QRP Plus Modifications – Notes and Corrections

Larry East, W1HUE
Tucson, Arizona

(contact author)

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Reprinted from the July 1997 ARCI QRP Quarterly


This was an unplanned continuation of my two-part article "Modifications and Enhancements for the QRP Plus Transceiver" that appeared in the January and April, 1997, issues of the ARCI QRP Quarterly. After those articles were published, I received valuable feedback from several readers who tried some of the modifications that I described. I have also had a chance to test some of the modifications on an "enhanced" QRP Plus. All of the modifications that I previously described were tested on my "original" QRP Plus.

I will follow my previous nomenclature and refer to the "original" QRP Plus as a "QRP+" and an "enhanced" model as a "QRP++".


A few corrections are required to the section "A Better Output MOSFET" in my April ‘97 QRP Quarterly article:

General Notes

Power Amplifier Mods

Make sure that the ferrite bead that you put on the cold end of the new output transformer is #64 mix (#61 is probably OK, but rarely used in beads); do not use a bead of any other material type. It’s not all that critical that a bead be used here, so if you aren’t sure what material those ferrite beads that you got a good deal on are made from, I suggest that you not put a bead on the transformer lead. If you are sure they are #64, go ahead and use one but make sure you get it on the correct lead! The type of bead used for the choke added in the +12V line is not critical. In fact, a bead made from #43 or #73 material might be preferable over #64 material for this application.

Make certain that you use a #61 mix binocular core for the output transformer and that you wind and mount the transformer correctly. The free end of the two turn primary goes to +12V (the bead goes on this lead); the free end of the five turn secondary (which is in series with the primary) goes to the output filters through the decoupling capacitors. The common leads (the ones twisted together) go to the FET drain.

It has been mentioned to me by a couple of people who have made the PA modification that the transmitter efficiency is not as good with the MTP3055E as with the original IRF510. The efficiency of the new final in my rig (a QRP+) is indeed poorer on 20M, 17M and 15M, but about the same on the other bands compared with the original IRF510. The new final that I recently installed in a QRP++ (see below) had poorer efficiency on all bands but 12M and 10M. Another difference: The new final in the QRP++ gave good linearity with an idling current of only 50mA, whereas the new final in my rig requires an idling current of at least 100mA. I attribute these differences to manufacturing variations in the MTP3055E’s and other components.

EPROM Upgrades

Although Index Labs. is no longer manufacturing the QRP Plus, they will hopefully continue to make EPROM upgrades available at a reasonable cost to anyone who needs them. They may also be available from third parties. I strongly recommend that you try to obtain the latest EPROM revision that is compatible with your radio.

NOTE added March, 2014:  I am not aware of any sources of EPROM upgrades at this time.
You might check with the QRP-PLUS Users' Group for information on possible sources.

The latest version that I know about is designated "Rev 4D" (see my January ‘97 QRP Quarterly article for hints on how to tell what EPROM you have). This EPROM eliminates many of the complaints about the internal keyer and provides more optimum low frequency cutoff values for the narrower SCAF bandwidths. It also apparently cures a problem that can cause a noisy mic switch to "crash" the microprocessor. The Rev 4C EPROM works in the QRP++ and later versions of the QRP+, but it cannot be used in very early versions of the QRP+.

If you have the Rev 4C EPROM in a QRP+, you should replace C54 on the RF board with 0.47µF as described in my April ‘97 QRP Quarterly article. This reduces the keying waveform risetime to eliminate mushy keying. (This change is not required in a QRP++.) The Rev 4D reportedly eliminates the need to change C54.

Note: You may also need to replace a couple of caps on the LO board to eliminate "chirp" when using the RIT. This mod is also described in my April 1997 article and applies to both the QRP+ and QRP++ (and is independent of the EPROM version). Further notes on curing chirp were published in October 1997 (read it here).

QRP-Plus EPROM’s can be easily copied using an EPROM programmer. However, the EPROM supplied with each radio contains some custom information for that particular radio. The most obvious is the serial number that is displayed when the radio is powered-up. There is also a "display offset" value that is used to compensate for the BFO frequency not being exactly 50 MHz. I used to have a list of the addresses where this information is stored, but I can no longer locate it! There may be other custom values that I don't know about.

