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January 18th 10, 10:33 PM
posted to rec.radio.amateur.boatanchors
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Need old meter case (broken meter ok)
Michael Black wrote:
On Mon, 18 Jan 2010, wrote: I am sure I have seen the 73 magazine issue, but that was back in 1975! Memory is not able to access a picture of the front cover. It was the last small issue, the second part was in the first larger size issue. It was a combined issue so they could get the larger format out before the competition, all the magazines switched over at the same time. The scope was on the cover, it kind of looked like a Tektronix 453? (it's wider than it is high), and if I recall properly, it had a built in battery so you could run it away from the power line. It was an impressive project, yet it was also limited, no really high frequency response. John W. Campbell's solution about 15 years before in "CQ" was to do away with amplifiers and run the scope tube off a relatively low voltage, which gave them better sensitivity. Feed the signal right into the plates, you'd get a dim trace but great frequency response. He got the idea from Bell Labs. Michael VE2BVW That's the scope. It did look like a tek 465/475 series that gotten shrunk down a bit (3" vs 5" tube). Actually the frequency response could have been as high as 50MHz with that circuit. As described the scope used transistors with an ft of 50MHz which got the scope to about 15-20MHz top end. The op amp used in the vertical preamp was good to about 90MHz. I corresponded with the author on some details and was told that he had later substituted transistors with higher ft (100MHz) and got 45MHz out of the vertical chain with some tweaking. He suspected it would go a bit higher by hand picking transistors / op amps. I have some 140MHz transistors I can use and have a bunch of op amps so I can hand pick those for best response. The 3ADP1 should in theory go higher in frequency than the 3ACP1 (and I have two of each). I suppose that today most would consider a 50MHz scope as not being really HF, but that's about high enough for most work with 8 bit microcontrollers and HF radio gear. BTW, this scope DOES run at low voltage, +/- 600 volts on a PDA tube designed for +/- 2000. The author suggested a mod for dual trace and I even considered this, but running the tube at low voltage to get the deflection sensitivity and bandwidth would make a dual trace option too dim to see well. Today the way to build a scope is with fast A/D converters and software. Digital scopes react to oversampling in a different way than analog scopes though. Instead of rounding off square waves and showing reduced amplitude they build false waveforms due to aliasing errors. (they lie). I suppose I'm guilty of the worlds worst case of procrastination on a project with this one! |
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