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DX400 Bi-Lateral Issues

Discussion in 'Amplifiers' started by skiman1, Jul 6, 2019.

  1. skiman1

    skiman1 Member

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    Hey Gang, Just picked up an DX400 off eBay, its an old Bi-Lateral with 4 - SRF2072 trannies and has 3 symptoms.

    The good, the power output seems fine, driving it with 1.5w's with AM button in out's 100w carrier and 400'ish with swing, just like it should. That's where the good ends.

    - When amp is on and tx'ign its normal power, the SWR is off the charts, like a 5, other amps I throw in line are fine, 1.2 - 1.5. I have the meter immediately following the amp, then into a Bencher LP filter. Meter is an MFJ-870, never any issues with the meter.

    - The preamp is not working, just goes deaf, doesn't affect the Xmit power.



    - When the SSB/delay button is pushed, its as if the output transistors/amp are taken out of line and I get stock power. When tx'ing no delay can be heard kicking the relay in and out, again like the amp is off.

    Honestly I could care less about the Preamp working, I've never used them anyway on every amp I've ever had, but obviously all the either issues are the real issues, but sure its connected.

    I'm sure just a couple of simple components, but not a super troubleshooter kinda guy. Any help would be welcome, thanks in advance!
     

  2. dave457

    dave457 Sr. Member

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    Try putting lpf between your radio and amp
     
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  3. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Put the SWR meter between the radio and the amp and see what the amplifier input tuning is like. The amplifier input might need some work.
     
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  4. nomadradio

    nomadradio Analog Retentive

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    SRF2072, the high-gain version of the MRF455, if memory serves. Prone to be unstable and oscillate. Amplifier builders liked the high wattmeter readings this would deliver, along with a 'soup' of other unwanted frequencies.

    Don't remember seeing this amplifier, but I'll guess that it lacks negative feedback on the transistors.

    Negative feedback generally does two things. Reduces the harmonic and spurious-frequency output from the transistors, and lowers the input impedance.

    Some pics with decent focus and lighting could be helpful.

    Best fix for the preamp is to jump around it, take it out of the circuit.

    If you're REALLY lucky, the preamp relay will have a clear-plastic top cover that can be snapped off the relay. Then remove the moving contacts and armature.

    A single strand from a piece of stranded hookup wire works best for this.

    [​IMG]

    This will bypass it completely and prevent problems with the "standby side" circuit that passes through the preamp relay.

    Easier than removing the circuit board to do this.

    73
     
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  5. skiman1

    skiman1 Member

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    See gut shots below, I don't even see that second relay, perhaps already modded?
     

    Attached Files:

  6. 999

    999 Well-Known Member

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    There was never a second relay in TS amps until you get to the sweet sixteen and larger amps.
     
  7. nomadradio

    nomadradio Analog Retentive

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    Shoulda said "Texas Star", and I would have put my foot back on the floor before it could reach my mouth.

    The SSB switch on this model does two things. First, puts a capacitor onto the relay to keep it from chattering. Second, it cuts the drive level about in half, more or less.

    That's the only way to make this kind of amplifier sound right on sideband. You can drive it almost twice as hard on AM before the audio quality will suffer. The attenuator side of the SSB switch is to hold down the drive level and improve the modulation quality.

    (edit) Just re-read your original post. Did you mean that the relay does not key up at all with the SSB button pushed in? That can't be right, clearly.

    73
     
  8. skiman1

    skiman1 Member

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    I know its not right, its why I'm here ha ha! :)

    With delay engaged, no relay click, no "amp" output, just stock power from the radio. like the amp is bypassed or off. Nothing in here smells burnt, all looks clean too
     
    #8 skiman1, Jul 8, 2019
    Last edited: Jul 8, 2019
  9. nomadradio

    nomadradio Analog Retentive

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    Looking at the schematic, the only explanation I can figure would be for C4 to fail as a short circuit. It's the 1000 uf cap that slows down the relay for SSB. If it's shorted, that should prevent it from keying.

    And eventually blow up the keying transistor Q1.

    73
     
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