• The lower audio frequencies 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

• 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).

• 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

• 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. And there are also frequency counters that allow an offset.
  The components are not too critical. It alos depends on the input impedance and sensitivity of the frequency counter.
  You need:
  20 cm of shielded cable
  68 pF capacitor (6.8pF...150 pF)
  100 Ohm carbon or metal film 1/4watt resistor (any value between 47...220 Ohm is ok)
  a RCA female plug
  Solder the resistor and capacitor in series to the receiver (look at fig.04)
  
  If I remember it correctly it is connected to switch S1d that goes to Q2 (for DX100L it is Q3).
  
  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.
  

• 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 4n7 capacitor. Connect the braid of the coax to the chassis of the receiver (fig.05). Finally place parallel on the SO239 a 1.5k 1/4 watt carbon or metal film resistor.
  SO239 backside

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.

• I noticed an improvement of the sensitivity of band 4 (SW3) by changing the cap C2 (C56) from 7pF to 15pF. The capacitor is found close to L7 (L5).

• DX100 Links
  
  DX-100 Service Manual.
• DX-100 Schematic.
• DX100 high-res picture of the radio
• DX100 inside view
• DX-100L Service points layout (do not attempt aligning the radio without proper knowledge and the proper service manual).
  
  More of interest:
  

• The fine tuning control range can be
  reduced by paralleling a resistor across the pot. This makes it much easier
  to tune in SSB. It can be added across the posts on the main board where the
  pot wires go. 5k for me made the control perhaps +/- 5 kHz.
  
  PS: who can help us? How is the mod "D10" done? Does anybody haveany info about it?
  Radio Shack DX-100 | RadioReference.com Forums

• Some complain of some backlag of the tuning knob. In most cases it is simply a matter of the tuning weel being a bit loose. This is mounted to the main tuning capacitor on the bottom of the PCB. Just tighten the screw and your done!