Notes Specific to the QRP++

Thanks to Dave Feldman, WB0GAZ, for being brave enough to let me try some of my mods on his QRP++ (an early production QRP+ that was upgraded by Index). Now that I have had some "hands on" experience with a QRP++, I can provide more specific information on making modifications to that version of the radio.

Applying the Audio Limiter Modification to the QRP++

My January 1997 article described the addition of a diode limiter after the SCAF filter to limit the audio level to a reasonable value until the AGC has time to react to a sudden increase in input signal strength. This is an effective cure for the "AGC thump" that plagued the QRP+. The approach taken by Index to eliminate the problem in the QRP++ was to add a limiter in the IF stage, before the SCAF. The limiter consists of two Shottky diodes, D2 and D3, across the output of the second receiver IF amplifier, U4, on the RF board. This approach produces faster limiting and less audio distortion, but it has a bad side effect: Strong signals outside the SCAF passband but inside the IF passband will activate the IF limiter and consequently reduce signal levels within the SCAF passband. It appears to the listener that strong signals outside the SCAF passband are "pumping" the AGC, but it is actually the IF limiter rather than the AGC that is being activated.

Bruce Franklin of Index stated in a letter to me that the IF limiter diodes also from the basis for SSB speech processing. However, these diodes are not directly in the transmit signal path, so it was not clear to me that they would have any great effect on the transmitted SSB signal. I used a ‘scope on Dave’s rig to investigate, and discovered that although there is indeed an RF voltage across the limiter diodes during transmit, it is too low to cause the diodes to conduct. In all fairness, I made this measurement after I had replaced the IRF510 final with an MTP3055E, and this may have resulted in less signal being present across the diodes (due to less drive required for the MTP3055E). In any event, removing these diodes resulted in no significance difference in the SSB signal generated by the new final. In my opinion, any "speech clipping" that was present resulted from "flat topping" in the IRF510 final, not the IF limiter!

I removed the IF limiter from Dave’s rig and installed an audio limiter following the SCAF. This completely eliminated the "pumping" effect of strong signals outside the SCAF passband, but had the side effect of allowing very strong adjacent signals to overload the SCAF. The overloading can be reduced somewhat by modifying the op-amp filter passbands as described in my January article, but I did not perform that modification to Dave’s rig. The problem can usually be eliminated (or at least reduced) by using the 20dB input attenuator.

Before installing the AF limiter in a QRP++, you must remove D2 and D3 from the RF board; otherwise, the AF limiter will have no effect. These diodes look like plastic transistors and are located between the second IF amplifier (U4) and T4. Do not remove any cap(s) soldered across T4 under the board. After removing the diodes, T4 must be retuned for maximum receiver output. It’s a good idea to install a 3pF to 5pF cap across the primary of T4 (use the solder pads for D2 or D3) to insure that T4 can be retuned for maximum signal.

Audio Limiter
Audio Limiter at Output of U4

Limiter added to output of U4

The AF limiter for the QRP++ is exactly as described in my January 1997 QRP Quarterly article (see the figure above). There are a couple of convenient grounding points that can be used for the diodes and 100K resistor. One is the ground pad for C26, which is located just to the rear of U4 (looking from the front of the board). The other is the ground pad of C42, located near R45 and Q10. You should increase the size of C28 to something in the range 680pF to 820pF to provide better filtering for the limiter.

I discovered that changing the first stage of the AGC amplifier/detector back to the circuit originally used in the QRP+ made the combined AGC/limiter action much smoother. I strongly recommend this change if you install the AF limiter. It is easily made (on the AF board) as follows:

  1. Replace the diode connected between the base of Q13 and +10V with a 2.7K resistor. This diode is shown as D17 on the schematic but it’s labeled D14 on the board layout drawing and the board itself, so be careful that you replace the correct one! Just make sure that you replace the diode that connects to the trace that goes to the base (center lead) of Q13.
  2. Replace R47 (10K) with a wire jumper.
  3. Add a 100K resistor from the base of Q13 to ground. You can solder the resistor between the R47 solder pad nearest C31 and C31’s ground pad.
  4. Check the value of the resistor between the emitter of Q13 and +10V (R49). If it is not 270 Ohms, replace it with 270 Ohms.

