IFR-1200S power measurement

Discussion in 'Radio Circuits, Repair & Performance' started by AD5GG, Dec 2, 2018.

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  1. AD5GG

    AD5GG Ham Member QRZ Page

    Hi all,

    I have started using a IFR-1200S which has been lying around for a while.
    My issue is the power measurement via the T/R port reads low until it has warmed up for about 20 minutes. After that, it reads just fine. I'm testing using a 5W handheld radio.
    As a troubleshooting step, I set the frequency of the spectrum analyzer to the same frequency as the handheld radio, and there is approximately 10dB difference between the value I read when I just switched on, and the value I read when it's warmed up. This is reflected also in the power meter (approx 360 mW just switched on vs 4W when warmed up)

    Is this normal? If not (I assume not) any ideas on what I should be looking at to fix it?

    I figured I'd ask here before I go delving into the service manual.

    Thanks in advance!
  2. KA5IPF

    KA5IPF Ham Member QRZ Page

    Have you considered it might take that long for the reference oscillator to stabilize? Every circuit is tied to it.
    AD5GG and K7TRF like this.
  3. AD5GG

    AD5GG Ham Member QRZ Page

    I have, yes. That's what I was going to put it down to if someone reported back that their IFR-1200S did the same thing.
    The behaviour is a little bizarre though:

    When the unit is first switched on, and I apply a few watts to the T/R connector, the relay in the IF block behaves differently depending on the frequency --
    At 144 MHz, it doesn't switch (REC RELAY line stays high).
    At 147 MHz, it chatters (I can scope the REC RELAY line at the IF block and can see the square wave causing the chatter).
    at 440 MHz, it switches (REC RELAY line goes low), and it power meter reads the low power I mentioned in the original post.

    I'm sort of assuming there's some power detection issue somewhere.

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