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#121
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Why should someone replace ALL the capacitors on old Tubeequipment?
On Wed, 15 Feb 2017, Clifford Heath wrote:
On 15/02/17 08:52, Michael Black wrote: On Tue, 14 Feb 2017, Ralph Mowery wrote: In article , says... I recently saw a notice that Rockwell Collins has stopped production of Mechanical Filters. Many of the newer radios are starting to use electronic circuits or more likely software fro the filtering now. Many newer radios do not have much RF circuity in them, mainly microprocessors doing most of the work. This trend started about a dozen years ago and is progressing more and more every year. Instead of being limiated to just 2 or 3 filters (that often cost around $ 100 each) you just turn a knob or move the mouse and set up any type of filtering you want. That's why in the past, some receivers went down to a 50KHz IF, if you wanted a selection of filters, it was cheaper using LC circuits down there than a bunch of crystal or mechanical filters at some higher frequency. So long as there is nothing 100KHz away from your desired signal, and no two signals spaced 50KHz apart that intermodulate in your RF front end. Imaging is a killer. Double conversion can help, but nothing beats RF selectivity. And nobody was dropping from HF to 50KHz, except in the very early days of the superhet. SOme of the receivers were modern enough that they had a "roofing filter" in the HF range or even above it. Some of the modern receivers are taking advantage of the availability of devices with much better linearity and headroom, which reduces the intermod - and they're using I/Q conversion to defeat imaging - but they cannot achieve the performance of a good communications receiver. There's nothing magic about using phasing to do conversion and getting rid of the image. It was certainly talked about in microwave circles fifty years back. But nobody seemed to apply it to low frequencies, though the RF phasing network probably put some limits on it. But most hobbyists thought in terms of the phasing method for generating SSB, and were unable to make the leap to downconversion. The Drake R8, did it come out about 2000? used the phasing method to go from an IF around 45MHz, straight down to 50KHz. Yes, the ultimate selectivity wasn't right after the first mixer, but you weren't likely to get an SSB-width filter at 45MHz either, so you'd still need to downcovernt Software hacks love SDR because it makes them feel like RF experts, but they're not. I'm more interested in a hybrid, the variable selectivity and ability to do different types of demodulation, rather than a magic IC that pretends traditional RF issues are no longer a problem. Michael |
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#122
Posted to sci.electronics.repair,rec.antiques.radio+phono
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Why should someone replace ALL the capacitors on old Tubeequipment?
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