General Category > Amateur Radio
Project 775
Josh:
Going from memory, everything txwise starts at the dsp if you have the dsp unit installed, some 775 did not come with the dsp card. The dsp will set the high and low ends of the tx (and rx) bandpass as you've noted. On rx you can pass ssb mode sigs through some very wide filters in both the 455 and 9 IF strips, say 6 to 14kc wide for each filter and the dsp unit will allow for up to 3.3kc rx bw. I'm unsure if tx is allowed via these wide filters, the filter truth table should show us what filters are used in ssb tx, I presume the FL96 and the FL80 to help suppress the lsb and carrier artifacts summing up to about 2.8 total bw. That being said, I saw some video or pdf of a guy opening the dsp unit and defeating the high and lowpass filtration for singlewideband (lol), so I suppose then the analog filters would set the overall bw per the switch matrix. There's a whole genre of HAMs trying to get wide audio on ssb, the ones I used to listen to were on 3630 at nite, 14178 days.
On the filtering, make sure to enable the FL80 and switch betwixt upper and lower, listening for tonal change.
Many FL80 filters fail in a way that causes their pb to shift a bit, say upper sounds pinched and peaky while lower sounds dull and smothered. There should be no substantial difference in audio passband going from upper to lower for any filter. I think all the FL80s that have passed by here have showed that issue to one degree or another, makes sense to spend on the inrad filters or do like me and swap in an FL70.
As stock I figure the dsp develops the mathematically perfect ssb signal, passes it thru the two xtal filters in the respective IFs on the way to the driver and pa, and then out to your antennae to be released to the world. Fly away little signal, fly away! Be free! The service manual will go into much greater detail, just look under the circuit descriptions and you'll get the facts.
For the Project 775 Care Package I've a copy of the extended qst review, an English translation of the Italian 775 mods pdf, and some other tidbits found along the way, just have to get them all into a zipped folder and sent to innernets for distribution. You mention a early 775, this can be good and it can be bad, in the sense of service updates and so on. I found the repair faq from Icom on the 775 and will include it - these are getting hard to find anymore, these faqs have service bulletins on the most common faults in Icom rigs, I've used them countless times to diagnose and repair issues in multiple iCom rigs with just the new part and faq alone.
Also, and I never saw this in the service or user manuals, cpu reset to factory defaults is; power off, press and hold "clear" button, and power on. This also nicely cleared up some odd glitch I had in the dsp and always wondered if I was going to be stuck with it. Before you clear it, make sure to look at the other guys saved memory freqs, might be some interesting stuff.
In your case with the inrad mod, you're not going to get much beyond about 6kc through the rig from antenna jack to speaker so the 6kc wide 9mc IF strip filter mod will definitely make sense, the one present in your 775 right now is 20kc wide, Icom did this to allow for am and fm use.
Dunno if I mentioned this before, but sherweng's never tested a 775 for whatever reason but has tested about every other solid state HAM rig. Apparently the Yaesu
FTdx-101D is a freaking amazing rx.
http://www.sherweng.com/table.html
Josh:
775 Care Package
http://s000.tinyupload.com/index.php?file_id=30671678912658575388
pjxii:
I found a 775 group and read these posts:
"You must bevare of the fact that FL80 (3.if 9MHz 2,4Khz) is bypassed when COMP is off.
In transmit that filter only works with COMP ON.
Else, the TX limit of 2,8KHz is in the FL96 (455Khz 2. if) and PSN is OFF in the menu.
With PSN ON you use the CWF455HT 6KHz filter + DSP settings to set the TX bandwidth."
"The FL-80 (with printed 2.4Khz on it) is more like 2.7Khz filter.
Even in the servicemanual it is called a 2.7MHz filter.
I replaced with FL-70 and made measurements and recordings before and after
replacement. Almost no difference.
And..also have in mind that during transmit the FL80 is bypassed when
compressor is off. (Stated in servicemanual too.)
I never use the compressor if I want good TX audio. (External rack.)
The DSP bypass-mod is more funny if good sound in SSB is the issue."
I don't understand that last line...anyone? That talk about the FL-80 is news to me.
I agree with you that matching filters makes a lot of sense when you have Twin PBT, so it'd probably be better for me to find an FL-44A to replace the FL-96, as DXing is far more important to me than ragchewing, or...would using an FL-70/96 and narrowing with Twin PBT be the equivalent?
I can just use DSP tx to go wider if I use the narrower crystal filters.
