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ALS-500M tripping

Discussion in 'Mobile Radio Systems' started by K5VV, Oct 20, 2015.

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  1. K5VV

    K5VV Ham Member QRZ Page

    I have run the ALS-500M for over a year now. It generally does a fine job, driven with 30W and an SWR around 1.5:1. Somtimes, it trips...I can hear the relay open...but then it will close again a few seconds later even without unkeying the mic. Sometimes it's a few seconds, sometimes it's five seconds. But it always keys up again without having to cycle the power OFF and back ON. It doesn't do it all the's very intermittent.

    Per the manual, the load fault kicks in when the RF frequency does not match the band-switch, or when the reflected power exceeds 70 watts. The load fault is only reset by cycling the power OFF and back ON.

    The thermal fault kicks in when the temperature exceeds a certain level, and automatically resets when the temperature is back to safe levels. seems to me that it must be the thermal fault kicking in. The amp is mounted to the floor of the truck (no space in between), but there is 12" of open space on four sides and above the amp. I do hear the fan kick in and it doesn't run all of the time. The amp chassis is barely warm to the touch, and I don't feel the fan moving excessively hot air. The A/C is generally on so the ambient temperature is around 70 degF.

    Any ideas? Open the amp up and make sure there's no dust and debris? RF on the keying line (I added a few turns on mix 31)? Bad temperature sensor? 30W is overdriving it (when I first installed the amp, I measured output power with 10-20-30-40-50W in and 30W is in the linear regime)?
  2. K0BG

    K0BG Ham Member QRZ Page

    I'd almost bet it isn't the amp! Without knowing ANYTHING about the installation, my guess it is RF getting into the amp via common mode. Do you have the lines to the antenna (coax and/or control lines) choked?
  3. K5VV

    K5VV Ham Member QRZ Page

    No control lines to the antenna. Homebrew center-loaded with cap hat and matching coil at base. Not a screwdriver.

    The antenna is fed with RG-58, and I've got 5 or 6 turns of RG-58 through two mix 31 ferrites about two feet from the antenna feedpoint. Solid connection between the coax shield and truck bed at the antenna feedpoint. Bed is bonded to frame and truck body; bonding between body and hood, body and engine block, body and exhaust, bed and exhaust.

    I haven't had any other issues with common mode or RFI, but I guess I can't count it out.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Most thermal cutouts will not reset in seconds, because of differential temperature needed to keep them from short cycling.

    I think it would be safe to rule out thermal fault.

    Good Luck.
  5. K0BG

    K0BG Ham Member QRZ Page

    Having been down this road more than once, it is time to check all of the connections. Including, but not limited to, the coax cable.
  6. KK4YDR

    KK4YDR Ham Member QRZ Page

    Try one of these of having dual 6 turn bead assemblies 2 feet form the feed point. Place this inches from the feedpoint. Also get an analyzer and test to see what your reactance is (X) which should ideally be as close to 0 ohms as possible. Also test your impedance (R) and try and tune your whip for as close to 50 ohms.

    I have a Larsen NMO 27 that I use for monitoring CB on road trips etc... also for 10 meters from time to time. I will go and measure it and see what numbers my analyzer gives me just for conversational reasons. I believe I tuned X very near 0 Ohms and R was about 51 ohms or something like that.
    Last edited: Oct 22, 2015
  7. K0BG

    K0BG Ham Member QRZ Page

    If you knew what was inside the RadioWavz choke, you wouldn't pay the price! Why? Because you can duplicate it for less than $10!
  8. KK4YDR

    KK4YDR Ham Member QRZ Page

    Yeah but I posted that assuming not everyone is capable of or wants to make a choke like that.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would think it is Not a choke problem if it worked properly for a year.

    It is most likely something loose, Like a connection. Could even be a bad solder connection in the amp. :eek:
    Last edited: Oct 22, 2015
  10. KK4YDR

    KK4YDR Ham Member QRZ Page

    Time to get MFJ on the phone :p

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