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Progress on the LDMOS Amplifier

Progress on the LDMOS Amplifier

A Fun Diversion

So OK I am easily distracted and once again I am off topic (only this is not political). Shown below is my beloved SWAN120 Single Band SSB Transceiver built in 1962. Yes, this is an appliance and not a homebrew radio. But there is a real story about the company that manufactured this radio and in particular about this specific radio.
In the late 1950’s, Collins Radio came out with an amazing product the KWM-1 a 100 watt, SSB transceiver covering the frequency range from 14 to 30 MHz in 200 kHz slices. This was followed by the KWM-2 which essentially covered the then five major ham bands, 80 through 10 Meters in 200 kHz band segments. This was a major departure from the separate receiver and transmitter that ruled the roost from day one. This opened up a whole new opportunity for mobile and portable operation. It was cutting edge technology with a cutting edge price tag! BTW it is rumored that a KWM-1 was used on Gary Powers’ U2 spy plane that was shot down by the Russians. So lots of intrigue here. Other companies were working on “Me Too” transceivers but the Collins rigs were by far ahead of the pack.
In true fashion there were hams working in garages the world over attempting to build a competing less expensive product. One of those visionaries was Herb Johnson, W6QKI (SK) who was living in Benson, Arizona ( a small town in the middle of nowhere not far from Tucson) who built one of those “competing rigs” right in his garage. The urban legend has that he convinced a radio parts supplier in Tucson to provide enough parts to build 10 radios. The concept was to build a single band radio based on operator preference and to concentrate on the portability aspect. In that lot of ten were several 40M and 20M radios. Ultimately the single band radios would operate in the 75, 40 and 20 Meter bands. Units were never commercially produced for the higher bands.
In true ham fashion Johnson bent the metal and painted the units right in his garage. Too bad Oasis Blue was not in the mix at that time. That first lot had an anodized (gold colored) front panel. Instead of a pair of 6146 tubes in the final ala Collins, Johnson used a TV sweep tube, a 6DQ5 which could produce 100 watts of RF. He also used the 7360 for the Balanced Modulator which added an air of cutting edge technology. Another innovation was to use a half lattice crystal filter operating around 5.0 MHz. Thus the same VFO range could yield 75 or 20 Meters. It was a no frills radio lacking refinements such as AGC or an S Meter. The essentially homebrew filter was a bit wide and the VFO was no PTO like in a Collins. But it was about 1/4 the price of a Collins. In addition to being a bit wide they were not as sensitive. But hey you were on SSB!
W6QKI produced several additional runs of radios at his Benson, AZ garage factory; but the popularity of the radio soon caused Herb Johnson to move to Oceanside, CA. The weather was definitely better but so was the availability of skilled electronic assemblers. Well the rest is history.
Now to my specific radio. It was built in Benson, AZ as indicated on the stamping on the back of the radio and is post the initial “gold” faced units but definitely built in the garage. Here is why I can make that claim. Take a good look at silk screen lettering –kinda crude and not well aligned with the controls or openings. The meter is much larger than ones found on later units and in fact later Swan units had a less expensive meter than the Triplett shown in the photo. In later production units to resolve the frequency display Swan used a second meter face that simply fit over the opening in the panel.
This radio was a fortunate find for me as it is in very good cosmetic condition and still works! So it is a keeper and yes I also have a Collins KWM-2.
But there is another story here, one that is perhaps almost as disturbing as the 2016 political process here in the US. Here is my concern. Swan was started by a single individual working out of his garage. He had a vision and that was to put an SSB transceiver in the hands of many hams who simply could not afford a Collins KWM-2. He also used innovations in his design. Ask yourself how many bands do you really work? I find myself spending most of my air time on two bands –40 and 20 Meters. When I operated mobile it was one band. One hundred watts is still one hundred watts whether coming from a pair of 6146’s or a 6DQ5.
The disturbing part is spending $7000 for a radio just so you can look at a 27 inch display to see if I have sparkle on my mid-range audio. BTW I can still repair that Swan 120 but not so sure I could tackle the new SDR radios. The disturbing part is, also, that there are very few hams with a soldering iron working out of a garage to produce a low cost radio that will provide endless hours of pleasure. I have a radio built in 1962 that still works in 2016 — that is 54 years. Where are the Herb Johnson’s of today? I can think of a couple (Ashar Farhan comes to mind) but in truth we are losing the skill set to try new ideas and thinking outside box to provide low cost radios that are frequency agile, stabile, and have a display but not a 27 inch screen. And the all important can be had for less than $200.
I am amazed at how well the radio still functions albeit a bit wide, slightly deaf and not too sensitive and did I mention “drifty”. Today we can build radios that resolve all of these issues at a mere fraction of the cost of that needed back in 1962.
Pete N6QW



In an earlier post I mention the Power FET Switch from W6PQL. I am now happy to report that the switch arrived and I built the kit. Quite a bargain and as supplied can be wired for a source voltage of 12, 28 or 48VDC. For my build I used the 48 VDC option. The build is straight forward and I soon will be giving it a smoke test. For info purposes shown below is the schematic of the switch and a photo of the actual built switch — only comment it is small. I now need to locate a heat sink rated at 24 watts.

Pete N6QW


More Discoveries Continued

Thanks to David GM4JJJ and his suggestion of using a power FET to quickly disconnect the LDMOS amp from the 48 VDC rail that will now be the approach. W6PQL, Jim, has a magnificent website where you can purchase parts for amps and one of his offerings is the FET Power Switch, which can be purchased as a kit or completely assembled. Variants include supply voltages of 12, 28 and 48 VDC. The kit version comes with the parts needed for any of these supply voltages.

The value of the blog is the bidirectional  sharing of information and I am most grateful for any and all inputs.

Pete N6QW

More subsystem testing and discoveries!

The real value of the breadboard test system is that it enables you to find/discover things that were not so obvious.
Currently I have the Start & Stop buttons wired to the Arduino. I also have all six of the Low Pass Filter relays enabling circuits installed and today I installed and used  LED16 which is the  circuit that must be engaged to sense the TR signal from the transceiver. Spent a little time getting LED16 to work properly as I had a bad relay.
Then I decided to use the actual 48 VDC power supply in place of the floor fan connected to the hockey pucks. The supply came on as desired when you punch Key #1 and does go off when you punch the 2 or 3 or 0 Keys. BUT I find that the going “off” condition does not happen instantly. The 48 VDC holds for a second or two and then slowly drifts down to 0 volts. That means the amp could be smoked by the time it gets to 0 volts.

This now requires using something such as a 40 or 50 amp automotive relay that has a 12 VDC coil fed off of the latch rail. This relay would have the 48 VDC fed from the supply  on to the contacts and then fed to the amp. As soon as the latch is broken the supply to the amp would drop. That adds another relay but the voltage “off” would be instantaneous. Had I not used the breadboard and done the testing I could have smoked the LDMOS Amp.

I have been doing some research on the SWR detector and talk about getting “wrapped in a knot”. That can happen really quickly. After some inquiries of those who know what they are doing I really don’t want to have a built SWR function. I have several external meters to do this including my treasured Drake W4. But what I do want is to detect the “reverse’ voltage coming back from a load that is mismatched to the amplifier output impedance of 50 Ohms. Helge Grangberg K7ESO/OH2ZE of Motorola fame wrote two articles in December 1982 and January 1983 QST regarding a 1KW MOSFET RF Power Amp. The protection of his amp was exactly that where a reduced bias would be applied to the amp stages as the Rev voltage increased.

Finally here is a shot of the control board. Additional control functions will be moved to a second board.

Pete N6QW