Wednesday, February 6, 2019

2019 ~ Retrofitting Old Boat Anchors with New Technology

Turning an Old Boat Anchor into a Silk Purse. Yes it involves the Arduino!!!!


I frequently tour eBay looking for bargain "Boat Anchors" that can be easily retrofitted to work the bands with rock solid frequency stability. Most of these old boat anchors suffer from drifting "ANALOG" VFO's. Yesterday I came close to acquiring such a rig. Early on Yaesu marketed some rigs that pre-date the highly successful FT-101 series of transceivers.

One such model is the FTdx-100 and I had one of those about 7 years ago and you can see how I "worked on that rig" on my website www.jessystems.com. Recently I spotted a Sommerkamp FT-100, virtually the same gear marketed by Sommerkamp in Germany along about 1966.



My complaint about the FTdx-100 was that it lacked DFMA.

[ For those who did not spend half their life in aerospace manufacturing DFMA is defined as Designed For Manufacturing & Assembly. Bottom Line you can service what you build without a total disassembly of what is built. I go into detail on my jessystems.com  website about how I was unable to replace the HV electrolytic caps because it meant removing a significant part of the under chassis wiring. Guess in true Japanese fashion they reasoned this will never have to be repaired because the quality was built in. Well caps get old and need replacing!]

But is was a marvel in that it was a hybrid rig that used HF Germanium transistors in the lower level stages and three tubes for the output stage. A 12BY7 driver and a pair of 6JM6's are in the output stages. BTW you can buy a matched pair of output tubes for about $25.

The FT-100 was particularly appealing in that it had a panel switch to select an external VFO or you could have crystal control of the receiver and transmitter.(Same for FTdx-100)

Now for the bonus part (and the Arduino). The primary analog VFO had a range of 8.9 MHz to 8.4 MHz. Yes it was a backward tuning VFO! There was a circular analog dial that had two sets of markings and these were 0 to 500 and 500 to 1000. On 80 and 10 meters you used the 500 to 1000 dial reading and on 40, 20 and 15 Meters you used the 0 to 500 scale. Thus if the dial real 250 that meant on 40 Meters you were on 7.250 MHz or on 15 Meters then the dial read 21.250. But at the same location if you were on 75/80 Meter you would read 3.750 MHz and for 10 Meters that would be 29.250 MHz for the reading. 

So now the trick is how to do that with an Arduino and an LCD display. Actually the effort involves building an external VFO that would have a 5 position band switch. The use of the band switch does two things: first is to signal and display what band it is on and secondly to choose one of two constants to perform some math functions. The only interconnect to the Boat Anchor rig is a small chunk of coax.

So how can we make the display (I chose an LCD) read 0 to 500 on some bands while reading 500 to 1000 on other bands. Keep in mind no matter what band, the VFO is on a constant range (8.9 to 8.4 MHz) and secondly works backwards (for the newbies that means at the low end of the band the VFO is at a higher frequency). 

A little time with an excel spreadsheet reveals the two numbers 8900000L and 9400000L. (The "L" is for large numbers that you identify in the sketch). 

So if you were on 40 Meters and wanted to read 250.000 (7.250 MHz) you would subtract the actual generated VFO frequency from 8900000L. Since .250 is half way between 8.9 and 8.4 the actual VFO frequency is 8650000L. When you are at 7.0 MHz then the subtraction is 8900000L - 8900000L = 000.000. So for three band positions the constant is 8900000L.

Now for the problem with 80 and 10 Meters. There we have to have a higher reading on the LCD for the same actual VFO frequency. Let us pick 3.750 MHz for our example. Yes the actual VFO frequency is the same as for our 40 Meter example 8650000L. But our LCD would have to display 750.000. Thus our constant would now be 9400000L. Thus if we take 9400000L - 8650000L the answer is 750000.

So our sketch logic would be to generate frequencies in the range of 8.9 MHz to 8.4 MHz and to limit the tuning range to that 500 kHz segment. There is a range limitation code in the software but you do have to make it 1 higher and 1 lower so that the display will in fact subtract and essentially read 0 to 500 and 500 to 1000. The other aspect is to decode the band switch so that the proper band is shown on the display and that the correct constant is used for displaying the range. As you tune the encoder the top line will show the operating 80 Meter frequency and the 3rd line the actual VFO frequency. As you change bands the 80M will change to the band selected and as you tune the encoder depending upon the band the top line will range between 0 to 500 or 500 to 1000.



Too bad I was not the successful bidder as the addition of the digital VFO would have made this a great boat anchor. The reason for the last line --later on Yaesu did make an FT-100 in a solid state version. I do not think it was around for very long.

73's
Pete N6QW/

2 comments:

  1. N6QW's jihad against analog oscillators continues. When will this senseless slaughter come to an end?

    ReplyDelete
  2. There is no jihad. Our hobby has room for all kinds of oscillator's and no real disparagement intended. But some analog VFO's do drift and in some cases quite a lot.

    Pete

    ReplyDelete

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