Index Labs QRP++ distorted audion in SSB TX

    Dear all


    I have just received an Index Labs QRP ++ from a friend of mine which shows a badly distorted audio in SSB. As far as I have read, I think this is a common issue both in the original QRP + and in the upgraded version.

    As far as I know, there is a very active group in Germany using and doing mods to the rig. I found quite a lot of information on the web regarding this issue. Larre East, W1HUE suggested to replace the original IRF510 by a MTP3055E but later on he suggested in another article to use the DL-QRP-AG PA which, as far as I know, is no more available.

    I thought about the possibility to replace the IRF510 by a Mitsubishi RD16HHF1 which is not pin to pin compatible (GDS in the IRF510 and GSD in the Mitsubishi), but would appreciate any advice from users with experience with this rig.

    There is not too much room inside the rig and although there are different alternatives to add a PA (QRP Labs, QPA-q from Hamradiokits, etc.) but again, the available room is an issue.

    Thanks for reading and for the feedback / suggestions.


    72/73


    Vicenç, EA3ADV

    Vicenc, I had this unit ,too several years ago. You can solve the problem by adjusting the idle current.. original it,s much to low. The IRF510 needs either a higher voltage (24-28v) and a higher idle current. Be aware the potentiometer on the back panel for the idle current is in the original version turning the current in an unusual way (right turning is lower, left turning is higher). With a higher voltage you can get up to 25watts output. With 12 V it,s around 5 watts. I remember in the very first issues of th qrp-report were several good mods for the qrp-plus.

    73 Uwe

    Good afternoon Vincent,

    in my first test review about the QRP+ in the German mag "FUNKAMATEUR" I wrote: "In SSB I got the Report AURORA Modulation" and that is still a good description of the SSB Modulation of the original :)
    The bad Modulation is caused by the non Linearity of the IRF 510 at 12V, In several Experiments we found, that the IRF510 needs at least 20V to operate linear.


    I did a lot of work together with Larry W1HUE in those days. Larry published some articles in QRP Quarterly, in German they have been published in our QRPp-Report. I did not find the Quarterly articles, but in my archive I found some original emails from Larry regarding the mods.


    I was not happy with replacing the IRF510 by MTP3055. I remember that I managed to kill several of them until I found a better solution - using only the QRP+ driver and adding a separate 10W PA. The was plenty of room for our extreme small Club-PA on the back panel. Unfortunately the small PA is out of order since several years because the bipolar transistors used in it have been not longer available someday.


    I did never measure if it fits but I think our actual 10W PA and also the QRP-Labs PA will not fit at the back panel, but eventually the 5W PA offered by BOX73 would fit.

    If you send your actual email adress, I will send you what I´ve found about the mods.

    73/2 de Peter, DL2FI
    Proud member of Second Class Operators Club SOC and Flying Pig Zapper #OOO (Certificated Kit Destroyer)

    Dear Uwe and Peter


    First of all, many thanks for your reply to my post.

    I have a complete file with all the available information about both the QRP+ and the QRP++. In fact I have all the articles Larry East wrote for the QRP Quarterly.

    I will see what happens when adjusting the bias as Uwe suggests. It seems that in some cases the distorted audio issue is solved while in some others not.

    In one of the last Larry's articles he described how he added the DL-QRP-AG PA to the QRP+ with outstanding results. Unfortunately as you mention it is no more available.

    I never faced the problem because I operate almost 100% CW but my friend likes SSB and from time to time he would like to use the QRP++ in this mode.

    Let's see what I can figure out regarding a reliable solution.


    Once again, many thanks for your advice and support.


    72/73 de Vicenç, EA3ADV