Project 775

General Category > Amateur Radio

Project 775

<< < (4/9) > >>

Josh:
In the spirit of keeping this rig as "Icom" as possible, it has been festooned with authentic Icom parts; from repair parts to modification parts, keeping with Icom quality standards has been a major consideration.

To wit;
The ac line input connector was replaced with the ac line filter from an Icom IC-765. The 765 had/has a noisy internal smps so Icom filtered it at the line cord. Replacing the internal smps with an external linear psu, I had the 765s ac line filter in the parts box for decades, knowing one day it'd be put to good use. The smps in the 775 is fairly quiet as smps go, I just want to make sure no rf makes its way into the rig via the ac line, and to make the smps as quiet as possible. This is especially important when tuning in the vlf band, as most smps spurs are strongest in the vlf band. Line filters like these can also be retrofitted to pc systems psu to reduce their emi footprint.

The DUALWATCH vfo tuning knob as featured on the 775 is a tiny affair, well weighted for tactile response but minuscule in diametre, and with no means to alter the tuning drag.
Pondering the tiny wheel one afternoon, I removed the existing knob and closely examined the vfo encoder shaft, it was the standard D notched affair with no set screw. Where had I seen this size control shaft before? Oh I know! The Icom IC-706 MKIIG! I had replaced the vfo knob on a 706 years ago as the shiny black finish had been chipped, and tossed the chipped but otherwise perfectly usable tuning knob into the parts bin.
Small as the tuning knob on a 706 is, Icom still included a freely rotating finger dimple, as well as a rugged rubber ring around the circumference of the knob, desirable features in any size tuning knob. The original DUALWATCH knob has no dimple, and as a dimple connoisseur, I demand dimples.
I fished the 706 knob out of the parts bin, gently affixed it onto the vfo shaft, and presto, a 775 with a 706 DUALWATCH vfo knob that looks like it belongs there, you can even see the skirt molded into the front panel behind the knob. And as the Handsome Al Fansome will tell you, knobs are important.

The FL23 9M20 9mHz IF xtal roofing filter, a device inserted shortly after the 775's 9mHz IF mixer to reduce IF bandwidth, will be replaced with an 9M15 xtal filter from an IC-706, passing 5kHz less bandwidth than the stock filter. This is a compromise between a 9M6 6kHz wide filter - wich would result in spectacular close in signal handling ability for ssb, cw, and digital mode work, but would more or less disable fm and am wide mode, so the 9M15 was chosen. This 15kHz filter still increases signal handling ability to a degree while passing am wide and fm signals.

The FL80 2.3kHz wide xtal filter in the 9mHz IF strip will be replaced with an FL70 2.8kHz wide filter that was excised from an Icom IC-R71A receiver. Icom, for whatever reason, paired the FL96 with the FL80, I assume they did this as the FL80 is considered a premium filter according to the shape factor, the FL30 used in most every other Icom hf rig was not in keeping with the flagship status of the 775. The existing (and desired) FL96 455kHz IF xtal filter is 2.8kHz wide, so the FL70 is a perfect match.
The FL80 is a premium part normally paired with the legendary FL:44A filter in Icom rigs of yore, but I've grown fond of wider bandwidth over time for being able to recover more intelligibility from a given signal.

Now to find a factory Icom tcxo!

Add some Icom MB19 rack handles, an unblemished front panel, bead blasted and powder coated cabinet, and a tuneup, and I'd consider this a project for the history books.

ChrisSmolinski:

--- Quote from: Josh on May 14, 2019, 1925 UTC ---In the spirit of keeping this rig as "Icom" as possible

--- End quote ---

And here I thought you were going to talk about low quality audio :)

Josh:

--- Quote from: ChrisSmolinski on May 14, 2019, 1935 UTC ---
--- Quote from: Josh on May 14, 2019, 1925 UTC ---In the spirit of keeping this rig as "Icom" as possible

--- End quote ---

And here I thought you were going to talk about low quality audio :)

--- End quote ---

On a serious note, Icom has long had a rep for poor audio quality.
The phrase "Icom audio" is well known by HAMs, especially those that run hf. I feel that the FL30 filter, wich is used to create lsb and usb signals before sent to the final amplifier is more to blame than the microphone and further audio processing. The FL30, present in millions of Icom hf (and other) rigs, is 2.4kHz wide at 6dB down, not wide enough for good audio quality in my mind. So take that poor audio bandwidth and splice on top of it poor microphone quality and audio processing and you have a textbook case of bad audio.

In the case of those stuck with xtal filters and no dsp modulation, replacing the filter is the only option for wider bandwidth tx.
If you have dsp modulation, you can open the dsp up to 2.8 or 2.9kHz lowpass in some cases, in other even more such as 6.6kHz wide, and 80 or 50Hz for the highpass side.

In testing, the 775 running dsp modulation, has 10dB more opposite sideband rejection than in analog modulation mode, so along with creating more mathematically perfect modulation, dsp can clean up your signal greatly.

Then there's the issue of filter faults, where an internal part loses contact with the rest of the circuit or changes value. This is often noted in the FL30 and FL80 filters, especially the FL80, where one sideband seems to pass much more audio than the other, even though these filters are designed and constructed to be as symmetrical as possible. Some HAMs have unsoldered FL30 and FL80 cases, found the culprit part, and resoldered or replaced the offender, rendering the filter fit for service once again. This is a tough nut to crack when the case is hermetically soldered along its entire seam and likely comprises an inert gas environment.

Josh:
A face plate in almost perfect condition has been sourced, and negotiations are ongoing for clean top and bottom cases. Could always bead blast and powder coat the case halves but it'd never be the same as the original Icom finish, same goes for the chipped tuning knob. Meanwhile, Project 775 sits here quietly monitoring hf utes and the occasional privateer.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

Go to full version