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Alpha 374A TX lights flicker when transmitting?

Discussion in 'Amateur Radio Amplifiers' started by N8ZI, Nov 19, 2019.

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  1. N8ZI

    N8ZI Ham Member QRZ Page

    Hello to all,
    Finally got the Alpha 374A in the shack and slowly becoming accustomed to it's use.
    Noticed today that when I increase exciter drive, the amplifiers red TX lights begin to flicker at around 500w output. When i lower exciter power the lights stop flickering @ 400-450w yet the flickering returns as power is increased again.
    It was in CW mode running 1.6KV HV @ idle and dropping to about 1500-1550KV when transmitting.
    Plate current was around 200ma and grid was around 25ma, both well within specs.
    I was wondering if it was starving for input current but I do not notice the HV moving while the lights flicker.
    Any suggestions or known issues?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    When they flicker, do the lights get brighter or dimmer when you "key down" (transmit) at the higher power?
    N8ZI likes this.
  3. N8ZI

    N8ZI Ham Member QRZ Page

    I was transmitting a steady carrier and increasing the drive level when the lights started flickering from normal brightness to slightly dimmer and back to normal. That makes me think there is a voltage loss issue. Possibly a component or connection on that PCB over the P/S capacitors.
    I believe the meter lamps operate on 28v? With only one TX and one RX light behind each meter the single bulbs do not do a very good job illuminating the meters in a brightly sunlit room.
    Last edited: Nov 19, 2019
  4. VE2GCE

    VE2GCE Ham Member QRZ Page

    Is this just a lamp issue ? I assume RF output is stable and not flickering.

    Check the following (very carefully, of course) :
    1) Is the 28 volt supply flickering during TX ?
    2) Check the base of the lamp driver transistor (Q103) ? Is it flickering ?
    3) Check the relay voltage on both sides of the K6 relay contacts which feeds the Q103 base.
    4) Also check the voltage on K6 coil. And then follow the troubleshooting chain to Q1 and related components, if the problem is here.
    N8ZI likes this.
  5. N8ZI

    N8ZI Ham Member QRZ Page

    Quite strange...... I have checked it on other bands and this is what I discovered.

    Something is so messed up that as I increase drive power, the red meter lights flicker then turn green. It is still putting out power as the lights change color from red to green. No fluctuations in power output or reflected, grid or plate readings.
    It was just producing 1300w in SSB position with 375ma plate and 55ma grid. That was on 40m in bandpass no tune position.
    Thinking the bandpass was adjusted favoring SSB or CW, I checked it in both frequency segments yet still the same lamp issue.
    I remember the manual says stay below 700ma plate and 100ma grid when tuning in CW. It says normal grid current is 40-90 but 100ma is acceptable for CW. In SSB adjust drive until grid swings no more than 50ma on voice peaks. So that is within specs and under control.
    I have three power/SWR meters inline. The ones built into the amp, a Daiwa CN-901HP and the meters in my Heathkit SA-2060 tuner and they all agree with each other.
  6. AI3V

    AI3V Ham Member QRZ Page

    Your dc input power (1550vdc @ 200 ma) gives you a input power of 310 watts.

    This is inconsistent with your claimed output power of 400-500 watts

    You need to read up about "dc input vs rf output of a linear amp" and get back to us with some realistic numbers.

  7. N8ZI

    N8ZI Ham Member QRZ Page

    After digging into the amp, I found out it has a mod on the plate meter. The meter readings are now 1.5x more than indicated.
    That puts my posted readings very close to the power output I specified. Perhaps you should educate yourself on the mods for the 76 series of Alpha Amplifiers.
    Happy Thanksgiving !!
  8. N8ZI

    N8ZI Ham Member QRZ Page

    Current update............
    After digging into the amp, I found out it has a mod on the plate meter. The meter readings are now 1.5x more than indicated.
    That puts my posted readings very close to the power output I specified.
    Also a few poor dull solder points were discovered on the PS/meter board and a pesky spider making a house between the PS filter caps. Removed the old solder and applied new, spider got the Hoover Wind Tunnel eviction, fired it up and the issue is resolved.
    Stayed home for Turkey day and worked DX with the repaired Alpha 374A.
    To those who offered good advice, thank you very much.
    Happy Thanksgiving to all !!

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