Fixing "Chirp" in the Index Labs QRP-PLUS

Larry East, W1HUE
Tucson, Arizona
(contact author)

Reprinted from the October 1997 ARCI QRP Quarterly

My dull little brain has finally figured out that there are (at least) two sources of "chirp" in the QRP-PLUS: 1) The PLL settling time when RIT/SPLIT offsets greater than a few kHz are used, and 2) the rise-time of the BFO offset switching signal. (The 650-700 Hz shift in CW transmit/receive frequencies takes place in the BFO, NOT the VFO/PLL circuit). Keying chirp due to both causes are more likely to show up after installing the "Rev 4C" EPROM, resulting from the faster QSK (especially when using an external keyer) provided by the firmware in this EPROM. For some reason (which my dull little brain hasn’t figured out yet), the chirp caused by the BFO frequency switching is more noticeable above 30M (20M and higher). There may be a third source of chirp; one QRP++ owner reported that my cure for the BFO chirp described below did not cure the problem in his rig. Stay tuned for further developments.

I understand that there is now a "Rev 4D" EPROM available from Index, even though they have stopped producing the rig. I have been told that this EPROM provides a longer switching delay between receive and transmit, thus reducing (but not entirely eliminating) the likelihood of the two sources of chip described above.

You can check for chirp by listening to the keyed output of your QRP+ (run it into a dummy load!) on another receiver, or by on-the-air tests with a local ham. It is most instructive if you can actually listen to your signal -- either in a local receiver or over the phone from another ham.

The good news is that both chirps can be easily cured; at least on the two rigs that I have experimented with. The bad news is that you have to remove the top three board from the rig to do it. Both fixes are accomplished by replacing some caps on the LO board (the third one down in the stack). The chirp fixes described below apply to BOTH the QRP+ (original version) and QRP++ ("new improved" or "updated" version).

Fixing Chirp due to BFO Switching

Change C3 on the LO board from 1.0uF to 0.1uF. Look for a green trimmer cap (shown as C56 in the schematic) near the front right corner of the LO board. C3 will be just slightly to the rear of the trimmer and very close to the right edge of the board; it's a small dipped tantalum, probably orange or yellow in color. Replace C3 with a small 0.1uF mono cap rated at 25V (or higher).

Fixing RIT/SPLIT Chirp

This mod was described in the April 1997 QRP Quarterly along with several other transmitter related mods. (See the January and July 1997 issues for more QRP+ mods.) However, to save you from looking it up, I’ll repeat the information here. Change C43 and C44 on the LO board from 4.7uF to 2.2uF. These caps are in the PLL low pass filter and are located near the right rear edge of the board. The original caps are dipped tantalum; replace them with 2.2uF dipped tantalum caps, rated at 16V or greater. After this change, there should be no detectable chirp for RIT/OFFSET splits below 20kHz (or more).

Fixing "Soft Keying" in the QRP+

Note: This Does Not Apply To The QRP++!

Just in case you haven’t seen this before... (it was in the April 1997 QRP Quarterly):

After installing the Rev 4C EPROM in my QRP+, I noticed that the keying was very "soft" unless RIT was used. I discovered that the problem was due to a long time constant in the +12V decoupling to the first driver stage resulting in about a 10ms rise in the keyed waveform (one would like no more than about 5ms). I fixed the problem by changing C54 on the RF board (the top board) from 4.7uF to 0.47uF. C54 is located in the rear left-hand quadrant of the board. The original C54 is a dipped tantalum, and I used the same type (but a factor of 10 smaller in value) for replacement. The voltage rating of the replacement cap should be at least 16V (preferably 25V to 35V).