Re: [TenTec] HERC II low power......ugh...

[Ken Brown <ken.d.brown@verizon.net>]

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


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