Sunday, January 28, 2018

A Little Weekend Update

Let me bring you up to date on the various projects and stories I've talked about. 

On the oscilloscope: the 5000 PIV diodes arrived yesterday and I wasted no time getting them into the circuit.  No joy.  I ran the scope on the variac box at 100V, but it has a "line voltage too low" light that was on.  I bumped up the line voltage until that went out.  The problem is that there appears to be no horizontal sweep of any kind, and the only time I see anything is if I press the beam finder button.  If I put a probe on the builtin calibrator (a 300mV pk-pk 1kHz square wave) and press beam finder, it spreads out to look like the correct voltage only without a sweep. 
These are the parts I changed.  The circuitry surrounds the horizontal sweep voltage and the high voltage, but it seems to involve lower voltage circuitry as well (I never see the sync light come on when I'm probing the calibrator).  I found test points for all the lower voltages in the unit (< 1000VDC) and all of those are good.  Troubleshooting will continue, although to really troubleshoot an oscilloscope requires another oscilloscope.

BTW, I got 10 of those diodes expecting to blow them as soon as I replaced them, and go through a few pairs before the thing worked.  The replacement diodes are still good. 

The miniature K9AY loop antenna has turned out to work "pretty well".  I have no comparison of a full-sized version, so I can't evaluate it that way, but I have a few contacts using it and measured some parameters like the front to back null as best I can.  On 10.1 MHz (30m), its targeted band, signals may be just a little weaker than my reference antenna, but it has about a 20 dB front to back ratio, which might be helpful.  Signals seem lower on other bands, but Signal to Noise Ratio is sometimes better; it probably depends on things like the exact angle of arrival, the specific noise or interference source I'm having trouble with, and other parameters that are hard to quantify.  Slightly surprisingly, it seems to work fairly well on 3.5 MHz (80m), which is the target band for the full-sized loop. 

The variac box has turned out to be really helpful and something I should have done before.  I've powered on all of my old vacuum tube gear (except an R-390A that I use several times a year) going through a protocol of leaving it on 90V, then 100, 110V each for an hour and finally the wall 120V.  All of the old equipment is behaving fine.  Nothing smoked. 

It took me a while to find a schematic editor that I like to produce a drawing for the variac box.  TinyCAD, totally freeware.   I have PCB123 for small PWBs if I design something that needs a board, but for this application I wanted something more like the plastic stencils we used to use in the years BC (Before CAD); I have no need for PWB component footprints for something wired point-to-point using duplex AC outlets from the hardware store. 
I upgraded the project guitar, giving it nicer tuning machines.  The fit, finish and functional smoothness are noticeably better than the $8 eBay specials I put on it first, but I honestly had no idea it would even survive having strings put on it.  I did a complete setup to the action to make it easier to play, and more like my best acoustic.  The work included setting the neck bow, leveling the frets, with my fret rocker and an 8" mill file, then adjusting the string heights with my string height gauge with nut slotting files at the nut and lowering saddle height at the other end. 

The other things that I've been dumping vast amounts of time into are probably too esoteric even for this crowd (I'm guessing a handful of you are intimately familiar with the Smith Chart, for example), and choosing the next project for the metal shop.  I haven't actually chosen that, but I'm getting closer.

And that's some of what's going on that I don't always write about!


  1. It sounds like the crud did not hit too hard! Good!
    It is fun to make and fix things, isn't it? I finally fixed the sewing machine I was working on.

    1. Thankfully, it was just a stomach bug. No big deal.

  2. RE: variac Box, What is the DPM module and SO4 ? I searched and found a "DC Charging Module" but nothing on SO 4. thanks

    1. SO4 is an AC outlet, dual, for a standard old 120V plug. They’re the kind you get for 75 cents at Home Depot.

      DPM is a digital panel meter. I’d call it a black box, but it’s not in a box, just open frame. There’s a toroid core on its back that the black wire goes through in order to sense current, that’s the through line on the symbol and two wires to go to the AC socket to measure the output voltage of the variac. I think I linked to the eBay auction where I got it in one of the posts I put up about it.

  3. SiG,

    If you look down at the right rear corner of your oscilloscope, you'll see a key with its handle folded flat. Unless you raise that handle and give the key seven or eight turns, you won't see a sweep on your screen. Hope that helps.

    1. That's the mouse's treadmill. The speed he runs on the wheel sets the sweep rate.

    2. So wake the little sumbitch up and put a cat in there with him.

      THAT will get things going right and proper in a helluva hurry...