Replacing the output MOSFET in the QRP++

This modification is essentially as described in my April article. Replacing R16 on the RF board with a 2.7K resistor (as described in the April article) will allow an output power of about 10W (with the new final) on the low bands. Leaving it at its original value (2K) will probably allow at least 7W output, so there is no real need to change it.

You should limit SSB output (via the output power control) on 15M and above to no more than 5W to prevent distortion. Check the SSB RF envelope on a ‘scope, if you have access to one, to determine the maximum output that can be obtained on various bands without "flat topping". If you don’t have a ‘scope, then make some on-the-air test with a local ham. Being able to listen to a recording of your signal is very helpful. I noted that the MTP3055E that I put in Dave’s rig begin to show SSB distortion at a lower power level on the high bands (15M and above) than the one in my QRP+. This may be due to slightly different characteristics of the two MTP3055E’s used, or it may be due to the fact that I did not replace the output filters in Dave’s rig. (The new filters described in the April article provide better output efficiency as well as much improved harmonic suppression.)

Other QRP++ Notes

  1. Apparently having an early production QRP+ upgraded to a QRP++ by Index does not necessarily mean that the RF amplifier board was modified to add a PA bias voltage regulator; Dave’s rig did not have that mod! I highly recommend that a regulator IC be added if your rig does not have it, and this modification is mandatory if you make the PA mod. Adding a bias voltage regulator is quite simple and fully described in my April ‘97 QRP Quarterly article.
  2. Early production runs of the QRP++ (and early QRP+/QRP++ upgrades) used 100K for R5 on the AF board. This resistor is in the "automatic" mic preamp, located near the rear center of the AF board. The 100K value allows too much background noise to be picked up by the mic and a smaller value (2.7K, I believe) was used in later units. Dave had replaced R5 in his rig with 10K, and on-the-air tests indicated no objectionable background noise. So, if R5 is 100K in your rig, I recommend that you replace it with 10K. If you still get reports of background noise, try a lower value.
  3. There was a problem with some QRP++ rigs being shipped with an incorrect value for R33 on the AF board. The value used in some rigs results in the signal level into the SCAF being too high causing poor AGC action. The schematic shows 3.3K for R33; Dave’s rig had 2K which appeared to work OK. I suggest that you check this resistor and if it is not in the range 1.5K to 3.3K, replace it with 2.7K.

A New Modification

Is your S-Meter too sensitive? Not sensitive enough? There is an easy modification that can be made to allow you to set the sensitivity that I forget to mention in my "receiver mods" article. You can use a calibrated signal generator to set S-9 to the "proper" value or you can set it to agree with your favorite rig. What is the "proper" value? The official answer is that S-9 should correspond to an input signal level of 50µV (or a power level of -73dBM at an impedance of 50 Ohms), and a one S-unit change in signal strength should correspond to a 6dB change in signal level. I found that setting the S-meter in my QRP+ to read S-9 for a 100µV input signal resulted in each S-unit corresponding pretty closely to 6dB. The S-meters in two QRP++ rigs that I checked seemed to be closer to 3dB per S-unit.

Adjustable S-Meter Sensitivity for the QRP+

On the AF board, change R55 (4.7K) to 3.3K in series with a 2K pot. Use a 0.25 in. diameter (or square) plastic trimpot and mount it so that it can be adjusted without removing the board.

Adjustable S-Meter Sensitivity for the QRP++

On the AF board, change R54 (1.5K) to 680 Ohms in series with a 1K pot. Use a 0.25 in. diameter (or square) plastic trimpot and mount it so that it can be adjusted without removing the board.

———— Copyright © 2005, 2012 by Larry East, W1HUE ————
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Page last updated on March 22, 2014