Along those lines, would it even be worth getting SSB filters for the narrow slots if it has Twin PBT? I'd probably get the Inrad 1.8s but that's another $250. Would the selectivity over PBT with the FL-80/44A be worth it?
This is getting confusing. Maybe I should have stayed with a simpler old rig, like upgrading my TS-130V to a TS-130S...
Thanks for the link to your "775 Package," I can't open it on my iPhone though. Maybe when I finally get a laptop (radios come first...)
Josh:
Icom has made 1.9kc wide ssb filters for the 9mc and 455kc IFs for a while now, if you want narrow, those are the ones to go for. Easiest would be the FL44A swap for the FL96, outstanding filter and matches the FL80 if you want no narrower than 2.4kc. My current understanding is for real narrow ssb work you want the FL222 and FL223 filters for 1.9kc width in each IF strip, a bit narrower when they are paired and aligned, meaning no pbt or IF shift activated.
The FL80 is a true 2.4kc filter, a bit wider at the top than the FL30, however the skirts are much steeper and likely go much deeper than the FL30, wich is why everyone wanted the FL80 to begin with, also there may be more poles in the FL80.
I'm gonna have to reread the service/user manuals to get a good grasp of what filter is allied with what buttons combination for rx and tx both.
Also if narrow bw operation's your goal, swap that 20kc wide second IF roofing filter with a 6kc one from a IC706, that might make for a worthy mod and is a drop in part.
Josh:
If modding the 775 for dx work and not wanting to do welding or other intensive surgeries on the 775 such as replacing an entire mixer circuit, meaning drop in, pin for pin, dx enhancing parts swaps, here are some ideas;
1 - Reduction of the noise floor and balance of the AGC gains
In the search for reduction of the total noise of the apparatus and improving the linearity of the amplification stages, the most critical stage is the result Dual Gate MOSFET Q15 3SK131.
This amplifier, placed at the end of the 1st IF chain, has a high gain of 24 db, and in the presence of strong signals loses its linearity. In order to reduce this gain and "lower the NF" said component has been replaced with a BF998R.
In view of its lower gain, about 4dB, the signals detected by the S'meter will be lower so it will be necessary to adjust the
trimmer relative to the total of the AGC loop gain, R198 (IF UNIT). Although there is a loss of 1 dbm on MDS, being the very sensitive IC775 receiver (MDS approximately -130 dbm), this loss is not appreciable although a lower NF makes it more pleasant listening. Probably the loss of gain could be compensated for if it were possible to weld two BF998R in parallel (one above the other), the configuration that would provide us with a higher transconductance and therefore higher gain, but the SMD components are not easy to a simple parallel-to-pin -pin.
NOTE
It is not recommended to introduce this change if you implement the changes occur repositioning of the roofing filter Inrad.
5 - ROOFING FILTER REPLACEMENT ON 2 ^ IF
Premise
To further reduce the intermodulation, NF, increase selectivity and MDS, could be replaced, the roofing filter post 2nd IF called FI1 (9.0115 MHz with BW over 20kHz), with a lower bandwidth. The choice fell on two ICOM commercial filters called FL102 6 kHz BW and / or BW FL103
2.8kHz. The first has a Fo = 9.0100 as originally used in AM, this filter having a different Fo will produce an asymmetry of the passband, although it is more than enough for the purpose.
FL103 ensures excellent selectivity at the expense of a lower audio bandwidth, if you are the SSB Hi-Fi enthusiasts do not use this second solution.
I'd replace the filter with a 6kc wide one from the 706mkiig as it's a pin for pin drop in instead of something that has to be spliced in with minicoax and have traces severed. Also since the filter is of the same design it should have the same in/out z and attenuation. That being said, the skirts will not be as narrow as if one spliced in a FL102 or oither 6kc wide multipole filter. The original filter for this position is one of the monolithics with only a few poles of selectivity.
Here's some info on monolithics;
https://www.electronics-notes.com/articles/electronic_components/quartz-crystal-xtal/monolithic-crystal-filter.php
The mods devised and discussed here stem from this pdf;
http://www.pensioneitaliacapri.com/ham/IC775-Progetto-Modifiche-2009_Finale.pdf
you can translate it into your lang of choice online
I recently noted that you can't really get a 3.3kc width on ssb rx unless you've installed a FL10X 6kc wide am filter in the 9mc IF optional filter slot, otherwise you get only the sum of the present 9mc IF and 455kc IF ssb filters.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version