AL-80B taking out finals....

Discussion in 'Amateur Radio Amplifiers' started by AE1P, Jul 3, 2020.

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Looking at the T/R relay board schematic there should be no measured resistance at all if there is nothing connected to the output SO-239 AND the transmitter is not keyed (receive condition). In the receive mode, the input is connected straight to the output connector.


    Pheel.

     
    Last edited: Jul 4, 2020
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    If you are making a resistance check "across" the Parasitic Choke it should read almost zero resistance because there is a very short coil shunting the resistor.

    Pheel
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    Nope, he is getting a proper reading most likely because if you are familiar with this amplifier and know, since we established it’s a newer model, it doesn’t have the typical resistor shunted “directly” by an inductor. The resistor has a series mica capacitor in series on each side of it to the inductor.
     
  4. W1QJ

    W1QJ Ham Member QRZ Page

    If by chance the mica cap opened or severely changed value that would cause a major disruption in the parasitic suppressor and that ain’t good.
     
  5. W1QJ

    W1QJ Ham Member QRZ Page

    A logical approach to this problem must be taken. I once had an amplifier which had a modification of a dual relay antenna TR switch system much like your newer model AL80b has. It was an aftermarket PC board with two relays on it. Ive seen that set up before work perfectly on other amps. This amp blew out the finals in a customers radio. I got the amp to repair it. I checked it over and the relay board was installed properly. Couldn’t see anything wrong. I tested the amp with my radio and when I attempted to load the amp as usual, my radio was instantly destroyed too. Finals shot and lots of burned traces in the radio. I had worked on dozens of these amps before and never a problem. The only odd thing about the amp was that relay board system. I removed it and replaced it with a regular stock relay and all was well. I never did figure out what was up with that relay board. I doubt you have an anode to cathode short which could be deviating but still you have blocking caps that should protect the radio. I’m leaning away from that and tests prove that’s unlikely. Only RF can get back through the blocking caps. That’s why I’m thinking major full blown oscillation.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Mine is a 2000 model (will be 20 years old later this year) but does have electronic bias.

    That's so sensitive, though, that just a few milliwatts drive will key it up and create idling current even if the output is basically nothing. To try a stability test when the tube isn't in cutoff, it should be possible to activate the bias circuit without actually connecting a transmitter to the input port. I'll take a look at that when I have a chance; probably others have already done this, as I have heard a few objections over the years re the factory circuit potentially clipping off the first milliseconds of the signal -- can't say I've ever noticed that, even at 40 wpm on CW. The output follows the transmitter waveform just fine, even on fast dits.

    The "time delay" for that circuit to activate may vary with component values or something. Seems very fast.
     
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I have the latest MFJ manual/schematics upload and it shows a conventional LR parasitic choke with two filter caps (C1, C2) in parallel on the parasitic side (High Side of the RFC) and on the B+ side cap C3 on the other side of the RFC. If you have the latest manual with schematics then please share since we are all interested in this problem.

    So you are saying the Parasitic Suppressor circuit looks like what I have here?

    Since in the receive mode the SO-239 connectors are simply an "In-Out" straight-through arrangement, the measured value cannot be correct according to the latest MFJ schematics.

    Pheel
     

    Attached Files:

    Last edited: Jul 4, 2020
  8. W1QJ

    W1QJ Ham Member QRZ Page

    Ameritron hasn’t updated their schematics in a long time so anything you probably looked up doesn’t depict what the suppressor actually looks like. I can’t open your file but suffice to say schematically it is a capacitor in series with a resistor in series with a capacitor shunted by the coil.
     
  9. AE1P

    AE1P XML Subscriber QRZ Page

    Here is pic from said AL-80B: Choke_IMG_2330.jpg
     
  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Okay, I have redrawn the schematic according to your latest description and it is a simple .PDF file but I don't see how any DC gets to the plate of the tube.

    I just saw Niels pic and without looking at the traces on back of the PCB it is difficult to determine exactly the configuration of the PSup.

    Pheel
     

    Attached Files:

    Last edited: Jul 4, 2020

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