Saturday, October 7, 2017

GSC 20A DC Power Supply Redo

This boatanchor was a find at the area radio store, as-is, think I paid 20 bucks for it, a buck per amp. Brought it home and did the usual lookover, found no issues, output was around 14V. Tried it with some VHF mobiles and ended up using it to power 3 at a time for monitoring and could transmit on one at high power with no issues. 

But not surprisingly, since this thing is old school brute-force (completely opposite from modern switching supplies) it just has a huge transformer, two diode rectifier (half-wave), a Zener diode regulator then a classic thee-transistor array to regulate the higher current flows. The transformer is simply bolted to the bottom of the case, the nuts had worked loose enough so it would buzz until i smacked the case. Plus these things tend to run hot, again, they are old school. 

Eventually I took the case apart again to analyze the design. Right away i realized most of the heat was coming from the two ECG5980 40A rectifier diodes mounted on a sheet of aluminum. This got me thinking how a full-wave would be nice to try in there, and make at least that part more efficient. The ripple tanks had been replaced from the original, for which there were still two ring mounts on the base of the cabinet, each measuring 1.5 inches diameter - like finding dinosaur tracks. We all know that vintage capacitors were much bigger than today and didn't age well, so it's not surprising these had been replaced with two 15,000μF axial lead in parallel. I also noticed there was a 510Ω bleeder resistor soldered across the outputs, so someone had done a decent job fixing it up. 

So in taking stock of what this thing offers, we have the dual-secondary coil that is presumably of 360 VA variety across both the windings. We also have the sturdy metal case that could be refinished, and the three TO-3 bays with heatsinks. Plus the main switch still worked fine, as does the fuse holder and rear binding posts. These would be the most expensive parts to start with from scratch, and so this makes it a worthwhile exercise to explore the PS theory I'd always read about. 

The panel light ran from the 120V side, and I believe the exact same lamp is available in the NTE line. But in this day and age there's nothing like the reliability of an LED, as long as the regulation works as it should. Plus, the LED acts as a bleeder circuit and helps indicate when things have gotten closer to steady-state.

For the rectifier I started getting a vision of the full-wave made of single diodes mounted on a breadbord that could be bolted to the case. Since this is dual-winding we'd go with parallel rectifiers, research shows nothing wrong with this. Quickly found the 10A diodes that would be decent to work with. 

As for the new ripple tanks, this part involved some of the biggest learning curve. I noted that my switching supply just had low capacitance and grounded two series ones at the middle, so that is what we tried first. Also at first I stuck with the original 15V Zener diode and replaced the original 270Ω 3W resistors with metal film equivalents, but wasn't seeing the regulation kick in. 

So I made a small breadbord tester with a 300mA transformer and some 3904 transistors, ran lots of tests, and concluded that a 7815 regulator would be the way to go. This would mean that all 3 of the 2N3055 power transistors would be in parallel and help share the current load, whereas in the original Zener circuit the first one sets the bias for the other two. 

But the regulation still did not live up to par, and just recently I got back to researching it. 

Realized it was time to consider high-capacitance for the ripple tanks. According to a couple formulas I found online it should come out to around 60-70,000μF across the regulated output. Well, the highest 25V capacitors I could find within a decent budget are the 22,000μF. (Note they need to be 25V since the regulators produce ~21VDC if memory serves.) Now a lot could be said about how these add up in the final circuit as wired, with all the series and parallel going on, but suffice it to say they seem to pass muster. 

When first test-firing it I noticed the panel LED stayed on quite a while, and realize i should have expected it given the massive boost in storage. Something made me check how much voltage that LED would take. The specs say 13.5 V max, and the nominal output is right around 14.2, so the finishing touch at this point is the 33Ω 1W resistor which brings it down to 13.25V according to my meter. Just to note I would have spec'd a 47Ω for the 2-volt drop @ 0.417mA but didn't have one. 

One also quickly notices the instant gulp as the transformer kicks in and loads up the capacitors, but I don't think it's enough to blow a lot of breakers.

