The Story of the Dream DX-394
The Portatop edition
Pictures at bottom
When I got a DX-394 a few years ago, I tried it for about 15 minutes and quickly declared it a piece of junk. A couple of years ago I started getting into modding and fixing radios for fun. Soon after, I read how easy the DX-394 was to modify and how much it was improved. I decided to get one and start experimenting. I tried almost all of the mods and decided how much of an improvement it made and whether it was worth the time, effort and expense, if any. Then I looked for other things or ways to improve the 394 and developed my own mods. The main things I look for in radios when I play with them or try to improve them are the audio quality, selectivity and fun factor.
This article will describe what I do to the 394 to make it my ultimate 394. This is the result of modifying a dozen or so in different ways, some mods were designed by others, some are of my own design. This article will not detail the mods themselves since this is already done in the YAHOO DX-394 group which you should join if you get one. Also, I will detail the custom mods if you email me.
Special note: I DO NOT recommend the 394 for SSB or CW work because the screwed up IF scheme used does NOT allow a quality narrow filter for these modes. More later.
I start on the outside because I find the 394 very bland, dull and unexciting to look at. I take the top and bottom cover and the four knobs and spray paint them satin (semi-flat) black. This gives the 394 a very sharp “pro look!” At this point I could either paint the tuning knob or I could go the next step which is to replace it! I replace it with a larger, heavier, higher-end consumer-grade metal knob, painted satin black of course. Now the 394 tuning shaft is on the short side so I had to cement a small knob to it to allow it to fit on the shaft. It is pretty cool now to look at and use!!
Before moving on to the inside I want to mention that the 394 is designed and built by GRE of Japan who have made many of Radio Shack’s scanners over the years and also the CLASSIC super-cool looking and fun to use SX-190 !
Moving to the inside, there are a few serious mistakes that GRE made in the design. Fortunately the 394 is very easy to take apart and it’s easy to buy the service manual. This has led to many mods from many people and a very large Yahoo group.
GRE forgot a couple of capacitors in the IF section that allows the IF signal to get into places it really shouldn’t go. A published fix is to add TWO caps, a fairly easy fix taking about 5 minutes. Next is a quick and easy fix to eliminate that annoying popping sound when using the tuning knob. This sound is known as chuffing and affects many radios, especially Sangean portables and even the famous Sony ICF-2010! All that is required is to either clip the purple wire or to remove it from the connector unit.
Since I am into improving sound quality as much as possible when it is easy enough to do, the next mod is to increase the audio bandwidth. This is done by increasing the value of a capacitor in the audio path from 100 uF to 470 uF. Next is the speaker. The speaker used in the 394 is probably worth about a quarter; an absolute piece of trash!!!!!! I remove it, smooth down the plastic that held it in place with a Dremel tool and then cement in a new one of much higher quality. The improvement is dramatic. When I use the 394 as a base station, I get fantastic sound quality from a large active speaker I made. It is a joy to listen to !!
The next change is for both sound and selectivity. The two stock filters are pieces of junk, typical of what most manufacturers use in their receivers. Even Sony used junk in their 2010! There are three options:
1) Replace the wide filter only with the best overall bandwidth for broadcast listening that allows good selectivity and good fidelity.
2) Replace the wide and narrow filters on the main board and do an additional mod that allows switching between the two filters
3) Replace both filters off the main board with leads to the filters and an external switch.
It is quite important to select a quality filter. I do not recommend the Kiwa filter since the shape factor is quite bad and does not offer good fidelity response or good selectivity. It is just another cheap filter with a tighter 6 dB response point to eliminate hets.
When one picks a filter, look for a large metal case from either NTK or Murata. The shape factor for 6 to 60 DB should be 2.0 to 1 at the most. shape factor explained more on filters here Most cheap filters are around 3.0 to 1 which stinks! The filters I used in my dream 394 are the Murata CFS455H (wide, 6kHz at 6DB) and CFS455J (narrow 3 kHz at 6DB). These are outstanding filters with excellent performance with shape factors of LESS than 2.0 to 1 !! I remove the wide filter and use thin coax to go to a switch at the back of the radio with the two filters attached. Look at the picture below and compare the Murata to the stock. Look at the difference and in this case size is very important. The Kiwa filter is the same size as the stock filter, very bad news indeed.
Probably the most radical mod is to change the power supply method. What I do is remove the power supply transformer and power cord and move the filter caps to the bottom (to make room for the bigger speaker I install). The 394 uses less than 200ma which is nice and low. I remove it to make the 394 portable (or portatop as some people call these kind of radios) and to make it lighter. With the power supply gone, there are two choices to power the 394 either an AC adapter or DC battery. I have made a 10 cell NiMH battery pack that should supply the 394 with over a dozen hours of field use. In the house I use a 7.2 amp gel cell that I got at a ham fest for $5 that should power the 394 for several dozen hours ! If I desired, a good 300 ma AC adapter could be used. Nothing beats hum reduction like using a battery !
Mods I don’t do:
RF gain mod. The 394’s I’ve owned never required the use of the RF gain control, so why mod it ?
Hiss mod. This goes completely against my desire for better sound. This mod was developed because of the problems with the cheap stock filters. Don’t use a 394 that requires the hiss mod if you like good fidelity !
AM AGC mod. I like the AGC of the 394 in the AM mode and would not change it. As for SSB, remember that I told you to forget SSB on the 394 !!! (At least if you have any interference !)
Shielding the 394. I have not found it necessary in my listening area, perhaps you need it, who knows ?
So what have I ended up with ?????
My 394 is a really fun radio to use and it works great. I own over 100 radios and have used a lot more than that, yet I still find my specially-modded 394’s a great overall radio. It is sensitive and it doesn’t overload any more than other radios in its price range for the past 25 years. It is selective; in fact, more selective than many of the sub $1000 radios from Japan over the past 25 years and having 3 and 6 kHz filters means I can work with almost any situation I end up in ! The 394 is FUN ! It has multiple tuning speeds, lots of memories and direct access to meter bands! It also has a really nice fine tuning control. I would say it has a better interface than many other radios I have used-----it is FUN !
Front Back 10 cell battery pack Upgraded filter vs stock filter
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Being so pleased with the results, I decided to go a step further and make one with more drastic surgery. In the unit I just made, I had to do some surgery to get the worlds largest speaker in a DX-394 ! I also put the filter switch on the front panel.
version 2 compare the speakers !!!!!
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Filter comparison chart !
murata part # bandwidth rating (poor-excellent)
stock wide (CFW455HT) 6kHz@6Db 18kHz@40Db poor/fair
stock narrow (CFW455IT) 4kHz@6dB 15kHz@40Db poor/fair
upgraded wide (CFS455H) 6kHz@6dB 15kHz@80dB VG/excellent
upgraded narrow (CFS455J) 3kHz@6dB 9kHz@80dB VG/excellent
To contact me email me at mikemaghakian@yahoo.com