Posts by DG0OFZ

    Hi all,


    just a short update from my workbench:

    My amplifiers are working now and they do not oscillate anymore. :)
    What I've done, was:


    - complete dismount the board from the heat sink
    - removed output circulator and both transistors from the board
    - cleaned all of them (circulator, transistors, board AND heat sink) very carefully with alcohol
    - mounting the board with all screws (changed to M2.5x4mm) on the heat sink
    - mounting and soldering output circulator
    - mounting and soldering transistors (bend the gate and drain pins to get good contact!) as mentioned by ra3s in this article
    - Implement the common BIAS voltage modification as explained by DH1DA in this article

    - connecting the GND from the power supply only to the heat sink


    I used a 10kOhm multi-turn trim pot to be able to set the BIAS voltage very exactly.

    I believe, in my case there was some residue from adhesive tape (used to fix the output circulator to the board for transport) on the circulator and the board. This and some small gaps between the transistors and the heat sink are responsible for having no perfect ground connection at those elements.

    So many thanks for all those helpful postings here, guys! :thumbup:

    Hi OE3GBB,

    Hi Thomas, I have the same problem. I do have a double unit and do want to combine them for double power. The right amplifier is doing well, but the left one is oszillating. Did you find a solution yet?


    No, no solution until now.

    I disabled the onboard bias voltage and fed it from external. So I'm able to slowly increase it from 0V on. But at some point around 1,7V the circuit begins to oscillate.


    Another try was to make the input impedance better through cutting the trace after the input isolator and soldering a small coax cable with 50Ohms in there. Circuit still oscillates.


    Used one FET or two FETs at the same time. Circuit still oscillates.


    I'm really frustrated. Both modules I'm having here are oscillating. Not only one ist stable.


    Next try is to clean the downside of the pcb and the heat sink for giving a better ground potential. Maybe there is some oil from the milling machine left there. I've done that before but now I'll do it again very exactly.


    Did you had a look at the other thread (https://forum.amsat-dl.org/index.php?thread/51-what-tx-amplifier-do-you-plan-to-use/) here? It seems, some guys there are having also the same problem as we are having.

    Hi all,


    I'm just trying to get my plates working. Did the simple modifications (removed input C, snowflaking and cuts at the drains) as per the documentation from SP8XXN and SP5XMU.


    If I put power on it, the current will be around 4 amps @24V. On the spectrum analyzer I can see that the amplifier is self oscillating around 2188 MHz. No matter if I have done the modifications or if I go back to the original state.


    Input and output are both terminated by 50ohms. All screws are in place, cover is mounted. No heat sink compound is used (have read the artcles from the beginning of this thread before). I've double checked for good ground connections.


    Any hints?