The following are modifications to the Realistic DX-394 HF reciever sold byTandy in the UK. All these modifications were taken from postings to theEuro-Scanner mailing list. Thanks go to the authors, although no names oremail addresses are mentioned here. To subscribe to the list, send an emailto ' euro-scanner-request@grid9.net ' with the message 'subscribeeuro-scanner' in the body of the email.  ------------------------------------------------------------------------The famous 'Purple wire' modIsn't it annoying, when you're fine-tuning the radio and as the frequencychanges, so it makes that awful 'Chuff Chuff' noise? Here's how to get ridof it.Take off the top cover (mind the speaker wire, it's a bit short). With thedisplay facing you, just by the left-hand end of the ferrite bar antennaare two connectors. The nearest one is CN1. The second wire in this ispurple, and needs to be cut. That's it!  ------------------------------------------------------------------------Frequency display calibrationNOTE: This is for SSB Only. (Not tried AM since doing the calibration)1. Remove the top cover by removing 2 screws at rear & then sliding up &back the top cover (Do this with the power off!)2. With the radio facing you locate 2 Blue crystals named x2 / x2. Near thefront. Next to these should be a trimmer capacitor with a small screwdriverslot. (Note- these are on the underside of the board in my own '394- Ed)3. The Left one is LSB The right one is USB. (I only adjusted the right onmine)4. Turn the power on.5. Tune to get the best reception to a constant signal (volmet around 5Mhzwill do nicely.) Now adjust the right hand preset to 'move' the signal up alittle. Re-tune the main tuning control toward the correct frequencyreadout - Do these a little at a time 'chasing' yourself until the signalsounds good & the readout is correct.This should be tried against a few signals to get the best overallperformance. Mine is now within .05 just about all over the range.  ------------------------------------------------------------------------Keyboard BeepsTo disable keyboard beeps, firstly do not take the lid off! Someonesuggested that the beeps can be disabled by removing a certain resistorfrom the main board; this sounds far too permanent for my taste. It's a lotsimpler.To disable beeps, power off and hold down CLEAR key whilst switching backon.To re-enable beeps, power off and hold down PGM key whilst switching backon.  ------------------------------------------------------------------------Fast AGCBridge C163 with 100uf cap for slow AGC, 47uf for fast AGC. C163 is locatedon the bottom of the main board with the radio knobs facing you. It is onthe right near center just below Q9 with four surface mount components tothe left.  ------------------------------------------------------------------------RF GainGain: Bridge a 4.7K ohm resistor across R184 (helps gain control).  ------------------------------------------------------------------------BandwidthBridge .01/.047/.22uf cap across R64 (narrower bandwidth/crosstalk).Bridge .1/.22uf cap across R91 (narrower bandwidth/crosstalk).  ------------------------------------------------------------------------Noise blankerHere are 2 easy mods that will make the DX-394 noise blanker performbetter. The performance on AM is marginal and doesn't work at all onCW/SSB. The mods will improve the AM blanker performance significantly andallow it to work on CW/SSB.1. Remove the bottom cover with the front of the receiver facing you.Bridge a 150K resistor across R180. This resistor is the third componenttoward the front from Q32 (don't count Q32). R180 should be black with 224(220K) on it (microscopes at the ready- Ed). This will re-bias Q33's baseto a more sensitive threshold.2. Find Q34. It is to the right of Q32. Solder a Schottky diode (1N5817 orsimilar) from the collector of Q34 to a ground trace. The anode of thediode should go to ground and the cathode (banded end) to the collector ofQ34. The collector is the single pin on the side of the transistor facingthe rear of the receiver. A 1N4148 or 1N914 will work but not as well. Thismod will now let the noise blanker work in the CW/SSB mode. It allowscapacitor C182 to reset (discharge) in between noise pulses.3. Replace the bottom cover and remove the top cover and find potentiometerVR1 (close to the front of the unit). Set the pot about midway and you arefinished (Leave the pot alone at first. Adjust later after you have checkedout the mods).  ------------------------------------------------------------------------This is a list of Mods from Deja News, the WWW gateway to uselessnet.Crosstalk Mods1) For those with DX-394s with adjacent channel "crosstalk" problems hereis a fix that will considerably reduce the problem. This applies to strongAM stations within 10 to 30kHz of the station you are receiving that "bleedover" and can be heard in the background. I am NOT referring to sidebandsplatter from adjacent stations caused by the overly wide stock CF1 AMceramic filter. The problem is caused by the fact that Tandy designers (orwhoever) forgot one of the CF1 bias decoupling capacitors.Anyone with a schematic will note that the node at the junction of R58,R61, R62 and R64 should have a bypass capacitor to ground from this point.Without it there is a resistive signal paththrough R57, R58, and R61 thatbypasses the CF1 filter and causes broadband bleedthrough. All that isnecessary is to solder a 0.047 to 0.2uf ceramic capacitor directly acrossR64 and the crosstalk will be significantly reduced.For those of you without a schematic you can find R64 by removing thebottom cover with the receiver knobs facing you. Find the silk-screenindent for D15. This is a four leaded surface mount, dual diode package.R64 is the third component to the right of D15 and has 222 (2.2K) stampedon it(you may need a magnifying glass!) This resistor is also just belowand slightly