On Wed, 11 Mar 98 09:50:41 -0800 Rich Measures <measures@vc.net> writes:
>>I was looking through the Amps archives and I noticed some comments
>about
>>damage to the 160-meter padding capacitor bandswitch contact due to
>Heath not
>>providing a
>>corona washer.
>Corona at 2800pV on a 5000pV capable bandswitch?
>> Sure enough, my SB-1000 has a burned bandswitch contact in
>>this place. I'd put off fixing it and attributed it to the former
>owner's
>>hot-switching...but maybe that isn't what happened after all.
>The SB-1000 is essentially an AL-80, and whose replacement parts
>happily
>fit.
>>Could someone please educate me with regard to this? What causes
>this
>>problem, and what kind of washer needs to be put where to prevent it?
>
>Replacing the
>>contact is going to be annoying (I really don't want to replace the
>whole
>switch) and
>>I only want to do it once!
>Open160m tune padder switch contacts tend to arc during intermittent
>vhf
>regeneration. at roughly 155MHz. If you couple a dipmeter to either
>side of the hv dc blocking cap. you just might see a sharp dip near
>this
>freq. You can find some photos of parasite arced bandswitches at:
>www.vcnet.com/measures
>- It is my opinion, based on conversations with the owners, that none
>of
>these bandswitches were ruined by "corona", "operator error, bad
>antennas, or by cheap coax". {Charles Thomas Rauch, Jun., W8JI} .
>The
>likely fix is to replace the burned contact, or to replace the wafer
>if
>the ceramic is blackened with metal vapour condensation, and to
>decrease
>the vhf Q of said vhf-resonant circuit.
>- later, Vic
>
>cheers
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K
The AL-80A is also known and prone to bandswitch damage, 160M is not
unique; it can happen on other bands also.
Ameritron addenda and tech support stress that this amp (and the Heath
almost-clone) must be heavily loaded and NEVER use the load control to
reduce power.
VHF Parasitics...highly unlikely. The more probable culprit is a
secondary reasonance in the output network caused by stray and actual L
and C. This can be confirmed with a wavemeter...dipmeter in diode mode is
fine. At various amp settings, tune the wavemeter and look for other
peaks...it is likely that you will see sudden peaks. Some may be
associated with harmonics and will be greatly enhanced at certain
settings.
A high level harmonic without a low impedence path will raise havoc in
the tank circuit and have absolutely nothing to do with parasitics. This
is why it is impossible to duplicate the problem into a 50 Ohm wideband
dummy load.
An actual non-harmonic related resonance is also possible....but it is
strictly confined in the tank circuit.
73 Carl KM1H
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