ken,
tnx for the reply..
the fault lite is constantly lit , but verry dim,,, i thought it was not on
, and therefore a clue to something else...'
i did build the 100 watt pa for the elecraft... so i understand the
concepts of the output sections and hybrid couplers used to gang together
all four output sections in the HercII...
i wonder if i will harm the herc / hybrids and other outputs if i run low
enough output power to test all four pa sections.....
i figure first verify dc power to all four sections;
then apply very little rf to see which one of the two pairs is not putting
out ...
off to the reflector archives
bill
At 02:13 AM 3/10/2004, you wrote:
Hi Bill,
I have never serviced an HF transistor PA. However I have repaired many
General Electric Mastr II UHF PAs and a few VHF ones and some other
miscellaneous VHF and UHF PAs too. I would say that more than nine out of
ten times the problem is failure in the base and/or collector connections
to the printed circuit board. Typically when you inspect it after it has
failed it is clear which connections are bad just by looking. The solder
will appear similar to a cold solder joint, and there is often carbon or
other signs of over heating or arcing. The initial cause of the failure is
probably poor soldering between the base or collector tab of the
transistor and the printed circuit board. A nice looking solder flow on
top of the tab and to the printed circuit trace can disquise a poor
connection underneath the tab. RF and DC currents flowing through a longer
than necessary solder path will eventually melt the solder. Then when it
recools it may become a cold solder joint, with higher resistance. Then
even more heat is generated when the current flows. Often this will damage
the transistor. Sometimes you get lucky and the resistance will increase
to the point where it limits the current sufficiently that the transistors
don't get damaged by overheating.
The way to fix this problem is to use a solder sucker and solder wick to
remove all the solder from both the transistor tabs (all of them) and the
PCB contact areas. Remove the transistors. Clean all of the carbon and
burned flux and other residues from the transistors and the PC board
contact areas. Straighten and flatten the transistor tabs. When everthing
is clean, check that when the transistors are set into place, flush
against the heatsink, the connecting tabs are also flush against the
printed circuit board contact areas. Then remove the transistors again and
tin both sides of their contact tabs. Tin the PCB contact areas too. The
tinning of the tabs and PCB should be thin, so that when that transistors
are mounted, the solder inbetween the PCB and the tab does not raise the
transistor off of the heatsink. Once all of that is accomplished, mount
the transistors to the heatsink with the appropriate screws or nuts. Then
solder the tabs to the PCB. It is probably best to start with the emitter
tabs first and then do the collector and base. Be sure to get a good
connection directly between the tabs and the PCB contact area. A large
puddle of solder on top of the tab is no substitute for a good connection
underneath. All of this requires a pretty good sized soldering iron. Not
the kind you use for most PCB work. You want a flat tip that is 3/16 to
1/4 inch wide (close to the width of the transistor tabs), not 1/8 inch or
smaller, and not a round pointed tip.
You might get lucky and be able to get back up to full output using the
original transistors. I have restored many low output PAs to full output
this way, without putting in new transistors. Labor and shop overhead
costs dictate that for commercial or public safety radio use, it is
generally best to just replace the whole set of transistors.
Have fun,
Ken N6KB
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|