preamp component smoked - RF Concepts 2-417

Discussion in 'Amateur Radio Amplifiers' started by KD2QED, Nov 17, 2019.

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  1. KD2QED

    KD2QED XML Subscriber QRZ Page

    Sorry for the delay, I had to order the resistors. They came in yesterday and I've just installed it now. Unfortunately it would seem there is more wrong than just the resistor, as the preamp still isn't turning on.

    I wanted to give that a try first as pulling the board to see underneath is challenging, but seeing as I don't have any other options I'll try to tackle that over the holiday.
     
  2. K6AQ

    K6AQ Ham Member QRZ Page

    Any progress on this? I have a 2-23 whose preamp switch significantly attenuates the signal when switched on (instead of actually amplifying it). I originally replaced the MRF966 GaAsFET thinking that was the issue but it didn't do much. Neither did tweaking the variable caps that are near that component, but that at least did seem to slightly affect the waterfall inside digital mode apps.

    Anyway, I didn't see any obviously burned out components or leaking caps, but I also only looked at the top of the board and possibly didn't take a close enough look at the relays. I might be able to look at the relays tonight but looking under the board might be a more significant effort.

    (I'll create a new thread for my issue if it's a completely different one than what KD2QED's having.)
     
  3. W1VT

    W1VT Ham Member QRZ Page

    If the preamp attenuates the signal but nothing seems broken, it could be that the amplifier is oscillating. An RF voltmeter using a 1N34A or similar diode can detect the unusually large signal on the output. A nearby transmitter can also drive a preamp into blocking with similar attenuation of a desired signal. Again, the voltmeter can spot this.
     
  4. K6AQ

    K6AQ Ham Member QRZ Page

    Would that be the case even if you're not pegging the S meter on the transceiver with preamp off? I'll look into making a RF voltmeter in any case.

    Also, I took a look again at the top of the board and nothing seems burned out around the two relays. Might need to get the board out of the case to see the bottom.
     
  5. W1VT

    W1VT Ham Member QRZ Page

    It is quite possible for the oscillation to be out of band and filtered out by the radio.

    I don't expect it here, but a three terminal regulator may oscillate if not properly bypassed and prevent a preamp from working.
     
  6. K6AQ

    K6AQ Ham Member QRZ Page

    Good point. There are a couple of three terminal parts in the preamp section as well (though I'm pretty sure they're not regulators). I don't recall offhand what their part numbers are, however, but it might be worth checking those too.
     
  7. K6AQ

    K6AQ Ham Member QRZ Page

    As an update, I poked around a bit more with a multimeter and I'm fairly sure it's the preamp relay that's dead--or at least one of the legs of it, anyway. I still need to get to the bottom of the circuit board to confirm, however, but removing all of the visible screws from the PCB didn't seem to let me do that (I was only able to pry up one of the corners of the heatsink and didn't want to force anything). Is there a way to do so without also desoldering all of the connectors/switches/LEDs (e.g. did I miss a non-obvious screw somewhere)?
     

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