[TenTec] HERC II low power...part II

[Bill NY9H]

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
>
>
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