I have made several MODS to improve selectivity for the Receiver portion and frequency agility for the Transmit section. Collectively these MODs have yielded  QSOs with 12 states in a relative short time and under casual operating efforts.

The transmitter is using a stock 2N3904 as it's final at about 120 mW with a stiff 9 volt power source.

Originally I built the single PC board kit from HSC electronics without MODs and did have some success.

The limitation of no TX offset and no RIT severely limited the success rate.

I initially put in the Doug DeMaw MOD for TX offset (i.e. also functions as two freq. transmit possibilities but decided that a full VXO mod would help the most.

I used the standard type of series LC in series with the ground end of the xtal and the var. cap was about 100 pF and the inductor was a toroid of about 10 uH. This yielded a 5 kHz VXO shift. Not bad.

I tried to add in a switchable cap to give Tx offset but the differences in the linearity of the end of the var. cap resulted in too little RIT on one end and too much on the other using fixed switch able cap. to pad the main tuning cap. So I wound up put two var. caps 100 pF each in with a switch between them to go from TX or RX and reverse. Now I essentially had two independent caps. so I could  zero beat the other stations' frequency and then adjust the other cap for correct RX offset tone. This not  only provided completely flexible Rx and TX operation but allowed me to set the RX tone to a fixed value on the upper or lower side of the other station's transmit frequency to take advantage of QRM  and  the use of a fixed  AF active OPamp filter (another MOD).

So looking at the  front of the MODed rig we see click thumb-nail for full-size picture(of all the pixs).

Notice the two black point dials on the left section of the panel and the small toggle switch below  and between them. This switch is the RX/TX switch.   This is  a back bottom view of the rig. Note the two variable capacitors at the upper (actually bottom) left edge of the picture.

This picture shows the three sections of the rig. To the left (of the picture) is the three notch Front-end filter, in the middle is the active AF filter and LM386 amp section (I didn't use the on board location because of the stability problems (oscillation, etc.) and to the right is the original HSC PIXIE board.

The Front-end filter was needed at my location to cut down severe local and foreign broadcast BCI. My longwire antenna is great at picking up a wide range of HF. The schematic of the filter looks like click the thumb-nail for full size schematic. The results of the filter are outstanding. I modeled the three notch filter on my ARRL Radio Design software and got the following output on the graph  Notice the great attn in the band I was trying to suppress. The filter only has about a 1 dB attn at the VXO frequency of 7.040. With the filter inplace I operate the rig without the cover on the enclosure as you see 

The Active AF filter is a two stage circuit from one of the DeMaw books. I used isolated pad technique for it and Dremel tool routing of foil on the Front-end filter.

The modified PIXIE has been a lot of fun to build, change and operate.

While it is ugly it works well and it demonstrates what a QRPP power transmitter and an absolute minimal receiver can do.

I would appreciate you comments at  [email protected]
 

72's and happy MODing,

Sam AE4GX