So for the first real test I try it with the IC-706MKII running a packet station. The nominal receive draws 2A and 50W ERP transmit now and then will test the filtering. I let it run this way for almost an hour. The unit understandably got hot, but now it's mostly in the TO-3 units in the back and just a little over the transformer (which no longer buzzes). Granted, this is not a normal scenario, since the 706 at full power will draw 20A and I had never intended to use this PS for an HF rig. 

But, this seems to be a successful test, as I could detect no faltering or AC hum when transmitting at full setting when listening on a separate receiver. 

So, yeah, this can easily go back to being a workhorse PS for things that need bursts above 15A but cruise much lower, and that only need to run occasionally.

Sunday, January 31, 2016

Astatic D-104 Lollipop Mic Modernization

Objective was to procure an all-vintage D-104 and make it appear to my modern solid-state rig as a modern stock active mic. My first desktop mic was a non-amplified D104 variant that I bought with a 23-channel CB from a friend, and I always loved the design. 

A lot of learned souls have written about these mics and I found a bevy of sage advice, special shout out to K3DAV. 

They boast a clear, strong audio, they just plain look cool, and are an icon of American radio culture.

The restoration was spread out over a year thanks to my own indecision at times and waiting for inspiration to hit...
  • The one I found was made to order for about 40 bucks on eBay. Chrome had some pitting but not going for a looker, this is about feel. This had been sitting for years, all original wiring and parts. We can rebuild it. We have the technology. Thankfully it doesn't cost six million dollars.
  • First order of bitness is to inspect and clean up,
    • Disassemble the barrel section and Deoxit the switch and mic plug contacts
    • Remove bottom plate an Deoxit the trimmer pot, Elec/Relay switch, etc
    • Exterior - tried some steel wool on the chrome and it took the pits down to spots looks some better, good enough for my shack.
  • The original crystal mic elements are known to not age well due to the formulation used back then, but replacements are available for about ten bucks. The one I got is physically smaller than the original, but thanks to some spare foam and speaker grille cloth it fits just fine up there in the ol' crow's nest.
  • To make it plug-in ready to a ham rig,
    • Replaced the cord - the original is high-quality but for a few bucks it was worth while
    • Get an 8-pin plug
    • Tap into the DC power supplied by the rig - for Icom this is 8V which is perfect for replacing the 9V battery, after all, what is that 8V there for?
    • Add a 33k resistor in series with the audio line going to transceiver, to bring it down from tube-drive to levels consistent with solid state equipment.
  • At this point we tested it and found some noise...
    • Part of it was crackling in the 5k trimmer pot. Took it apart to find a worn spot in the paint, right where it needed to be set. Found it easiest to just swap around the end connections so the sweep would rest on solid carbon.
    • After the trimmer was back in play, found it still had low audio, bad fidelity, and a popping sound. Basically it was like someone talking from the other room while someone near by was drumming on an empty beer can. I think I cleaned the switch contacts once more and took some emery to them to make sure, but that didn't clear it up.
  • Finally decided the amp board had to be bad and to rebuild it -
    • Subbing a 2N3904 for the original 2SC945, which appears to be a legacy part these days
    • Wired on plain breadboard, since that's what I had. Some have made very nice etched boards for this but the redneck way will do just fine, if...
    • ...there's a way to mount the board and provide a chassis ground, as the original board does with lugs. At some point realized some 12-guage solid wire would do the trick, just need to drill holes in the board and bend it properly, and avoid cracking the board in the process. The solid wire is in a U-shape with the ends looped for mounting stubbs and the crossbar serving as a ground strip for the circuit.
    • Somehow this all came together, and once the new board was in place we are in bitness.

Should note, I don't do a lot of voice mode, mostly digital, but I've gotten some great reports with this mic, even on FM, which doesn't lie. Seems I no longer have to repeat information as often and sure is nice not having to worry about the battery.

Preamp Circuit:
  • Note the wire colors are for my particular base, which should be fairly universal but at least useful for reference.
  • The 5k variable is the trimmer that's accessible from the bottom. Generally this is just set once then left alone.

Base schematic sticker for this TUG8 stand:

73 de N8WWY