[AMPS] some basic questions

Tom Rauch (W8JI) w8jitom@worldnet.att.net
Fri, 07 Mar 1997 23:28:31 -0500

w1vxv@juno.com wrote:

> fail badly!)  Last spring I purchased a new appearing LZ500ZC and drove
> it on RTTY and SSB with a TS440S.   Antenna tuner was a KW tuner by MFJ.
>   All went fine for about 5 months with good reports and results.  Then
> arcing occurred in the tuner so I stopped using the amp.  Investigation
> shows some pitting on the variable capacitor, which I know that I can
> smooth out.

You have to micro polish the air variable plates. Normally that is done
by tumbling them in ground Walnut shells. You will never polish the
plates well enough by hand to restore full breakdown voltage.

I assume you are talikg about the tuner's plates??   
> I suspect parasitics were the cause and I have purchased a kit from Rick
> Measures which I have yet to install.  My questions are these:

Put the kit away in a drawer, and forget about it. There is no possible
way a parasitic could have damaged your tuner. The tuning capacitor in
you amplifier is a virtual short at VHF, and the inductor in the tank a
very high impedance. The loading capacitor is another virtual short, and
the voltage breakdown of the capcitor in the 989 tuner is about 4000
volts peak.

The most damage a parasitic ever causes is sometimes a burnt resistor in
the suppressor itself. Anything else is generally due to other causes.
> 1)      Why did all go well for a while and then go to he#$  in a
> hand-basket?

Becasue you probably set the tuner with too little capciatnce. That
raises the operating Q, and the peak voltage in the tuner. ALWAYS set
the tuner for the maximum capaciatnce and minimum inductance that
permits a match.
> 2)      Are parasitics the probable cause, and will the Measures kit
> solve it permanently?

No. The Measure's kit lowers the HF Q of your tank circuit, and it
barely changes VHF performance or Q. The most noticable change is
smoother tuning of the tank, caused by the lowering of HF Q. I think the
LK-500Z's Q is already low enougfh, I wouldn't reduce it more.

> 3)      What else should I be doing?  I am NOT using a scope during
> tune-up; some comments indicate that is strongly recommended.

Tune the capacitors for maximum C that permits a match in the tuner.
> 4)      Could the TS440S be overdriving the LZ500ZC?

On peaks it could, be sure you load the amp heavily enough.

73 Tom

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