Finally! Amp Troubles Resolved

02 May

I may be early with my enthusiasm, but I appear to have solved the riddle of my amplifiers intermittent problems. This one took a while and as usual I made the problem too difficult. The amps output tended to sound like a relay was dropping out at the remote end of the radio circuit. With an aging piece of hardware like this 80’s vintage QSK amp (Amp Supply LK 550 ZC), anything can and usually does go wrong. So I laid in a supply of hard to find sealed relays and girded my loins for a lengthy battle. Part and parcel to fixing an issue like this is having it fail. So I removed the top and began hammering away with a scope probe dangling near the open wire feed lines. After about fifteen minutes of testing on CW I began to see a step function reduction in output power as reflected in my scope trace. It was as though the amp had become unloaded somehow and grid current rose in step with the output power loss. Grid current rising was the only sure fire way I could tell it was failing while the cabinet was buttoned up. As discussed, the output dropped in sync with the grid current rise. After tracing signals all over the QSK board, bias circuit etc. I began to see the problem come and go with probing (Oh No)… I was able to isolate the problem area to a vacuum relay that I had replaced almost 18 months ago. With lots of light and a magnifying glass I was able to see what looked like a cold solder joint on the input side of the amps output vacuum relay. As I probed the area I could make the problem come and go so I re-flowed the connections as best I could with a smallish variable temp soldering station and since have been unable to see or hear any symptoms. Also it turns out to be a problem of my own making! Clearly I need a heftier soldering pencil. Next time I hit the tool store Ill add a larger Unger Pencil to the battery of tools. Lessons learned. It should have been apparent to me to revisit the last things I touched FIRST as this is normally where things go wrong. I did get almost 13 months of service out of those solder joints before troubles showed their nasty faces though. The joint went bistable after a while transmitting. I can only suppose that due to (Voltage, Current or Heat) caused the joint to go open and thus unload the amps output and thus change grid current upwards. At this juncture its my theory and Im sticking to it unless or until the problem reoccurs!

Thanks for reading my Blog. Best, Chas W5PG


Posted by on May 2, 2011 in Radio


2 responses to “Finally! Amp Troubles Resolved

  1. KF6U

    May 2, 2011 at 3:54 pm

    You are indeed and ACE Technician there OM. Job Well Done!
    Last fall I repaired an LK-450 by Amp Supply and found numerous solder joints that
    had become intermittent as the solder aged and started to decay. The ones that seemed
    to be the worse were the larger wire sizes that require a fair amount of heat to
    make a solid connection. I agree, a high wattage soldering iron is essential when
    working on QRO equipment. 73, -Gary

    • w5pg

      May 2, 2011 at 4:19 pm

      Thanks Gary….yes it was interesting. Time for an Unger 80 W Element! Best, Chas


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