Tuesday, March 4, 2014

Making progress on the UW VFO..

I diddled with the KayPro some over the weekend and have been led to believe that re-capping the Micrex video board might solve my problem. I will do a more lengthy post on that as  reference in the event anyone's searching. But, honestly, my impression of the knowledge out thee on the various Retro Computing boards is miles wide and millimeters thick. I sure as hell ain't no double-E but some of the folk lore is just absolute fantasy land presided over by witch doctors. I'll dig up some of the incredible excerpts when I do my post. One of them is a real side-splitter.

Anyway, I had a few moments this evening and wanted to engage in some idiocy of my own, so I turned back to the Ugly Weekender VFO. What follows is something that outstrips the stupidity of the KayPro posts and leaves them panting in the dust!

You are warned!

I put it in the form of an e-mail to WB6JDH:
Dick,

Per your advice, I rewound the coil: 42 turns tapped about 10 turns up from ground. It measured out at 8.45 uH and, when strapped in alone without the variable cap, hummed along at about 4.810 MHz. Adding the variable cap dropped it to 4.700-4.780 MHz (swinging the cap). Here are some pictures:

Source of Q1

From there:

Collector of Q3

No wonder we use a transformer after Q3. How does it eliminate all of that hash?

Output of T1

Note that I swapped the 10K resistor for a 5K and that only gave me the 650 mV above. When the 10K was swapped back in, it rose a bit. I still don't understand why that stage is in there unless it's to buffer the VFO from the next stages.

Anyway, I lashed in the variable cap and just for grins measured the total capacitance across C2 (i.e., the entire L/C circuit) and got a 180-270 pF swing across the range of C1.


Probably not kosher; tracing that out on the nomograph yields about 3.200 MHz.


If I do some rough calculations, the two caps in parallel yield 100+10 pF to 100+120 pF or a range of 33-39 pF. That in parallel with the 100 adjacent to C2 (which is not in the circuit) yields 133-139 pF, I guess. Probably wrong; maybe not. Look what it does on the nomograph:


Using the nomograph again, I gotta wind a toroid a little less that 4 uH to get the VFO in the 7.000-7.070 MHz range. That's about half of what the coil is no (42 turns) -- or 21 turns.


Not really sure what will happen if I don't land on 7.000 MHz going up -- So I am going take a deep a breath and probably add a padder at C1 and see what that that does and *then* rewind the toroid. That way I can zero he VFO in.

So that's my story and I am sticking to it.

Am I crazy?

~Bill
-72-

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