He does a pretty job of explaining and demoing this feature. I post it here as a convenience for myself. There's also this video on how to check one of these beasts:
..and some more:
Enjoy; I shall.
I am compelled to weigh in with a less-than-sunny evaluation of this product. I had written a previous review (removed by eham.net) based upon my experiences with the Idiom Press people and their failure to not only not deliver their product BUT TO REFUSE TO ACKNOWLEDGE MY ORDER AND RESPOND TO E-MAILS AND PHONE CALLS OVER A SIX-WEEK PERIOD. (I was not alone in travails like these; another ham had similar contemporaneous problems and likewise cancelled his order.) I can understand the previous review being removed because it was not about the product per se.
Anyway, aside from the fact that I did not have my order honored and ultimately cancelled with them, I bore the actual product no malice. In fact, because I was able to pick up one at a swap meet -- one requiring repair work, by the way -- I want to now write of my experiences and offer caveats to anyone contemplating purchasing the product or the kit. Here goes:
Firstly, the unit I got at the swap did not work. Attempts to find a manual were fruitless until a fellow ham graciously sent me a copy of his. Upon commencing the repair work I noted that there were radical differences between the manual and the actual unit I had on hand. While the board may have been accurate per the manual, the front switch was not. My unit had a DPDT with six tabs and the manual had nine tabs. The wiring on the unit I had bore only the remotest resemblance to the wiring in the documentation.
Please note that the instructions were published in 2006 and there was an addendum of corrections outlining a means of modifying the LM386 circuitry so it would perform better. (I think circa 2008) It was a very complex series of "four instructions up from the bottom change x to y" steps and occurred to me that perhaps the owner should have REWRITTEN the original to reflect the changes and tied them more tightly to board/kit revisions.
When I finally got my unit working, I was pleased -- BUT ONLY MILDLY IMPRESSED -- with its performance. The circuit is merely a low pass filter comprised of the two SCAF chips driven by a 555 timer. It ain't rocket science. Frankly, the NEScaf kit by the New England QRP club (http://newenglandqrp.org/nescaf) and the HI-PER-MITE (http://www.4sqrp.com/HiPerMite.php) offer products with as much promise, are very well supported, and are much more reasonably priced than the Idiom Press SCAF-1 unit.
Finally, previous "I bought mine several years ago, built it, and love it" reviews might be nice and fuzzy and heartwarming but you will be spending anywhere from $95 to $130 for one of these things and, if so, you had better be prepared for
(1) At least six weeks of being ignored after your order is taken.
(2) The prospect of a kit whose instructions are ambiguous and possibly out-of-date.
(3) A pathetic record when it comes to support and service.
The 2 is for the performance of the unit. Were I to rate the overall experience with Idiom Press with respect to the SCAF-1, it would be zero.
Color me unimpressed.
(1) The two Zener diodes to derive the +5V and -5V display run hot (very hot!) when a phone jack is plugged in the back and 12.5 VDC is applied.Now, I have no Earthly idea why all of this happens (which is why I am going to WB6JDH's QTH Saturday) but, when
(2) The unit does not work (i.e., complete silence) when powered by 12.5 VDC and switched on. See above regarding hot Zeners.
(3) The unit will not work with the right rear PCB mounting screw put in place and tightened down. It's intermittent. Again, see #1 and #2 above. This also happens when the box lid is mounted.
So, the bottom line is that common ground problem exists for anyone who buys the SCAF-1. Also, here's a tidbit: the SCAF-1 circuit was ripped off a 1995 QST article (reference upon request) that has a Far circuit board made for it. I was gonna get that but, now that I know the dual-ground problem, I think I'll live with my other swap meet treasure -- the Ramsey AF-1 -- and that should be fine.
Just a word to let you know that there is a potentially serious bug (thank you for the schematic and manual) on the SCAF-1. It will not affect 99% of the people who use it, but it bit me in the fanny.
As you know there are two grounds in the unit. One is the power supply common ground and the other is the "artificial ground" in the circuit to derive the +5 VDC and -5VDC voltages for the chips. Turns out, if you have a common power supply for your SCAF-1 and your HF rig, then the two grounds on the SCAF-1 touch pee-pees and cause the voltages to get all hosed up and the Zeners to get hotter than blazes.
It affects my station because I use an Astron RS-20M to drive my IC-745 *and* the SCAF-1.
First noticed it when I was researching why the board seemed to have "microphonics" when I buttoned up the board and put on the cover. At the risk of boring you to distraction, I kept the cover off when I was testing it in the station and noticed every time I plugged in the station PS,that the unit stopped working and the Zeners began to sizzle. I then powered it with a separate gel cell and everything worked fine.
