al 800 multimeter problem.

Discussion in 'Amateur Radio Amplifiers' started by W4RS, Nov 29, 2018.

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  1. W4RS

    W4RS Ham Member QRZ Page

    well my 20 year old al-800 has a slight problem. when you turn it on in standby, neither lights come on. now the grid/plate is suppose to be off till the 3 min timer runs its course. but the multimeter, the frd/ref/hv is dark. ok, the hv is correct at 2700. it tunes great on 40 the grid is .30 for 700 watts cw. the multimeter is showing 2500v on hv, 700 watts fwd, 60 watts ref. the multimeter is still no light.
    then when I send the fwd is jumping up to full scale, the ref is jumping to full scale. but on my bird panel its still 700 watts fwd 60 watts ref. when I say full scale they meet at the top.hi hi . yet the grid and the plate are still all ok? the best I can seem to think of is the r-117? or the light bulb are causing this? now I understand the light is now led? or so its listed on ameritron sight, but no explitation of how to replace it and no expilation for the meter acting this way.
    any ideas? and as many of you know I am in the Philippines so it not a run down and get a new bulb kinda place. hi hi
    thanks to all, love this forum,
    jim w4rs, du3jh. du3r, dx3r, dx2r
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    unlike many other amp metering circuits that use "actual" driving voltages and shunt resistors to calibrate meter readings, this amp uses some amplification and comparators to give meter readings. By the nature of doing this we introduce transistors and op-amps that could go bad and give full scale readings if they short. If you look at the meter board schematic for this amp you can clearly see how the meters, in some positions, are driven by the output of the op amp. Meters usually respond to only millivolts of voltage or micro or milliamps of current. If the op amp shorts out internally it is possible to put Vcc at the output of the op amp and thus cause the meter to pin hard at full scale. Now based upon the fact that the meter lights do not work, it could be that a shorted IC or transistor could be pulling the 12 volts down on the meter board.
     
  3. W4RS

    W4RS Ham Member QRZ Page

    thanks lou,
    I guess I see now why ameriton sell the meter as a whole. not any user parts on it. it appears it has a metering board that drive the meters so the problem may be there?
     
    Last edited: Dec 2, 2018

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