I wasn't going to post an answer on towertalk, but I will. The amp
reflector is useless for good info, because it has such a poisonous
environment!
I designed the SB-1000, and two other Heath amps.
> Being as mentally challenged as I am, I have had to repair the
> bandswitch on my SB 1000 TWICE..both caused by not haveing sufficient
> loading as I tuned for max output.
Underloading a PA is the SINGLE most common cause of switch failures.
Second are arcs in the antenna systems, causing the amplifier to suddenly
"unload" when faced with a high SWR in a direction that causes grid current
and anode voltage to skyrocket. Peak anode voltage and tank voltage can be
several times the dc supply voltage when an amp is underloaded!
One VERY common cause of this is lightning discharge devices that fail or
falsely trigger under peaks. I've seen dozens of cases of this first hand!
I intensely dislike use of gas discharge tubes on the output lines of high
power PA's.
Another is exciters that put out sharp envelope peaks or spikes. My ICOM
706 whacks out almost 200 watts upon key closure (even if the drive is set
for 30 watts or less), overdriving the PA and forcing the grid current way
up. The effect of this is the same as unloading the PA. That's another
reason why you ALWAYS want to overcouple any PA to the load!
Also be sure the antenna relay is properly sequenced, so the antenna
contact closes just before the input contact when the relay is pulled in.
You can do that by tweaking the contact running to the tank, bending it up
towards the armature contact a tiny bit (use needle nose pliers to
carefully bend it).
> I called Ameritron Service Dept.(see
> any Ham magazine for phone no.) They sent me a new bandswitch. I removed
> contact sets from the new bandswitch by drilling out the rivets and
> replacing the burned contacts on the last wafer of the SB1000 switch.
You don't even need to do that. The entire wafer will slip right on the
shaft, and be properly indexed! Just be sure you don't flip it 180 degrees,
it fits two ways and be sure to keep the back to the rear.
Also be sure you add the anti-corona ring Heath left off. They also
loosened the specs up on the transformer, so the voltage regulation is a
bit worse. All of this aggravates any loading or driver spike problem you
might have.
73 Tom
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