ALC high on the FT-450D

Discussion in 'Radio Circuits, Repair & Performance' started by N1QWI, Nov 19, 2018.

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  1. 2E0LPL

    2E0LPL Ham Member QRZ Page

    Ok that's reassuring.

    Even setting the RF power to 5w still gives the same high ALC in CW mode.

    I have spoken to Yaesu about it in the past...not sure they 100% understand what the radio is capable of...he suggested limiting RF power in CW mode as its hard on the radio...his opinion.

    Hope you can understand why I am so apprehensive.
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I do. If I'd blown a couple of sets of finals in a new rig I'd be apprehensive too. But stressing about high ALC metering in a constant carrier mode is barking up the wrong tree :)

    That said, slightly limiting CW power with the power control does reduce risk to the finals so that's a way to buy a bit of insurance, just don't worry about what the ALC meter is reading until you switch to SSB mode. It might help to switch the meter to read power output (PO) while transmitting CW so you don't get hung up on what the meaningless ALC meter is showing.
  3. 2E0LPL

    2E0LPL Ham Member QRZ Page

    Thats very true...its very distracting...I normally leave the display in the SWR mode.

    All good advice...appreciated.
  4. G3YRO

    G3YRO Ham Member QRZ Page

    This is the problem with rigs with Transistor PAs . . .

    Back in the day, with Valve PAs, ALC (automatic linearity control) was introduced to prevent overdriving the PA stage on SSB, ie to keep it within its linear range, otherwise there would be distortion, with resulting splatter.

    On CW (or FM) it's just a carrier, so ALC is unecessary, and would usually be disabled.

    However, on rigs with Transistor PAs they nearly always use the ALC circuit to control the power output level . . . even when you adjust an internal pot to set a lower power, that rarely reduces the drive signal - it just makes the ALC circuit to reduce the gain of the ALC stage.

    Roger G3YRO
  5. 2E0LPL

    2E0LPL Ham Member QRZ Page

    Hi are you suggesting I still need to be careful and look into the high ALC I have with this new rig.

    The one it replaced was the same...but my pals has an ALC that does not go above the max meter marker...its over 3 years old.

    Odd that Yaesu gave me a brand new rig when I complained I was concerned about the high ALC on the old one that had blown 2 sets of finals...both times repaired under warranty....maybe a QC issue?

    The old rig was still working fine when I returned it to Yaesu...but I did not risk an CW on it...
    They replaced it without question...I cannot fault them.

    But before I contact them again I need to know if I do indeed have a problem with this new rig...I don't want to be making a fuss over nothing if no problem exits.
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    Automatic Level Control ( Heathkit called it "Triple Automatic Level Control". Drake just called it AGC (Automatic Gain Control).
    Last edited: Jul 28, 2019
  7. G3YRO

    G3YRO Ham Member QRZ Page

    Sure, ALC in the context of a Tape Recorder is Automatic Level Control . . . (obviously a much more widely used acronym than for SSB Transmitters)

    But proper ALC in an SSB PA was designed to stop overdriving the PA, ie keeping within the Linear part of the curve . . . so it stood for Automatic Linearity Control. (such circuitry was never designed to maintain a certain Level - that's what a Speech Compressor or Processor is for)

    Unfortunately, some of the rig manuals when printing out the term ALC in words then used the wrong acronym. (ie the tape recorder one)

    Anyway . . . regarding the excessive ALC readings on the FT450 on CW, I wouldn't have thought it would be a problem . . . but I agree that it is rather odd !

    Roger G3YRO
    Last edited: Jul 28, 2019
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    Yep, it [is supposed to] prevent nonlinearity by controlling/limiting the level. Thus, MOST often, ALC stands for "Automatic Level Control". :)
  9. W2VW

    W2VW XML Subscriber QRZ Page

  10. G3YRO

    G3YRO Ham Member QRZ Page

    Yes but the PURPOSE is to prevent non-Linearity . . .

    The purpose of ALC (On Transmitters, not Tape Recorders) was never for Level control. As I said before, that was the job of a Speech Compressor/Limiter.

    You'll find that lots of acronyms end up misused in this way (ie the wrong words being put to the initial letters that were the original acronyms)

    A proper ALC meter on a rig starts to move when you're just starting to overdrive the PA, ie when you might be going into the non-linear part of the curve, so it produces a voltage to an earlier stage to reduce it's gain, and thereby reduce the drive. The more the meter is reading, the more "overdrive" voltage it is producing.

    Roger G3YRO
    Last edited: Jul 28, 2019

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