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Author Topic: Project 775  (Read 4061 times)

Offline pjxii

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Re: Project 775
« Reply #30 on: November 16, 2019, 0104 UTC »
Josh, I can't believe how well you've gotten to know this transceiver! Thanks for all your work, it's greatly appreciated

Offline Josh

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Re: Project 775
« Reply #31 on: November 16, 2019, 1921 UTC »
Thanks, kinda a icom rx nut from way back (arigato Inoue-san!) and they all seem to share a lot of the same design philosophy so you get to know part numbers, expected gain, etc. Started out with an R70 in '89 and a few months later with a 751. Had 735, 745, 751 and 751A, 765, 761, 756Pro and ProII, and lastly the 775 for tx, R70 and several R71s plus the R7000 for rx.

I wonder why the Italian HAMs who devised the 775 mod pdf don't recommend replacing the 3sk thingy with the bfr thingy for lower nf in the IF amp if you splice in a inrad roof. Pretty sure you can come up with an additional 3 or 4db gain with the inrad roof filter amp (let alone a spot on alignment) so could make up for the lost gain. I'm in a quandary over mine, should I forsake the am rx side and mod for ssb work or leave it possible to rx wide sigs like am and fm? This thing sits on ssb all day day in day out and I've several other rigs that sound better on am.
« Last Edit: November 16, 2019, 1923 UTC by Josh »
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Offline pjxii

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Re: Project 775
« Reply #32 on: November 16, 2019, 2335 UTC »
I wonder why the Italian HAMs who devised the 775 mod pdf don't recommend replacing the 3sk thingy with the bfr thingy for lower nf in the IF amp if you splice in a inrad roof. Pretty sure you can come up with an additional 3 or 4db gain with the inrad roof filter amp (let alone a spot on alignment) so could make up for the lost gain. I'm in a quandary over mine, should I forsake the am rx side and mod for ssb work or leave it possible to rx wide sigs like am and fm? This thing sits on ssb all day day in day out and I've several other rigs that sound better on am.

Can you give me details about that mod regarding the Inrad roofing filter, please? 

Personally, since I have other receivers that sound great on HF I'm not worried about how AM sounds on the 775. I would think that if you aren't using it for AM or FM tx optimizing it for SSB would be the way to go.
« Last Edit: November 16, 2019, 2336 UTC by pjxii »

Offline Josh

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Re: Project 775
« Reply #33 on: November 18, 2019, 2331 UTC »
Here's a link to the pdf on the mods, I translated it into English and it came out very well;
https://www.filemail.com/d/vdwnpnjhonrxdbl

You can open a pdf right?
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Offline pjxii

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Re: Project 775
« Reply #34 on: November 19, 2019, 0022 UTC »
Yes, got it. Thank you!

Offline Josh

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Re: Project 775
« Reply #35 on: December 02, 2019, 2023 UTC »
Sometimes one comes across a bad FL80.

These filters are found in the 751, 751A, and 775 as well as other Icom rigs, and any that have a FL80 installed as an option.

The FL80 was sold as an upgrade option for any rig that had a FL30; same pin out, IF, and similar bandwidths but the FL80 skirts went deeper and steeper than the FL30.

A simple way to check for a bad filter is to note the passband response of the rig in ssb mode, then flip to the other sideband and note the response, they should be similar if not identical. Perform this test with no optional filters enabled or the results will be for those filters and not the FL80/FL96 pair.

A bad filter will display a markedly asymmetrical response, I note in most cases usb is pinched while lsb is fine to a bit too bassy. Never seen a bad FL96 or FL70, but surely it can happen, almost always a bad FL80 is lurking in that Icom.

A specific part in the FL80 goes stale over time and changes the passband. For all I know there's only xtals, caps, and inert gas inside one. A HAM opened one to fix it and found a bad silver mica cap (dread silver mica disease!), replacing it brought the filter back into spec. FL30s also fail in a very similar manner, some even stop working altogether.

