HFU HF Underground
Technical Topics => Equipment => Topic started by: Under30 on August 23, 2020, 2351 UTC
-
Hey all, I just received my DX-440 :) I had one back when they came out, but was not too impressed back then.
So fast forward I am quite happy with it.
I think I want to do the anti-chuff mod, but have heard some folks don't fully like the results.
I hear things like a whistle or whine, especially on FM, and poping noises when hitting certain buttons.
So coming from an engineering background I'm thinking there must be a reason for that chuff.
What are your opinions?
-
The chuffing doesn't really bother me, since I mainly use the direct keypad enter for FREQ's. That was the big selling point of that portable back in 1989 - 1990, and the easy that it can do so. Therefore, I've never done the anti-chuffing MOD. The noise on FM? Do you have a CATV, or DSL, MODEM with wi-fi? Do you have various appliances with those cheaper switching PS wallwarts? They can added to FM noise. Take it outside and see if your FM cleans up. That portable is quite sensitive on FM, like around 2uV. That was good for a portable back then, with most being from 7uV to 5uV. BTW, what are you powering that portable with? Switching PS, or transformered wallwart? If you can, ditch the switching PS. The popping from buttons pressed? Yep, mine does it too.
-
I don't have any whine or popping. But I have heard some folks say these noises appear after doing the mod.
-
The noise likely originates from the PLL circuit. Its noise is lower than the whatever threshold is set for the mute feature, thus why it is not normally heard.
Do the mute mod, and you no longer have that threshold, thus potentially improving the chance of finding weak signals.... including the weak signals generated internally by the radio itself. ;)
-
Chuff never bothered me on any radio, whether he DX440 or any other SW digital portable.
The whine can be switched off by flipping up the lock switch. That is, once you're parked on a frequency you want.
For mods I would forget the anti-chuff one and add protection diodes between the antenna 'post' and the negative side of the battery. The 803A/440 has no diode protection for its SW RF amp front end. If you're going to dig into the radio I'd do that instead of worry about chuffing....
Good luck either way. Enjoy the radio. SW sounds terrific on headphones on these radios, by the way. Back in 2002-2003 when I was SWLing every AM with my 440 and the bands were still packed it was a dream.
-
What's a good pair of diodes to use?
I think I may have some 1N919's lying around.
-
What's a good pair of diodes to use?
I think I may have some 1N919's lying around.
Any diodes will work better than no diodes. I don't remember what I used, probably a couple germanium ones from the standard pack I got from Radio Shack in the 1990's I had lying around. 1N414's or whatever they're called.
All they have to do is drain static discharge from the whip -- when you touch it -- to keep it from zapping the SW RF amp.
Also gives a layer of protection from external antenna static bleed. Of course, in lightning regions, you'd want the radio disconnected from an external antenna anyway during staticky conditions -- be it extra dry winter conditions (when antennas can apparently develop static during wind or whatever) or when there is t-storm activity....
I used hookup wire, soldered the diodes together (reversed polarity from each other), and wrapped the diodes with shipping tape after soldering them together and to the length of hookup wire, to keep them from grounding out on anything nearby. I forget which end the diodes went to (probably the battery negative), and the hookup wire end went to the antenna solder pad on the PCB. I'm going by memory here, but it's an easy thing to do if I can do it, and protects the radio.
-
On my variant of this model, I included a switch so that the chuff/no chuff could be turned on or off. A small slide switch was mounted discretely on the side of the case. The noises heard with anti-chuff mod implemented are to do with the native 1 kHz base frequency at which the PLL circuit operates. It generates a lot of phase noise as one makes frequency steps, until the PLL lock stabilises again. IIRC it is not so noticeable on quiet bands, but acute when tuning on strong signals.
The whine, I remember well enough. I seem to remember curing it with better shielding between the CPU/controller board and the ferrite rod antenna. It was a long time ago since I made that mod, so details are a bit sketchy, but it did involve digging quite deep in to the radio, removing the RF board.
Once, just once I was able to get the set to do something weird and neat that I have since seen reported elsewhere. With some odd combination of battery change and application of power, it forgot it's tuning limits, and let me tune long-wave below 150 kHz, all the way to zero on the dial. Whether this was any use or not, I don't know, but it was cool at the time.