So you want buy an HF linear amp.

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K4ICL
Posts: 663
Joined: Wed Mar 05, 2003 1:07 pm
Location: Greenville, SC
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So you want buy an HF linear amp.

Post by K4ICL »

How will you know if one is any good?

Here is a suggested check list that will be suitable for most HF linear amplifier prospects.

WARNING--this thing can kill you. If you are not comfortable working with high voltage equipment, have and experienced person do the following steps for you.
  • · Unplug the amp from mains power.
    · Remove the top cover; all screws should be the same and none stripped.
    · Check for excessive dust, webs, etc.; if excessive, clean before continuing.
    · If the amp is equipped with a safety interlock circuit, check all interlock switches for proper position and not having been disabled.
    · Turn the load and tune knobs, they should track without slipping.
    · Turn the band-switch through all bands; it should “set” with a positive click and the contacts should align properly for each band.
    · Check the band switch for burns and arcing, melted solder and other physical defects.
    · Look at the plate choke; it should not have burn "rings" on it nor should part of it be discolored.
    · Check the parasitic chokes connected to the top of the finals; they should not be burned.
    · Check all boards; there should be no burned or loose components.
    · Check the cabling from the band switch to the input board for loose wires, damage, or overheating.
    · Check all visible wiring for incomplete or poor soldering; reject if found.
    · Check the receive/transmit relay to be sure it has not been modified or disturbed.
    · Check the fuses; they should not be larger than 30 amps on 120VAC mains, or 20 amps on 240VAC mains. If so, find out why.
    · Check for rust and corrosion; there should none or very little.
    · Check the electrolytic capacitor date codes on the finals and on all HV rectifier circuits, if possible. Very old electrolytes will need to be replaced.
    · Do a general inspection of the insides of the amp; question any apparent modifications or repairs found.
    · Replace the top cover, using all the previously removed screws; don't over-tighten the screws.
    · Check the meters; they should be on zero when the amp is off and should work properly when the amp is on.
    · Hook the amp to a suitable exciter and a resonant antenna or a suitable dummy load.
    · Connect an appropriate RF power meter between the amp and the antenna/dummy load.
    · Be sure there are proper fuses then plug the amp into proper mains power source.
    · Power up the amp and check HV; verify using operator’s manual.
    · Be sure the fan runs smoothly.
    · Check to sure, when turned on, the amp "cycles up" to normal "ready"; in some amps the process takes about a few minutes, verify with user manual.
    · Without any exciter RF drive, Key the amp with the relay control (short to ground) and check the no-signal idle plate current, it should around 100 mils or less but not zero and not negative.
    · Using the Load and Tune knob setting in the manual, load and test the amp for power output on 20 meters; the amp is capable of legal limit and should produce full rated power output.
    · Time and suitable antenna loads permitting, test power out for remaining bands. Power out should meet published specifications.
    · If you have access to an infrared heat-sensing device (temperature “gun” or similar) check the temperature of each final tube; widely varying tube envelope temperatures can indicate a gross mismatch of finals, the hottest tube is doing the most work.
    · Power the amp off with the Grid/Volts meter in the Volts position; the HV volts should bleed off within a minute or so.

    You probably should not buy any amp that does not pass the above checks. Especially, avoid any amp with poor soldering, poorly done wiring, undocumented modifications, burned components, broken or missing parts, damaged components or does not produce full power.
If you see smoke at any time during your testing, shut the amp off and run, not walk, away from the deal.

De

K4ICL
ku3x
Posts: 37
Joined: Sat Mar 24, 2012 5:43 pm

Post by ku3x »

Great post, all good information.
Barry, KU3X
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