to the right of the third pin (from the left) on CF1. Itshould be noted that with real strong adjacent signals there still may besome crosstalk, and reducing the Fr gain will help even more.Good luck and be careful - the parts are all tiny surface mount.Another note for those who listen to AM only on the 394 - if you swapceramic filters CF1 and CF2 you will have a narrowwer AM bandwidth forcrowded band conditions. Unfortunately, the SSB and CW will now be wider!2) Here is another DX-394 modification that will further reduce adjacentchannel crosstalk from strong short-wave stations (see above). This one maybe more effective than the previous one and, used together, crosstalk isreduced to an insignificant level.With the bottom cover removed and the front panel knobs facing you find thesilk-screen indent for Q13. It is a black surface mount transistor withthree leads (2 on one side, 1 on the other) midway toward the front of theunit and left of center. Resistor R91 is just below it and should be markedwith 102 (1K). Bridge this resistor with a 0.1uf or 0.22uf ceramiccapacitor (keep leads short) and you're done.IF signals preceding the ceramic filters are injected into the noiseblanker transistor Q13 and are somehow amplified and then capacitivelycoupled around the ceramic filters via this circuit path. I have notfigured exactly how this happens yet. Adding the 0.1uf capacitor aroundQ13's emitter resistor R91 (it already has a 0.01uf cap across it - C81)seems to decouple the bleedthrough signal. The operation of the noiseblanker is not affected at all.  ------------------------------------------------------------------------AGC ModGot the service manual and found the cap that sets the AGC timing! This modwill give a nice attack and release time (otherwise slow it down) and onlyinvolves putting a 100uf / 16V (or better) electrolytic cap across C163. Ichose this value to my liking but you may speed it up or make it slower byincreasing or decreasing the value. C163 is located on the bottom of themain board with the radio knobs facing you. It is to the right near centerjust below Q9 with four surface mount components to it's left. Put the +side of your caps towards Q9. A shielded cable could be brought out to asmall switch and back to one of the cap's leads to select fast and slow ifyou wish- I am currently using 47uf for fast and 220uf for slow with aSMALL mini spdt switch mounted just above the headphone jack. It's a tightsqueeze but entirely possible!  ------------------------------------------------------------------------Gain ControlThis modification will improve the linearity of the DX-394's RF Gaincontrol and make it easier to use the RF gain as a volume control for SSBsignals. It will give the gain control better resolution so that the rolloff won't all occur in the last 40 degrees or so of the knob (in the CCWposition). To implement the mod solder a 4.7K or 5.1K resistor acrossresistor R184. This resistor is located in the right rear of the main PCboard when facing it with the bottom cover removed and the knobs facingyou. One end of it is connected to pin 4 of connector CN4 (has 5 pinstotal), which is located just in front of the 12 volt dc input jack J5 onthe rear panel.  ------------------------------------------------------------------------Timer ModsOne glaring omission on this otherwise fine (with mods) little radio is thelack of an output jack to switch on a tape recorder for use with the 5built-in timers. I have added a small 12-volt relay that is energizedwhenever the receiver is powered-up, and a jack on the back tied to therelay's normally open contacts. The switched 12 (13.8 actually) volts iseasily obtained across C140.With the receiver on its back, C140 is in the front right corner of theboard, and there is a handy metal shelf nearby that is perfect forattaching a small relay. I simply glued the relay on its back on that shelfand ran the coil wires across C140, and the contacts out to the jack.A few caveats:*Use a small relay so as not to leach off TOO much juice. Most taperecorder's remote inputs don't need a large set of contacts, and besidesspace is limited.*Make sure that you put a clamping diode across the coil contacts if thereis not one inside the relay already. The cathode (banded) end is tied tothe + side of C140.*This ultra-simple mod gave me fits for a while trying to get it to workwith my cassette recorder until I figured out that the remote input needsisolated switch contacts, and didn't like one side of the of the jack tiedto the radio chassis (ga-DUH!). I had to enlarge the mounting hole for thejack so it didn't touch, and also mounted the jack between 2 pieces ofrubber to keep it off chassis ground.With this small addition the 394's timers become truly useful. I alwayswondered that their point was if you had to be at the radio anyway to catchthe program you set the timer for! Now the set-up is automatic and I canrecord whatever, whenever.....a simple mod that should have come standard.  ------------------------------------------------------------------------Antenna InputIf you look at the schematic you will find that the Hi-Z input is coupledthrough a 15pf capacitor which will have significant attenuation whencoupling to a random wire of the length you are using, so using the Low-Zinput is actually best. As far as longwave goes the Hi-Z input probablygives better coupling to the first RF stage since it bypasses the front-endRF coils' primary winding. The 65' total antenna length is very shortelectrically for LW so the antenna appears very Hi-Z itself and theimpedance match is more appropriate. Find a 2 to 5 mH (that's millihenry)Fr choke and put it in series with the antenna for LW, connect it to theLow-Z input and you should see a marked improvement in signal to noise.

Current time in Deltona, Florida, United States [28.9002N 81.2419W | Grid: EL98jv] is Fri Jul 30 18:11:15 2010