General Category > Amateur Radio

Project 775

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Josh:
I see you've a 718, nice rig, kinda a grandchild of the 775. I was going to offer a cw filter but the 718 doesn't have a 9Mhz IF strip, just 455Khz and all I have to spare is the 9MHz filter, if I come across one for 455 I'll let you know.

So far in Project 775, Icom parts has the relays I need to replace those popped off in shipping, but the display tube's no longer available, neither is a face plate. The external cable with the broken connectors isn't a xverter jack, it's for an external receiver and I never use those, so as the solder points on the pc boards are broken I'll just link the two points internally with coax.

Some HAMs have used leds to replace the problematic display tube;
https://www.youtube.com/watch?v=jp5qyuBeIdA

What the original looks like;
https://www.youtube.com/watch?v=0z4hd0LSFho

Matt285:
Oh well, I'll come across one somewhere. Yes I really like the 718. Its a simple rig with a nice screen and easy layout.

Josh:
Starting the cleanup and disassembly as well as perusal of service manual. One nice feature noted in said manual is the IF strip is an honest 15Kc wide in wide filter am mode, freaking ossum for ambc audio quality. In ssb you can go 3.3, 6Kc total but it's dsb, not ssb then. The medium filter setting nets you 6Kc width in both 9MHz and 455Kc IF strips, the 9MHz IF filter being optional, and I have one on hand, perfect for swling.

(These wider bandwidths are why I miss my TS870, the 870 ambc audio sounded like you were in the studio, the downside to the 870 is the filtering isn't very sharp.)

Then the 775 detracts from that potentially ossum audio quality by not employing dsp to demodulate that am signal, it uses a diode, and diodes are noisy. Dsp can produce mathematically perfect modulation and demodulation sans distortion = why you want dsp for this. The 775 dsp only modulates and demodulates bfo modes; ssb, cw, and rtty, not am or fm. Oh well, diode rectification can sound good, just replace the diode with a schottky/hot carrier type, bias it a bit perhaps, adjust the output capacitances to recover lows and there you are with studio quality audio out.

Josh:
Recalled that some Italian HAMs took their 775s to nerdvana with some serious mods;
http://www.pensioneitaliacapri.com/ham/IC775-Progetto-Modifiche-2009_Finale.pdf
You can translate it online if you wish;
https://www.onlinedoctranslator.com/translationform

Gian and Nick carved up their 775 and replaced or modded quite a lot of the inners of the 775, to include mixers, active vco filtering, roofing filter swaps in various IF strips, etc, read the pdf if you want to know more.

One of the takeaways I got from the pdf was the 9MHz IF roofing filter I'd not noticed. This filter sets the 9MHz IF bandwith to 20KHz right after the mixer, they replaced it via a factory Icom part that was only 6KHz wide. This is great if all you want to pass thru that IF strip is 6KHz wide or less, in itself this should greatly increase tolerance to strong nearby signals while not compromising the desired signals.

Having a filter right behind a mixer is a good idea, because in most mixer cases you send in two frequencies to mix and you get four out (due to images) when you only wanted one freq.
The way to properly terminate a mixer is with a diplexer circuit that terminates the undesired product freqs gracefully. The filter is a brute force means to that end, as well as reducing noise in the IF strip as a typical IF strip can be 10s or hundreds of KHz wide.

The 6KHz wide 9MHz IF filter is too restrictive for someone who wants to have a truly wide bandwidth like 10 or 15KHz, but Icom has other filters of the same case and pinout design and the same center frequency but with different bandwidths. 9M6A1 FL-116 is the 6kHz BW filter - the Icom IC706 employs these for wide am, part number 201 0000 950 if you want one from Icom. The standard IC706MKIIG has a 15KHz filter, this is the one to order from Icom if you want to pass ambc sigs yet still get some tighter filtering over the standard 20KHz wide filter. I'll install a 15KHz filter here, so glorious wide ambc and hfbc sigs can flourish. Flourish I say!

Josh:
Preliminary inspection complete.

First off, I'm not set up to work on a rig this massive! Good thing homes have floors.

This thing's missing a lot of case screws, mostly for the faceplate, wich can be an indication people have been rooting around inside and not bothered to replace the screws. Simple enough to remedy and Icom has the screws, I checked. Icom likely still uses many of the same screws in all its rigs.