The FL70, wich matches the illustrious FL96 in bandwidth as well as steep and deep skirts, is a choice option to replace the FL80, unless one wants a narrower passband.

The shame of it all is Icom went with plug-in optional xtal filters in the rigs newer than the 765 series, meaning Icom don't make a pin for pin solder in replacement for the FL80, one has to use up an optional filter slot to run a Icom narrow ssb filter if so desired. And the solder pins of the current plug in filters do not match up with the traces/holes on the circuitboard, so one is obliged to use jumper wires to splice in a newer filter.

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Offline Josh

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Re: Project 775
« Reply #36 on: December 06, 2019, 2110 UTC »
Monitoring weak (realllllllllllly weak) cw sigs has allowed me to test if an 80Hz filter helps or hurts weak cw signal detection. Some weak sig cw enthusiasts say "find your weak sig, then open the filters up wide".
That may work for some ops but when I tried that on some truly weak cw today they melted into the band noise and the cochannel crap, enabling the 80Hz filter made them pop back into detection, time and again on every sig coming in.
And they were just at the perception level in the 80Hz filter.

I don't recall the narrow filter I normally set in the 756 Pro2 (100Hz) being as effective, mebbe it has more ringing, will have to compare both rigs at the same time same signal.

The diff is the 775 has high quality matched 500Hz xtal filters in the 9mHz and 455kHz IF strips that then feed the dsp demodulator/filter unit.
The 775 has 5 frequency mixes/conversions from antenna jack to audio out. A ADC16071 16bit adc does the a/d conversion in the 775.

 The Pro2 has a several kHz wide 455kHz IF ceramic filter it passes all sigs through prior to the mix down to 36kHz and right into the adc and dsp. The Pro2 has 3 frequency mixes/conversions from antenna jack to audio out.
The DSP in the IC-756PRO/756PROII employs a 24-bit A/D
converter. The logical value of the dynamic range of a 24-bit A/D converter is 144dB, however the actual value of the analog performance is smaller than this and performance may differ considerably, depending on the type of A/D converter used.

An interesting effect I noted with the dsp filtering in the 775 is you can open the IF filters wide as in ssb wide and the dsp filter does almost as good a job despite no longer having the protection afforded by the preceding narrow xtal filters, despite being a 16bit adc.
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Offline Josh

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Re: Project 775
« Reply #37 on: December 12, 2019, 1958 UTC »
Was wondering how to fill the option filter slots for the 9mHz and 455kHz slots. Already have narrow filters to pass anything 500Hz and narrower.  Wanted something for wide ssb, but Icom branded filters for such are rare or unobtainable.

Then I recalled I have a FL-33 6kHz wide xtal filter I can splice in the 9mHz slot, but what about the 455 slot?

Hoarding parts saves the day once again!

Back in the 80s a customer (I used to build and sell ambc and 160m tabletop loops) gave me something from his Icom IC745, a small pcb with caps and xformers that held a 455kHz ceramic IF filter in the slot of a 455kHz xtal filter.
To install a optional xtal filter, you remove this pcb/filter assembly and discard it.

The caps and xformers on the pcb are to match the different in and out z of the filter itself to the IF circuit as it expects to see a xtal filter in that slot, not a ceramic with different characteristics. Mismatches lead to spurs, attenuation, and uncharacterised bandpasses as well as loss.

Much rather have a FL-257 3.3kHz wide xtal filter but those are rare and costly.

So since I have the pcb to swap in a ceramic filter, as well as several ceramic 455kHz filters to choose from, I decided to run with that.

The filter of choice is a Murata CFR455H, a 6kHz wide, 9 pole, metal cased affair that is perfect for wide ssb and narrow am, this will match up with the FL-33 for use in hfbc am listening too, not just ssb.

Best part is all are Icom original parts, continuing with my all-Icom parts theme/fetish.
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