WMartin@SIMTEL20.ARPA (09/18/84)
From: William G. Martin <WMartin@SIMTEL20.ARPA> I think the antenna-trim on the CR-2021 is just changing a variable capacitor in series with the antenna input; that's what "antenna tune" controls on other radios I've seen have done. As for the whip vs. external, the whip is always active, I am sure. This can cause problems if you try to use an external medium-wave loop antenna for Broadcast-Band DXing, but otherwise shouldn't have much ill effect. I have some Uniden literature on the CR-2021, and it gives no technical specs at all, so I don't know the bandwidths or RF attenuations. As to modifications, sounds interesting, but I have heard of none yet. By the way, has anyone out there gotten a SERVICE MANUAL yet on the CR-2021, and, if so, please tell the list the price and where to write to order one. I have the Sony 2001 manual, which was only $2.00 from the Sony Kansas City parts depot, but, even if the circuit is similar, the mechanics are different enough that the real service manual would be worth having. Will Martin -------
jhs%Mitre-Bedford@sri-unix.UUCP (09/18/84)
Not knowing anything about the radio you are asking about, I will speculate that the "antenna adjust" knob is like those on other radios since time immoral: a tuning capacitor provided to resonate the input tank circuit for maximum signal transfer to the first amplifier, mixer, or whatever. Traditionally, these "antenna trimmers" are used to allow manual correction for any detuning effect of an antenna whose feed-point impedance differs from the ideal value contemplated in the design of the front-end resonant circuit, i.e., is highly reactive. Some receivers in the old days provided a low-impedance input arrangement for "real" antennas plus a tap on the top of the coil for a "short" antenna. An especially popular and useful arrangement was to have a center-tapped primary coil giving your choice of 75 ohms unbalanced or 300 ohms balanced input, with a convenient grounding strap to make the selection. (Remember that impedance varies as the square of turns, so that a center tap gives a 4:1 choice of ratios, not 2:1.) Often the short antenna was provided as a built-in whip, as in your radio. It might well be left connected at all times on the theory that if you collapse it and use a "real" antenna, the whip's small residual capacitance can be corrected for by twiddling the antenna trimmer. Still sticking to my position that full-blown antenna tuners are a waste of time on most receivers, I will allow as how the combination of several impedance taps and the trimmer gives a "mini" antenna tuning capability which is useful. In fact I suppose if one of these newfangled receivers didn't even give you a trimmer capacitor to adjust, a tuner would be a reasonable way to correct the designer's oversight. 73, John H. Sangster, W3IKG jhs at mitre-bedford
Taylor.WBST@XEROX.ARPA (09/19/84)
I GOT THE CR-2021 PROMPTLY FROM THE VA. SUPPLIER-- LIKE THE UNIT! I AGREE WITH WILL-- THE ANT. ADJUST IS PROBABLY JUST A CONVENTIONAL SERIES RESONATING CAPACITOR. I WOULD ASSUME THAT THE FINE ADJUST ON SSB IS JUST A VARIABLE BEAT FREQUENCY OSCILLATOR, SO IT WOULD HAVE NO EFFECT ON AM. I WOULD GUESS THAT THE ATTENUATOR STEPS ARE OF THE ORDER OF 10 DECIBELS EACH. IF IT'S IMPORTANT I CAN MEASURE IT. REGARDING THE BANDWIDTH, I RECALL +,-, 5 KHZ AND 2.5 KHZ FROM THE RADIO SHACK CATALOG FOR THE DX-400. GOOD LISTENING! JIM, W20ZH
gino@voder.UUCP (Gino Bloch) (09/25/84)
[if this line is present please report to the office] Re schematics with electronic gear: the Shack DX-400 comes with a schematic and costs $300; the essentially identical Uniden CR-2021 comes without one and costs $90 or $100. Take your choice. I chose the latter; I'm trying to find out if a service manual is available. If it's under $200, I'll probably buy it. -- Gene E. Bloch (...!nsc!voder!gino)
jcp@brl-tgr.UUCP (09/28/84)
The Antenna Adjust knob on the CR2021 probably does what most antenna tuners do, control a variable capacitor or inductor (probably the latter), forming 'transformer' which is used to match the input impedance of the radio, (probably 30-50 ohms) front end, to the impedance of the antenna (which varies with frequency, and for a short whip is probably several hundred ohms). From comparing the 'sound' of the filter on the CR2021 with my McKay Dymek DR-101, I would suspect 4Khz/8Khz as the filter widths, although it could be 3/6 as well. The 'narrow' is definetely less than 5Khz, which is a standard 'station spacing' on shortwave. The RF attenuator acts like about 10db per step, (which is pretty standard for attenuators). The 'fine tune' knob controls the frequency of the BFO, which is totally divorced from the IF tuning frequency generated by the digital synthesizer. I suspect getting it to act as fine tuning on AM is difficult. -JCP-
AWalker@RUTGERS.ARPA (09/28/84)
From: *Hobbit* <AWalker@RUTGERS.ARPA> I'm of the opinion that any piece of hardware should come with full documentation [service manual, source code for operating systems, etc], free. That is one thing that Rat Shak does right; everything they sell comes with a schemo. Other outfits want *money* for the things. Come on! If they spent less on fancy packaging and silly plastic frobs, and more on printing up schemo copies.... _H* -------
karsh@geowhiz.UUCP (Bruce Karsh) (10/04/84)
> [if this line is present please report to the office] > > Re schematics with electronic gear: the Shack DX-400 comes with a > schematic and costs $300; the essentially identical Uniden CR-2021 > comes without one and costs $90 or $100. Take your choice. > Gene E. Bloch (...!nsc!voder!gino) I went out and bought the DX-400 for about $200 a few months ago. What I'd like to know is whether there is any difference at all between the DX-400 and the CR-2021. A friend of mine is very anxious to get the DX-400. Wouldn't he be a lot better off getting the Uniden? Bruce Karsh
wmartin@brl-tgr.ARPA (Will Martin ) (10/04/84)
It's doubtful that you could say that someone would be "better off" buying the Uniden CR-2021 than the Radio Shack DX-400. They are essentially the same radio, with minor cosmetic and S-meter differences. The main difference is the price and warranty situation. You trade off the Radio Shack warranty and easy access to service for buying by mail order at a much lower price. (Unless you happen to be located in the area of one of the Uniden discount sellers.) It's been my experience that modern electronics that work when you turn them on continue to work, at least through any warranty period. Only long extended (5-year) warranties on expensive hifi gear seem to be worthwhile to me, so I personally discount the advantages of buying the Radio Shack. By the way, in addition to the two $89 sources of the Uniden previously posted (Missouri Radio Center & National Tower), Grove Enterprises is now selling it at $99 with free UPS. EEB still is including the free headphones but charges for UPS. Competition reigns... Will Martin seismo!brl-bmd!wmartin or wmartin@almsa-1.ARPA