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Author Topic: Project IC-R71A  (Read 1106 times)

Offline Josh

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Project IC-R71A
« on: December 15, 2022, 0129 UTC »
Whilst perusing qth (for parts for yet another project) I ran across an ad for The Ham Station, and I had to take a look. Dan's linked url displayed his wares and I noted something dear to my heart, namely an Icom IC-R71A for sale, and for sale cheap! I called to see if he still had it, he did and mentioned some details, it was clean but had a few issues. I made arrangements to meet and purchase, we met behind his old building in Evansville as if he was my pusher and me a radio junkie needing a fix. I mean who would turn down an Icom IC-R71A for a lousy $135?

So I get it home and fired up, immediately noting it seemed to have a narrow cw filter installed (yay!) and the bespoke issues. The volume/rf gain knob is fooched so the volume pot has no rotation stop, and the pbt knob was a bit bent, I surmise both were victims to shipping via one of the "drop it or you're fired" companies.

I've been a fan of these rigs since dirt was new, had like 10 of them over the years, and often wondered if I'd ever get rid of the surplus of parts to mod them with, to include a rom board with programming to go from 10kc to 32mc plus a newer battery, stock they tune 100kc to 30mc. The issue here is many Icoms of the era need a good cmos battery or the radio literally forgets how to run if the battery dies before replacement.
Icom stopped building these things a few decades ago and some of them are from the mid 80s so if the cmos battery is original, well, you have been warned. So I opened the rig up to install the rom board and see if it had a cw filter, sure enough an optional Icom cw filter (worth about $50) was in place and the innards were very clean to boot.

Something I almost always do with a new rig is check to see if the various screws are snug, and this time the chassis was fine but each and every pcb was just a bit over finger tight. I wondered how that works if it was like that all the way from Hirano-ku, Osaka, 547-0003, Japan, but I tightened them up and it seemed to quiet down some odd but brief pll/vco/cpu noise it had at first startup that has yet to reappear.

The addition of the new rom board went fine and the initial startup after brain surgery showed it worked fine too, still had the same memories it had when I put the board into a static proof bag years ago. Now this baby tunes all the way down to about 9.4kc in cw mode. Copied wwvb (60kc) last nite with about 10ft of wire fed into a Grove TUN3 (if you remember those) and am fairly impressed with the rig, shamefully lacking a fish finder unlike most of my other rigs.

So this one is going to get the knobs sorted, a fresh alignment, and various mods, some of wich still have parts for.
One of the mods is to replace the pn rf bandpass switching diodes with pin types; the illustrious MI204 PIN diode, bought a mess of them from Icom years ago. These diodes have a much longer carrier lifetime than standard pn diodes, if the rf passing thru the diode isn't high enough frequency, the diode can contribute to imd.
See here for some empirical data on diode caused imd;
https://www.radioamatore.info/attachments/508_TS-940%20-%20PROMOTIONAL%20BROCHURE.pdf
Above say 11mc the as-installed diodes work great, below they start to contribute imd to a slight degree. Replacing the bandpass diodes for all ranges below 11mc will improve performance but not to a startling degree. More or less I just want to use the parts for what I bought them for.
Below 3mc, Icom used the MI204 diodes so they don't have to be replaced, just 3mc to 11mc bandpasses.

The next mod will be a better IF filter for am mode, better as in there is none as stock. The Icom R70 had enough slots for filters for most every mode, the 71 gives you two optional slots, one in the 9mc IF strip and the other in the 455kc strip. The filter I propose here will be around 6kc wide and as it's a monolithic filter, looking like a large canned crystal, will be soldered in the 9mc strip, replacing a cap. I do have a proper Icom 9mc 6kc wide xtal filter but I don't wanna have mini coax going all over, wich that would necessitate.

Then the neat mod, an infinite impedance detector in place of the simple diode detector as present. A good diode detector is fine but a infinite z detectors finer.
See here for more;
http://www.pan-tex.net/usr/r/receivers/elrpicamdetect.htm

Another mod is the nb diodes, the 71 has much better nb action than my other project, Project IC756, and that should not be, yet it is. The 71 hardly distorts sigs when nb is on and it does a great job filtering out the leaking pole insulator down the block but the 756 grossly distorts sigs when nb is enabled and anything over medium or weak sigs are present. Some schottky or hot carrier diodes may make for superior blanking, the 71 will be the test bed for both rigs nb sections.
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Offline Pigmeat

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Re: Project IC-R71A
« Reply #1 on: December 15, 2022, 0251 UTC »
Evansville? Home of Falls City Beer and it's upscale brother Drummond Brothers? Falls City tasted like hopped water straight out of the Ohio River, but Drummond Brothers wasn't half bad. It was malty in a time when most American beer tasted like tap water.

Offline moof

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Re: Project IC-R71A
« Reply #2 on: December 15, 2022, 2320 UTC »
Falls City? BLLLRRRRg.  Get the real Sterling or Mickeys.  My granny worked for Sterling.  Now all you can see on the innernet is urban spelunking of the old building.  Probably no longer there.  I remember it vaguely growing up.

Offline moof

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Re: Project IC-R71A
« Reply #3 on: December 15, 2022, 2333 UTC »
Oh and listening to the Sterling Mystery Theater late weekend nights secretly on a tiny silver AM transistor radio in bed in first or second grade?  Living the high life as long as you were not 9v battery deprived.  Which we were 2/3 of the time.

Offline Pigmeat

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Re: Project IC-R71A
« Reply #4 on: December 16, 2022, 1552 UTC »
I remember Mickey's. There was a serial killer on the West Coast in the late 70's who decapitated a woman and sent a picture to the local cops with the woman's head on top of one of those "Big Mouth" bottles with the caption "Mary Ann lost her head over Mickey's." He had a sense of humor.   

Offline Josh

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Re: Project IC-R71A
« Reply #5 on: December 23, 2022, 2256 UTC »
And in other news,  Project IC-R71A is currently having a contest to see what detector circuit to best replace the time-tested single diode detector with, and you are NOT invited to participate. Many types are worthy of consideration, "high precision, infinite impedance, pll, synchrodyne (poor mans synchronous), synchronous, in analog or digital renditions! Beers will be consumed! Eskab and Edvis (who doesn't remember those guys?) used to make a PLAM (Phase Locked AM) board for the R70 and R71A, would be nice to find but doubt any loose on the market right now would fall under the current funding guidelines. A look at the am stereo chip used in the Sony  2010 might be a good idear. I know you only get circa 6kc of am audio bw, and it IS an Icom after all, but those 6kc can be glorious if you set your mind to it despite all efforts to make you see the light.

Recently the R71A was submitted to pll/vco alignment and it passed with a few caveats - all ranges were out a few tens of volts, one being an visually suspect heat damaged part, the expert inspection/evaluation will have to await the New Year due reasons of being sick of my projects.

Also looking for an FL-96 2.8kc wide 455kc IF xtal filter for excellent bespoke pass band tuning as well as outstanding matched filter performance when that filter set is selected.






Next time you quaff a Guinness think kindly of the R71A.
We do not encourage any radio operations contrary to regulations.

 

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