Realistic DX-100L

(model20-9228 - Belgian FCC approval number: RTT/ONL/X0031)

By ON6MU www.qsl.net/on6mu

Realistic DX-100LRealistic/Tandy DX100L mods

DX-100L details  
General Coverage Communications Receiver Rating (1-5): H (1)
Made In: Taiwan? 1981-1984 (27 may 1981) Voltages:12VDC 4W - 117/220 VAC 6,5W
Coverage: 150-30000 kHz (All HF bands) Readout: Analog
Modes: AM/SSB-CW Selectivity: One Position
Circuit: Single Conversion Physical:12x5.7x8" 10 Lbs. - 30x22x11 cm
Features: " Head. Jack, S-Meter, ANL, Dial Lamp, Standby,
Fine Tuning, Longwire antenna connection, Telescopic antenna.
 
New Price: $100  
Comments: The DX-100 also operates from 12 VDC. A basic receiver. Please note that Realistic is a registered trademark of Tandy Corporation.
Note: There is also a DX-100 version without longwave reception:
Important: the components numbers doesn't match on the schematics of both versions

 

DX-100L MODS (de ON6MU)
Place take caution when performing these modifications.
I can not be held responsible for any damage or loss of waranty leading from opening your receiver and performing any modifications.
Use them at your own risk.
Remember: there are some schematic differences and some changed part numbers between DX-100 and DX-100L!

Improved (bass) audio quality
improved bass audio response modfig.01
  • The lower audio frequencies (bass) are not well presented. You can improve the audio quality by adding some thin foam on the back (remove front panel) of the speaker and glue some on the inside of the cover. There is lots of space at the speaker side of the cover. You can use some padding too.

    This requires some soldering but gives the biggest result:
    - DX-100L: Replace the 22nF capacitor C61 with 220nF
    - DX-100: Replace the 47nF capacitor C77 with 220nF
Improved BFO SSB detection
modification for ssb dx-100
fig.02
  • The BFO signal to receive SSB signals is somewhat on the low side. IT works but you need to tune (with an ATU or ATT) the SSB-signal to almost no signal S1 to be able to tune to SSB signals and understand what they are saying HI. By increasing the BFO output we can improve the detection of SSB signals.
    Folow the little black coax cable that comes from the BFO knob.
    - DX100L: Replacing C38 (C41 in DX100) with a capacitor of 330pF worked fine in my set, but Bill G4KKI reported capacitor of 100pF to be ideal in his DX-100, so experimenting is probably needed.

    After the mod you should get a continues S-meter reading of about S5 in SSB (no antenna connected).
Improved VFO drift stability
fig.03
  • This mod doesn't require any soldering skills, but has a large impact on the stability of the receiver..
    A very effective way to decrease the frequency drift is extra cooling of Q9. I've found that Q9 gets hot after some time which tend to change the voltages as it heats up causing extra VFO drift. Mount an extra larger heatsink on the transistor and don't forget to put some thermal grease on it (fig.03).

A Youtube movie of somebody who did the mod (drift before and after):
http://www.youtube.com/watch?v=qZCZXwopeC8

Connect a Frequency Counter
frequency mod dx100fig.04

  • You can connect a frequency counter to the DX100! The frequency read-out is about 455kc off but gives you a perfect readout of the tuned frequency. You could (as I did) modify a "low budget" frequency counter to give the correct readout.
    The components are not too critical. You need:
    20 cm of shielded cable
    6,8 pF capacitor (6.8pF...68 pF)
    47 Ohm carbon 1/4watt resistor (between 47...150 Ohm)
    a RCA female plug
    Solder the resistor and capacitor in series to the receiver (look at fig.04)

    Note: DL4YHF reported a different value worked beter (100nF). So the value of the capacitor may change according to the input impedance of the frequency counter used.
Connect a 50 Ohm coaxial Antenna
fig.05
  • By default the DX-100L comes with a telecopic antenna and a 300 Ohm external antenna connector (fig.07). Those antenna connections goes to the preamp via a coil L1. L1 is a base loading coil for the build-in telescopic antenna. We do not need a loading coil when using an external antenna.
    Connect the centre part of the coax before L1 and couple it with a 3n3 capacitor. Connect the braid of the coax to the chassis of the receiver (fig.05). Finally place parallel on the SO239 a 1k 1/4 watt carbon resistor.
    SO239 backside
fig.06

You need:
1 kOhm carbon resitor
3,3 nF capacitor
20 cm of 50 Ohm coax
SO-239 chassis UHF connector (female counterpart of the PL259)

<- This is how I lead the 50 Ohm antenna connection and the frequency counter connection to the outside world (fig.06). Lots of space to mount the extra connectors there.

fig.07
  • DX100 Links

    DX-100 Service Manual.
    DX100 high-res picture from Radio Timeline website.

    More of interest:

DX-100L: My very first "real" world receiver. Enjoy this little old radio!

The youtube made by David shows an example of some of my mods done by David:
http://www.youtube.com/watch?v=29p_OvircsQ
and
http://www.youtube.com/watch?v=Tw3UpUx3EiY

 

ON6MU
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