Kevin Custer wrote: > > mch wrote: > > >I think much of the problem is in the tuning caps on both the exciter and PA. > > The Spectrum equipment seems worse than any other, and the > problem *may* stem from the quality of the original parts used.
That certainly is the case with Hamtronics (at least the early 80s UHF stuff). I suspect the same may be the case with Spectrum. I think it was Lee who said it was also partly design. I was usually able to tune the spurs out of them, so I think it was mainly a component issue. But, the components are admittedly part of the design. > The biggest reason manufacturers use capacitors for tuning rather than > inductors is it allows for greater operating (tuning) range with no > change in parts. In other words they can tune a greater span of Mhz > with the same part values. This 'feature' is nice because some > commercial radios will more easily tune the ham band and retain > original specifications, but at the expense of tuning stability. This is also not the type of 'feature' you want in a repeater. The tighter, the better. One thing I particularly like about the Micor and MASTR II is the inability to hear a signal several MHz away from the operating frequency. While this makes these units harder to tune to a distant segment, it also means that even without duplexers, and with a crystal operating on frequency several MHz away, it won't hear much on that frequency. That greatly reduces the chances for intermod, and in the case of the TX, even if you do get a spur, the unit design itself is working with you to ensure that it doesn't get anywhere - as opposed to a broadband unit that will pass it as if it were on on-frequency on-tune signal. Broadband is great for internet, but not for repeaters. > The GE Mastr II is a great radio, and is very popular because it > tunes most of the ham bands without much, if any, modification, but > they certainly don't have the stability of radio sets that utilize > inductor tuning elements. Still, it is a much better unit than most similar units. > All of the Mastr II's I have, have required retuning during the same > time frame as above, (usually one a year for optimal performance) but > they too use variable caps for the tuning. I can't say I've had that kind of maintenance requirement with the MASTR-II, but your point still stands. Then again, I suppose I have with respect to the amp losing a joint or two - usually on the output of the transistors. Maybe it's because I usually give it a once-over at that time anyway that I never noticed such a tuning problem. > It only stands to reason that something that is limited to certain > tuning 'ranges' will provide better stability. And better rejection of 'out of range' segments. To touch upon my earlier point in another way, scanners are great receivers, but they are designed to receive almost anywhere. As such, they have a tendency to... well... receive anywhere! They are much more prone to intermod and overload since there is little to no front end on them. Repeaters, by design, should not need to receive or transmit on more than one frequency (one each for TX and RX). As such, sensitivity is needed only on one frequency - not across an entire range. The more your repeater rejects other frequencies by itself, the less 'stress' you place on your duplexer and other filters. The more broad your repeater is, the more you have to rely on your duplexer rejecting out of band signals - both from the 'outside world' AND from your repeater. > Spectrum purposely designed their equipment to tune from DC to light, > and while you can make them work very well, just don't expect them to > stay..... Actually, I've found that most Spectrums do stay within that range (DC to light) ;-> > Oh, to answer the rest of your question, in my opinion, every part of > the Spectrum Radio is unstable, whether it be the receiver, exciter or PA. > > Kevin Custer True, but if your receiver is a POS, at least you're not going to affect anyone other than yourself. Joe M. Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
