Can I make a joke along the lines of: if it's Federal, it's probably not good for you? I guess I just did. On that note, did you know that the archaic form of the word federal is foederal? I suppose that label was deemed a bit too transparent (cue laugh track). But, I digress, I'm not interested in talking about commonly accepted popularity contests or congregations of self important assbags, I'm more interested in rectifiers at the moment.


Here's a close-up of the selenium sandwich stack in a Navy ME-6D/U electronic multimeter (which will likely be covered in a future installation). Ease of identification is selenium's greatest attribute, it can typically be spotted in well under a minute of having cracked the case. The component pictured above is a less common version without the oversize heatsink fins. My philosophy is to change selenium out when I see it. You can read about the failure modes elsewhere on the net, here's a fine write-up: http://yarchive.net/electr/selenium_rectifiers.html. This is the rectifier block in schematic:


And, extrapolating from that, the electrical orientation of diodes in the stack:


In terms of out with the old and in with the new, this cannot be easier to wire; the main concern is going to be introducing some sort of framework that will avoid a free form rat's nest sculpture that's rectifying the B+ voltage (I'll be sure and photograph this section when I post an update on this unit).

In terms of electrical characteristics, selenium and silicon are not identical, and depending on the installation, a low value high wattage resistor might be beneficial in terms of nudging your B+ down a bit. Also keep in mind that average wall voltage has crept up over the past 50 years (I think evidence of selenium in the power supply typically dates the unit to that age at least) so the combination of higher AC mains and reduced internal resistance at the power supply might place undue stress on the circuit. A bucking transformer at the power cord is generally a good idea (even with tube recitification). http://www.philcoradio.com/phpBB2/viewtopic.php?f=11&t=2247

Regarding the selenium pull jobs, I have the sick notion of dropping them into ring modulators. Owing to the greater forward voltage drop (read as lower efficiency) I expect I'll need to build boosters on all three legs of the circuit. We'll see what shakes loose on that front.

IT LIVES!

After hacking away at this thing FAR MORE than I had intended to, the diode test fixture works, again. Having displaced the guts enough to tackle some metalwork, I bored out the power supply connector hole to accept a DB-9 connector that will support all power feeds, though I've retained the banana connectors for the variable supply for convenience.

After plumbing that in, I applied power only to learn that I'd lost a channel. Following a bit of hair pulling (from the front, naturally), I isolated a poor solder joint on one of the three breadboards. Closed it up and discovered that while I'd fixed the channel I was working on another channel took a shit. Traced that to the same board, repaired it.. gently tucked everything away and closed the lid. You'll just have to live without gutshot of the most current incarnation, sorry. Imagine a lot of wires that have been cut and spliced numerous times and three ugly breadboards.

Here's a couple face shots:

These numbers are meaningless in terms of absolute voltage. I have traded accuracy for precision. Here is a shot that demonstrates the sort of swing one gets from a change in temperature, my finger throwing off the readings of channel D (it took about 20-30 seconds to drop that far and it hadn't finished dropping).



That is why all matching of semiconductors takes so long, one must allow the part to thermally stabilise or any readings are essentially useless.

Sporadic updates, I'll try to be a better archivist.. it won't be difficult.

Things are looking good for my diode test rig, only 1.5 years in the making. To recap, it's a comparative machine that tests four at once. I was having a hell of a time calibrating it (across 4 parallel voltage dividing resistances) so I added a bank of MOSFET transistors to decouple each branch from the power supply, and eliminate the pesky crosstalk from wreaking havoc on my calibration process. But, before I go into that, here's photo-documentation of the earliest revisions:

In the event it hasn't occured to you by looking at that pic, this phase was a HUGE pain in the ass to calibrate. It entailed flipping the lid (which required mental inversion of position, since everything you see there is actually mounted to the lid) and gingerly turning the screwdriver slotted into a pot that is suspended by wires. Simply locating the correct pot for display and range was bad enough, but this mock-up proved the theory enough to pave the way to revision 2:
This was a huge step forward, all calibration being routed into the base of the unit allowing for adjustments with the test fixture closed and operating. Now, being able to physically engage in the act of calibration, I could come to the conclusion that from an electrical standpoint calibration was impossible.

In a nutshell, here's what's going on: An external variable voltage is fed through 4 identical load resistances which in turn each feed a diode in a conductive position; from the legs of the DUT is a three tier resistance ladder comprised of precision metal film resistors and trimpots (my basis of calibration) for each step, a millivolt meter is fed from taps along this ladder.

My theory is that the variable internal resistance of the diodes under test was large enough to throw the four parallel loads out of balance, making calibration impossible. So, the easiest (attempt at a) fix I could come up with was to slap some buffers on the variable voltage feed and hope to eliminate or at least reduce to an immeasurable level any cross talk between the test channels.

My first attempt at this was during a beer fuelled ElectronFeed meet, in which I electrically placed the MOSFET buffers feeding the DUT directly, on the wrong side of the load. This resulted in my polarity protection 1N4001 on the 12 volt meter feed acting as a fuse.

After sorting that out the meters wouldn't hold a steady measurement. Fearing them cooked, I disentangled one from the device, only to discover it worked fine. So now, with the inclusion of the buffers in a logical manner my metering has gone to shit. I'm a rat's whisker from gutting the entire thing and starting fresh when on a whim I disconnect the meter supply from common ground. Worked like a charm. So now, since I haven't any DC to DC converters laying about, I'll dedicate three power supplies to this thing. The variable bench supply (now only a reference thanks to the buffers), a 12 volt supply for the buffers sharing ground with the variable, and a 12 volt floating supply for the meters.

This weekend I hope to plunge a larger hole in the case to support the multipin PSU buss so I can escape the web of supply wires in order to... begin calibration.

All in the name of ring modulation.