Ameritron Amplifier Problem

Discussion in 'Amateur Radio Amplifiers' started by K9JBX, May 9, 2020.

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  1. K9JBX

    K9JBX Ham Member QRZ Page

    A few days ago I purchased a used Ameritron AL-80B amplifer that was supposed to be working correctly.
    I'm having a problem with it so I thought I would ask for help from anyone who has worked on this type of amplifier.
    I connected it to a 300 watt dummy load (the biggest I have at the moment) and to my Icom 7300.
    Then I connected a cable between the 7300 and the amplifier to be able to key the amplifier
    I set the tune and load to Ameritron's suggested levels to start with and band switch to 20 meters.
    I set my 7300 to 20 meters and 25 watts output in rtty mode.
    While in standby mode on the amplifier, I keyed my 7300 and the output meter on the amplifier shows aproximately 25 watts.
    I go into operate mode on the amplifier and key my 7300 and the grid current shows 60ma and the plate shows 150ma however the power output meter shows nothing. While keyed, I adjust the tune and load slightly and it has no effect on the grid or plate readings on the meter so I'm asuming there is nor rf coming out of the tube.
    Also when I key up, the plate voltage drops slightly from 3000 volts.
    The problem is, I sold my older external wattmeter and I have a new one ordered so I don't have one to put between the amplifer and the dummy load at the moment to see if it's putting out any rf.
    The dummy load doesn't get warm at all but then again I didn't key the 7300 for more than about 20 seconds.
    This amplifier was built in 2001 and it looks like it has the original Amperex tube in it.
    I did notice that there was a slight amount of heat coming out of the side of the cabinet where the tube is but not much.
    One other thing.
    A couple of times when I would turn the amplifer on, I heard a slight sizzling sound coming from inside the rear left side of the amplifier. It was kinda like the sound of a rocker switch if you just press slighty on the switch just enough to break the contacts in the switch.
    I researched similar problems like this on the net and I did see a couple of answers that suggested the tube was bad somehow.
    Anyway, any suggestions on where to look would be helpfull.
    Thanks!
    73
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Remove the cover (unplug the amp first) and check the two relays up in the upper left corner near the rear panel.

    Sizzling when you first power on the amp may be caused by the step-start relay contacts not making solid connection as they should within just several milliseconds after flipping the power switch to ON.

    No output power and no change in plate current as you tune the PLATE control under drive (operate, drive power applied) conditions may indicate bad relay contacts on the 3-pole T/R (transmit-receive) relay.

    If the AL-80B internal wattmeter indicated 25W drive when you applied 25W in STBY, obviously the wattmeter circuit works. It's the same circuit, and the same meter, when the amp is in the OPR condition. So, I doubt you need an external wattmeter.

    Once the amp is working, you WILL need a bigger dummy load. Most 300W loads will overheat and be damaged within literally seconds when the AL-80B runs full output power.

    Carefully check the contacts on both those relays to assure the contacts properly align; try pressing on them (gently) to assure the spring-loaded contact arms can operate freely. Use a magnifier and strong light to inspect the contacts on those relays as closely as possible.

    If you can see relay contacts that look dirty or not "shiny," you can clean them by wiping a dollar bill through the contacts while pressing them closed (lightly) with a finger. If I'm feeling rich, I use a $100 bill.:p But seriously, a dollar bill is a good contact cleaner; don't ever use a burnishing tool, emory cloth, light sandpaper or anything abrasive -- the contacts are very thinly gold plated and anything abrasive will rub the gold right off them, damaging the relay forever.

    [If a relay is truly "bad," Ameritron sells new replacement relays for a very good price; last I checked, around $12.95 or something. Re-wiring a replacement relay is a bit of effort but can surely be done.]

    The problem you describe doesn't sound like a "bad" tube to me. After 19 years, it may easily be a "weak" tube, but would still produce power and let you tune up...until it's really dead.
     
    Last edited: May 9, 2020
    WA7PRC and K9JBX like this.
  3. W1VT

    W1VT Ham Member QRZ Page

    It may also be time to upgrade your antenna system. Most tuners and RG-58 work fine at 100W, but often start fail when you add the amplifier.

