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DRAKE R4B MODS
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by N8XP on September 17, 2002
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DOES ANY BODY OUT THERE IN RADIO LAND KNOW OF ANY COMPANY/PERSON WHO MAKES/SELLS RECEIVER MODS FOR THE DRAKE R4B RECEIVER? I USE MY R4B DURING CONTESTS, AND THE R4B FILTER IS PRONE TO FILTER "BLOW BY" DURING HEAVY QRM IN TETSTS. NO SHERWOOD DOESN'T MAKE ANY FILTERS FOR THE R4B. I HAVE CHECKED!
I'VE LOOKED HIGH A LOW FOR ANY HELP FOR THE R4B AND HAVE MOSTLY COME WITH NOTHING,ZIP, ZERO. OTEHR THEN FILTER PROBLEMS, THE R4B IS A VERY FINE UNIT.
THANKS FOR ANY HELP!!!!! ROGER, N8XP
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RE: DRAKE R4B MODS
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by n6tr on September 17, 2002
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I think a few people did modify the R4B to put filters in, but it is a very extensive modification and I wouldn't even attempt it now.
By far - the easiest way to solve this problem is to use e-bay. Simply sell your R4B for whatever you can get for it (maybe $100 if you are lucky) and then find a R4C to purchase. You will probably end up spending $150 or $200 to "convert" your R4B to an R4C - and this is less than you would spend buying the crystal filters you would need.
73 Tree N6TR
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RE: DRAKE R4B MODS
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by W8NF on September 18, 2002
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Hmmm, good question. First, don't jump on N6TR's suggestion to just replace the "B" with a "C". The R-4C was, it seems, somewhat hastily placed on the market. Due to inadequate mixer injection in several stages, the R-4C, stock is a very poor receiver. Between W8JI and Sherwood Engineering, you can learn enough about the R-4C to "fix" it, and turn it into one of the best receivers available for any price, but don't buy an R-4C thinking it's going to be satisfactory "stock". You MUST upgrade an R-4C to get a decent RX.
The R-4B has a first IF of 5.6 something MHz, with a crystal filter. It's a fairly wide roofing filter. I think you can put in an R-4C-intended roofing filter there, and you can even get one that's 600Hz wide and with steep skirts - but then you have a CW-only receiver. That's not uncommon for those who work mainly 160 meters.
The R-4B has a second IF of 50kHz and the filtering is RC. So it has "gentle" skirts. I can compare my R-4B with my Icom IC-735. The '735 is no dream rig, but the skirt selectivity is far better than the Drake. I don't think you actually get filter "blowby" in the Drake - I think the R-C filters just have rather gentle skirts to them.
The principal reason that a filter upgrade on the R-4B is so difficult is that the 50kHz filters have tunable center frequencies. That "passband tuning" knob that selects LSB and USB is actually tuning the filter. In a rig with crystal fitlers, the "passband tuning" (or IF shift or equivalent) is actually tuning an oscillator, which moves the IF signal up and down inside the bandwidth of a fix-tuned filter. Just try homebrewing a crystal filter with 50kHz center frequency, and, say, 400Hz 3dB bandwidth, and then being able to adjust that center frequency over about a 2kHz range. I'm a pretty decent RF engineer, and I would not attempt that design and claim to make it work in production!
For a brief period, I used a Timewave 599 DSP unit outboard on my R-4B. It really improved the skirt selectivity. But the CF of the Timewave is not adjustable, and once accustomed to true passband tuning, one doesn't want to go without. There's a higher end Timewave unit that offers adjustable filter center frequency, and when the budget permits, I'm going to go looking for one.
The other mod on the R-4B worth looking at is the product detector upgrade from Sherwood. The slow AGC setting is far too slow, but the fast setting snaps and pops badly, because the product detector is momentarily overloaded when a signal comes in. The product detector upgrade from Sherwood overcomes this overload issue. But you'll still have the gentle filter slopes to contend with.
73,
Dave W8NF
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