On the shipping damage front, I was very worried there would be broken boards due the connectors soldered to said boards being pushed in and broken off. Boards are fine, just the leads to the components have sheared off, an easy fix. The external antenna connector jumper apparently hit the floor, broke off two different boards, and in one board pushed two relays far enough that their leads were broken. So far I need 3 relays that Icom says are 11 bucks each, will try to find them cheaper. I suppose I could repair the relays easily enough but the leads are really thin so new is likely best.

On removing the bottom cover, I noted some weird buildup that at first reminded me of mud dauber wasp activity, but tasting it revealed a distinct waxy flavour. Na jk, I touched it and it's wax. So where does wax come from in a 775? As expected, the stalagmites of wax (or are they stalactites?) emanate from the pll and dds area, as provenanced by replacing the cover and seeing how the wax stalagmites and vco/dds openings aligned. This indicates the rig either ran for long periods of time, or the wax wasn't very temp stable. The worst part of this will be everything inside the vco/dds cans might be wax covered due to capillary action, to include the vco tuning caps, then again this wax just might be a beneficial preservative in nature. Have to open them cans and find out.

The reason companies apply literal gobs of wax to vco/pll/dds circuitry is due the nature of solid wax; easily applied in liquid state and then hardens to a stiff support structure. The reason we want to stiffen the vco/pll/dds circuitry is to keep mechanical but not electrical vibration to a minimum. Another issue some waxes and glues present in electronic circuits is that they can become hygroscopic over time, meaning they can absorb humidity from the air and retain it, causing changes in the tuned state of nearby electronics. The Kenwood TS 440 and 940 were/are notorious for this. Removing and replacing this wax is a common procedure in hf rig repair, often hot glue or epoxy is used to replace the wax. I prefer to use something that can be more easily removed than epoxy.

This secret and forbidden love for water that wax seems to have is one of the reasons why one must replace every wax capacitor inside old radios, the wax has had decades of absorbing moisture, and the acids in the paper the cap dielectric's made from have come out and are all combining to transmogrify (lol "transmogrify" is actually in the spell checker!) those caps into a very poor form of resistor.

On the vco/dds front, have to get the rig fired up and warmed up before vco/dds tuning can commence, as well as a complete alignment as a rig this glorious warrants. Every used rig I've ever had needed at least vco/pll.dds alignment, and many could use a full alignment. These full alignments can take hours, but are kinda satisfying to see a rig that before seemed a mite deaf come out with much better than than published sensitivity and selectivity.

Here we should have a talk about manuals since we talked about vco/dds.
To do any real service work on these rigs you need a service manual. simple as that.
I've had about every Icom hf rig, and since getting involved in the madness that is personal computing, have aggregated gigs of user and service manuals on pdf. If you have a rig, its service manual is likely out there on the web for free. If you want a hard copy, it's so easy to take that manual via flash drive to your friendly local printers and have them do one up for you, complete with holes for your three ring binder. Try to splurge and get full color rather than black and white as some schematic overlays use two or more colors to show underside traces and so on.

This faceplate really needs replaced, it's scarred and scratched and looks like someone spilled milk on it. I suppose a trip to the bathtub with hot soap and water will make it look as best it can, considering. I don't trust chemical cleaners such as the spray on types people use to clean electronic devices other than say bar soap. hot water, and a dish rag anymore for cleaning items like radio faceplates due the chance the chems might wipe away printing, or eat plastics.

On checking the various board screws for tightness, almost all of them so far have proven to be about finger tight or loose. The inside is very clean overall other than the wax issue wich is contained to the bottom cover and the insides of the vco/dds cans, so I wasn't expecting every fing board to be loose but that's trivial to deal with. Every rig I get, new or used, has all the screws loosened and tightened or at least checked for tightness. I guess after traveling across the ocean from Japan in some huge creaking container ship and decades of use have temperature cycled the rig, causing the screws to be loose, at least that's my fantasy.

On the top and bottom covers, no spraycan from walmart will be restoring the deep gouges and rusted spots, these things will have to be bead blasted and powder coated or spray painted.





This has been cathartic and fun so far, that's it for now!

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