Also, my unit was one built by one of my SK friends and it had 4-40 screws from the chassis to the board which -- after a little abrasion -- shorted the two grounds together. So I replaced them with nylon screws to avoid any problems.
Probably obsessing over this, but would not feel right if I did not inform you. Oh, and I am NOT a EE but I got a friend who is [one and is the smartest ham I know when it comes to fixing things: WB6JDH] and he confirms [my analysis].
What a lot of folks don't realize is that a great many of the amateur radio suppliers, and especially the QRP type suppliers, are "one man operations"! As such, they don't have a particularly organized and sophisticated ordering and shipping process. Accordingly, if you are in a particular hurry, or need something quickly from one of these very small companies, you should talk to them personally and make sure you are going to receive your order in the timeframe you are expecting. There are a thousand reasons why it might take longer than you expect to fill your order. Most of these little companies are operating out of their garages, or similarly small space, and parts coming in versus things going out can be an issue. I'm not making excuses for bad service, but so many of these operations are reliant on their receiving the necessary parts, which they really can't control. They can't afford to maintain large inventories of parts, so a sudden rush of orders probably means a lag time that would normally be considered unacceptable.You go in business, advertize a product and you either deliver or you respond to e-mail inquiries as to timeliness of the order-filling process or an explanation re parts shortages, etc. I would just salivate to see W7AQK order something form those folks and then try to phone them up for a status. That's the point, dummy! They do not respond.
Bottom line, if you are in a big hurry, call them and talk to them personally to find out what delay might be in the offing! I know, some of
these operations promise more than they can handle, but most of them, I think, try to be reasonably forthright in their advertising. In any event,
if time is of the essence, I'd be verifying things them personally.
(1) Speed of light = 186,000 miles per second.
(2) 186,000 x 5,280 = 982,080,000 feet per second.
(3) 982,080,000 / 1,000,000,00 = 0.98208 feet per nanosecond.
(4) 0.98208 feet = 11.78 inches.
(5) Light travels 100% in free space but travels only as fast as the velocity factor along a cable.
(6) The velocity factors of popular cables are:
CABLE - VF
RG-8 - .66
LMR-400 - .85
RG-8X - .84
RG-11 - .75
RG-58 - .66
LMR-195 - .83
RG-59 - .82
RG-62 - .84
RG-174 - .66
RG-213 - .66
RG-214 - .66
RG-217 - .66
RG-218 - .66
RG-316 - .79
RG-400 - .695
LMR-500 - .85
LMR-600 - .86
1/2 HARD - .81
7/8 HARD - .81
(7) I got 2 hunks of cable I want to determine the length of. One is a long piece of RG-8X and the other is a shorter piece of RG-58. I have another hunk of coax that I want to verify the velocity factor of.
(8) For the longer hunk of RG-8X, I assume that the velocity factor in 84% so that means that when an electron travels through it it's only going 84% as fast as its cousin in free space. Consequently, it is traveling 84% of 11.78 is 9.98952 inches. So let's say 10 inches for grins.
(9) I lashed up the scope and the function generator like these two lads did and found that the RG-8X cable's out-and-back trip was 2.4 divisions at 100 nanoseconds per division. So 100 x 2.4 = 240 and half of that (only want one-way) is 120 nanoseconds. Thus, the cable is 120 x 10 inches or 1,200 inches long. Obviously, 1,200 inches is 100 feet. (Do I really have to?) And, guess what, that's what the guy at the swap meet sold to me: 100 feet of RG-8X.
(10) The shorter hunk was 1.6 divisions at 100 nanoseconds which works out to 160 nanoseconds. The one-way time, of course, is 80 nanoseconds and, since it's RG-58, the distance traveled in a nanosecond is 11.78 x 0.66 or 7.78 inches. So electrons traveling 80 x 7.78 inches or 51.8 feet.Let me get a tape measure and check that number.
(11) Not for the third hunk of coax, I know it's 22.75 inches long and I know that it "scopes out" to 0.7 divisions and 100 nanoseconds. Half of this (one-way trip) is 0.35 x 100 or 35 nanoseconds and 35 x 11.78 or 412.3 inches or 34.36 feet.
But that's the distance in free space, and I know that the coax is 22.75 feet long. So the velocity factor would be
34.36 / 22.75 or 0.662107 or 66% -- which is what they say RG-58 should be.
Funny thing, that!