A Tale of Two Old Analog VFOs -- Artificial Intelligence's Role. PLEASE COMMENT!

The Kenwood VFO under test, with 2 Yaesu VFOs Standing By 

I have sung the praises of the VFOs taken out of the old Yaesu FT-101s.  I am using them in two homebrew 20 meter SSB transcievers,  one in the Dominican Republic, the other here in Virginia.  We started buying these things just for the anti-backlash reduction drives. But the vendors would send along the reduction drives AND the entire VFO module.  These circuits were all analog, and all discrete component.  I understood them, and had completely homebrewed circuits very much like them.  So I felt comfortable using them. 

Somewhere along the line, I got the VFO from a Kenwood TS-820s.  My motivation was the same:  I wanted the anti-backlash reduction drive. But as with the Yaesus, the entire VFO box came with the purchase.    The frequency on this one was much lower than the Yaesu.  the Kenwood ran from 5-5.5 MHz, the Yeasu was up around 9 MHz.  On this alone, the Kenwood seemed better.  

I recently put the Kenwood on the bench and began testing it.  I used Artificial Intelligence (AI) in the tests.  The results were good and bad.  Sometimes AI helped, sometimes it made matters worse.  Mike WU2D recently used AI to look at the tubes of an old receiver he was working on.  He too was impressed by the usefullness of AI, but I think he too had some misgivings. Here are some of Mike's conclusions: 

Let's start with a fundamental question:  I asked Gemini which of the two "ages better."  Gemini said that the Kenwood was superior in this regard.  I disagree.  Here is why: 

-- Tuning on the Kenwood was very jumpy and very scratchy.  AI correctly said that this was probably due to corrosion over the years on the connections between the main tuning capacitor and ground.  Strike 1! AI recomended cleaning with DeOxit. As a last resort they recommended putting some solder wick between the rotator shaft and ground.  I did that and it fixed the problem (you can see the solder wick in the picture above), but I never had to do that with the Yaesu VFOs.   

-- The schematic of the VFO shows a .9 V RMS output from the VFO.  But it does not specify a 50 ohm load.  Here is the Yaesu schematic: 

AI kept insisting that I use a 50 ohm load.  When I did, the output was reduced down to 54 m V rms. I questioned AI about the load, but it kept insisting on 50 ohms.  Only later, did AI point out that in the Yaesu, the VFO would feed into a HIGH IMPEDANCE load.  Strike 2. 

-- Gemini kept warning about the Q3 and Q4 transistors. They were 2SC460 transistors with weird (ECB) pinouts.  And they were prone, it said, to a horrible sounding disease: "Original Hitachi 2SC460 transistors are infamous in vintage ham gear for silver migration ("black foot disease"), where the lead plating oxidizes, turns black, and causes severe noise, leakage, or intermittent oscillation failure." AI happily recommended replacing them with 2N3904s.  I did this, but the voltage on the emitter was way off.  AI blamed this on the new transistors (that it had recommended!) saying that their hfe was too high (so, uh, why did you recommend them?).  It then said that the direct coupling from Q3 to Q4  was at fault;  it recommented putting a .001 uF cap between these two transistors, along with two resistors for the biasing of Q4.  I did this.  I am not sure if it was necessary.  Strike 3. 

-- Gemini said I should check D3. It was germanium, you see, and "notorious"  for going open.  It was open.  I replaced it with a wire, but AI had a fit.  It worried that voltage from the follow-on circuits in the Yaesu with mess up the voltages on Q4.  I told AI not to worry about this -- all I had was the VFO!  Strike 4. 

-- I still wasn't getting good output.  Here AI was very useful.  First it told me to check the voltage on the emitter of Q4.  It was too high.  Then it told me to look at the resistance to ground from the emitter of Q4.  Uh oh.  It was open.  I quicky went through the connections to ground.  L6 was open.  I relaced it with 100 uH inductor. This corrected the situation.  I now had 1 volts AC rms at the output with no load and .8 V AC rms with a 1000 ohm resistor in the load.  SCORE ONE FOR AI. 

So, looking back, I came to several conclusions:  

-- Gemini was just wrong in saying that the Kenwood VFO ages better than the Yeasu VFO.  The Kenwood had Germanium diodes, weird transistors that get "Black Foot" disease, main tuning caps that get scratchy due to corrosion, inductors that go open.  Yuck.

-- You have to take the recommendations from AI with a grain of salt (see WU2D's conclusions above).  Sometimes they are right, sometimes they are wrong, and sometimes they recommend things that might not be necessary.  Dean KK4DAS points out that AI's seem to do better when you upload the schematic.  Maybe this lets AI analyse it using a simulator.  I had not done this.

-- I will probably use the Kenwood VFO in a 17 meter SSB transceiver.  I recently bought some 13 MHz crystals for the filter and the BFO/Carrier Oscillator from Mouser.  And AI confirms that this combination should not produce any problematic spurs.  We'll see.  The ultimate test is still real-world construction. 

What do you guys think about using AI in projects like this?  Will AI improve?