    A quick and easy solution that I used was to switch over to resonant half wave dipoles. Then I switched over to multiband antennas that could handle the power.

    Zak W1VT
     
    AC0OB likes this.
  4. K9JBX

    K9JBX Ham Member QRZ Page

    WB2WIK...
    I cleaned both relays.
    The step start relay was worse than the T/R relay.
    Now I have a nice clean solid click with no sizzling when I turn on the amp.

    I removed the spring on the T/R relay and lifted that section of the contact assembly out to clean them and get a better look at all of the contacts.
    They didn't seem to bad to me but a little dark material did come off when I cleaned them.
    They seem to be aligned just fine.
    I put the relay back together and checked continuity on all the contacts while pressing the relay manually.
    I also checked continuity form the rf output jack on the back of the amp to the point just past the two parrallel capacitors coming from the plate cap.
    When I press the relay manually I get continuity from these two points.
    I tried the amplifier again but still no output.
    This time I keyed the transmitter for 30 seconds and there way considerable more heat coming out of the side of the amplifier where the tube is.
    Have you ever seen a tube to draw grid and plate current and load the plate voltage down but not put out any rf?
    Thanks!
     
  5. W1QJ

    W1QJ Ham Member QRZ Page

    Make sure that the verniers are not slipping. You must look and see if the variable caps are full mesh when fully CCW. Make sure the caps are turning when you turn the dial and the when the dials are fully CW the caps are now full open unmeshed. If either cap is not turning properly yu will never hit the resonant spot and get any output. The fact that the HV sags down is a sign that the tube is drawing current but the tank is not resonant. There could be other issues but first check to see if those caps are turning properly with the dials.
     
    AG5CK, WA7PRC and K9JBX like this.
  6. K9JBX

    K9JBX Ham Member QRZ Page

    I checked the verniers and the capacitors.
    It seems like mine are exactly opposite of what you suggested.
    When the plate adjust venier is set to fully ccw (pointer at 9 o-clock or zero) the capacitor is fully open.
    When it's set fully cw (pointer at 3 o-clock or 10) the capacitor is fully meshed.
    It's the same with the load vernier and capacitor.
    Any suggestions?
     
  7. W1QJ

    W1QJ Ham Member QRZ Page

    It’s backwards, the presets in the manual are based on both caps being fully meshed T 9 o’clock. So your amp is fine. You haven’t found the right setting to resonate the tank.
     
    K9JBX likes this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I agree, I think.

    "Someone" re-set those capacitor shaft couplers. What Lou said is absolutely correct. I have an AL-80B (vintage 2000) and have worked on more of them, and when the pointers are at "0" the caps should be fully meshed, and at "10" they should be fully open.

    But if someone went to the trouble of doing that, my guess is they were working on something in the amp to fix it, and maybe didn't really fix it.:p

    Since amps are heavy and expensive to ship, I'd never buy one I didn't see working in person so I could just give it a comfy ride home in the back of my car; although buying from a pro dealer is different and I'd do that if there was a reason to. Pros who ship amps all the time know how to pack them -- many others don't.:(
     
    K9JBX likes this.
  9. K9JBX

    K9JBX Ham Member QRZ Page

    As per your suggestion, all I had to do was remove the knobs and move the red arrows to the opposite sides of where they were mounted and then turn both knobs back to zero and both capacitors are now fully meshed at zero.
    I set the plate and tune to the recommended settings and it's now putting out rf. The verniers weren't slipping as far as I could tell but the plate vernier was a little stiff ( I couldn't turn it without the knob being mounted) so I put a dab of wd-40 with a toothpick right where the shaft enters.
    It's now easy to turn and much smoother. I tuned it up but 400 watts is all it will do. I guess the 9 year old Amperex tube has seen better days. I have a new tube I'm going to install later this evening and see what happens.
    Thanks guys for all your help!
    73
     
  10. W1QJ

    W1QJ Ham Member QRZ Page

    How much drive did you put on and did you tune for maximum output with load and tune each time you added more drive?
     
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