldn’t be nice to spare your vocal cords and the antenna tuner the wear and tear and just press the damn “TUNE” button?  Too bad there ISN’T one.

 

Sidebar:  One of the steps when you program a frequency into memory, is an option for “A3H?”. Most people think “what the heck is that?” and move on.

A3H is a communications mode that’s essentially no longer in use. When you transmit a conventional SSB signal (A3J) the carrier and one of the sidebands is fully suppressed.   With A3H, one of the sidebands is suppressed, but the carrier is only partially suppressed (by -16dB). While I’m not entirely sure, I suspect this reduced carrier was utilized by certain types of “old school” receivers in order to facilitate accurate tuning of your transmitted signal. Frankly, I’ve never crossed paths with a SSB rig that needed to see a carrier for proper demodulation, or seen a listing on a frequency chart that stated the receiving station was operating A3H, so I really don’t know what the point is. The bottom line. This carrier is essentially a waste of energy and is totally unnecessary in the vast majority of circumstances.

For you tech heads: The 222 makes use of 2 “open drain” electronic switches embedded in one of the PLL ICs to enable A3A operation (a -6dB carrier required by law for 2182 KHZ) or A3H operation (assuming you entered “1” when the display read “A3H?” when you programmed a frequency into memory).

u11

When you enter a frequency, the microprocessor sends a data burst to the PLL not only setting the frequency, but turning on one of the switches (if the programming calls for it). Pressing the EMER button sends the data needed to tune the PLL to 2182 KHZ, AND turns on the “A3A” switch on the U11 pin15. FYI – When the switch it turned on, it connects the pin to ground.

These open drain switches ground the bottom end of a pot when activated. The pot control the voltage variable attenuator – u32, adjusting the level of a 6.4 MHZ  “I.F.carrier” into the signal stream. This signal is later translated into a carrier on the frequency that’s you’ve selected. You can adjust the pot to produce a key down RF signal from zero watts up to the maxim power out ~175 watts – no modulation required !

222CRA

 

Cutting to the chase, It occurred to me that a tune button could be added to the 222 by installing a DPDT momentary contact switch  – essentially in parallel with the existing switching circuitry. One side of the switch would ground the PTT signal (as the mic does when you key it) and the other side would simultaneously ground the A3H control line from the PLL IC.

R30 – the A3H carrier adjustment pot – is set so the rig produces 35 – 50 watts when activated.

In the photo below, I’m pressing the newly installed TUNE switch. Note the watt meter is indicating nearly 50 watts. Also note the mic. It’s not being used.

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After wiring it up and proving that the process worked, I needed to permanently mount the switch. I dismantled the front panel and set up my drill to cut a hole in order to mount the switch. When I could see the back side of the front panel, I noticed 2 holes already in the aluminum, but were covered by by the graphics label part of the keyboard. After careful inspection it became clear that all I had to do was cut through the plastic over the hole and the switch would go right in. The alignment was great – in line with the bottom row on the keyboard – and nicely centered between the volume control.

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Like it was meant to be there…

In the highly unlikely event that you needed to communicate with an ancient station that required A3H, you still can produce that kind of signal. The carrier will be a little “hot”, but that won’t have a detrimental effect on the receiving end.

I tested this with a 1612 auto tuner and it works brilliantly. The tuner endures much less stress durig the tune process, the radios power amp is spared having to endure the high SWR levels presented during the tuning process, and your vocal cords get a break. Once the tuner memorizes the parameters for the frequency, you’ll only have to press the TUNE button for a second.

 

 

 

 

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