Thursday, August 16, 2018

Saga of the Suck 100 Antenna Analyzer

So here's the deal: Dick came by the yesterday to calibrate his Aussie Antenna Analyzer and check things out. That process was was moderately plain sailing until I pulled out my Sark 100 analyzer (hereinafter referred to as the SUCK 100) and we did some comparisons.

Well, it actually was not the comparisons that was the problem, it was the fact that my analyzer had developed a display problem. Seems that a row of bits in the second in the second line had decided to take a siesta and, on occasion, caused some ambiguity in the SWR and Z readings. Here's what I am talking about:

Quick glances at a VSWR of 1.90 might lead you to believe it was 1.00 or something. Anyway, the fact that I paid 80 Simoleans for this piece of shit and the display failed after about two-three months of minimal use and great care in a docile environment. (I was not building or measuring any antenna at McMurdo Sound or in the Sinai Desert.) So, when Dick folded up his operations, our attention turned to the SUCK 100 and the possibility of fixing the display. And, long story short, we reasoned that possibly one of the two-line displays I had on hand might be a suitable replacement inasmuch as they had the same pinouts and footprint.

After jimmying one in parallel with the existing display and putting power to it, we saw that it did light up and some characters seemed to appear intermittently. Anyway, it was enough for Dick to wick out the old display (he's better at that than I am) and replace it which led to success! We had a unit with both lines intact and robust and, although the backlight was not as brilliant, it was more than adequate. However, another problem arose in that the evil "Error Vf Level" message appeared and subsequent operation of the unit was inconsistent bordering on pathetic. So Dick departed for home and I spent the evening hours thinking and researching.

Luckily, I turned up the assembly manual here (the Sark 100 started out life as a kit) and it contained some troubleshooting tips:

Well, this was at first dismaying because my SUCK 100 was all SMT and had four or five of those tiny chips. However, pressing further, I discovered the following video which breathed life into the process above:

So, emboldened, I took a lash at tracing the problem and found that the output of U2 -- the AD8008A dual op amp -- was pretty puny and that it would probably have to be replaced. resigned to this, I noted that the chip itself was actually pretty hefty and a second look at the SMT parts proved to not be as scary as I originally thought. Sadly, the AD8008A was not readily available. The only sources were the eBay Chinese vendors with their questionable quality and four-month delivery times or Digikey who wants $3,100 for a boxcar full of these beasts. So I decided to see if the local houses had any of these in the morning.

But, while casting around for a source for the AD8008A, I happened to notice that one of the SMT caps near a display shim nut was empty and looked as though I had excised it when I wrestled the nut off the board. So I got a through-hole replacement, tacked it in place and powered the SUCK 100 up -- WITH NO VF ERROR MESSAGE!

And checking the output of U2, it was restored to the 1.5 Vrms per the procedure above. However, the analyzer still acted a little inconsistent so I subjected it to a re-calibration process (the 50-ohm, 150-ohm, 274-ohm mambo) and all seems to be right with the world!

An additional note: I savaged the crappy ON/OFF switch in the process of extricating the board (to get at the display) and found that one of those small SPST switches fit precisely in the slot on the board and could be tacked into place. (Dick took care of this as he is a wizard at such things but it was pretty easy to do if one is careful.), not too bad considering 15 hours ago I was going to toss the SUCK 100 onto the shit heap!

No comments:

Post a Comment