[TowerTalk] YCCC 9 circle preamp failure test

[Wes]

Why all of the complications?  The preamps are unity gain.  Doesn't the output 
voltage tell you something about the input voltage? If they saturate during a 
measurement, turn the power down until they don't and then calculate the worst case.

Wes  N7WS

On 9/24/2019 12:07 PM, jimlux wrote:
> On 9/24/19 11:09 AM, VE6WZ_Steve wrote:
>> Yesterday I decided to measure the actual voltage at base of my short HI-Z 
>> verticals while I transmitted on the TX array.
>>
>> Since I first posted about the coupling capacitor failures in these units, I 
>> have received private emails form 4 others with the same problem.
>>
>> My question is what has caused these failures? Is it a lightning event,  is 
>> it high-potential wind static on the vertical elements or is it from TX RF 
>> overload.
>>
>> TX overload seemed possible at my location because my array is less that 100’ 
>> from the TX antenna.  I built a diode detector probe and in the field 
>> measured the actual voltage when TX.  The results indicate at most I see 50 v 
>> at the pre-amp.  The coupling caps are 63v rated units.  This would indicate 
>> that RF overload is not the cause, but I would really like thoughts and 
>> opinions from others.
>
> This is the 0.1 uF input capacitor? What kind of capacitor is it (there's 
> hundreds of 0.1 uF, 63V caps out there)
>
> I'm not sure that your measurement probe (shunt diode, 0.1 DC block, 100k 
> load) is telling you the right values -> since it's just wires hanging out in 
> space, could there be parasitic C (or L) that's fouling things up?
>
>
> If I were going to measure the voltage at the antennas, I'd put a known 
> voltage into the preamp (say, 1mV) and measure the output voltage, so I know 
> the gain of the amp.  Then I'd fire up the TX and look at the voltage you're 
> seeing at the output of the preamp.
>
> FWIW, 50V is huge - I'd be using 100V capacitors there, but I'd worry more 
> about the opamp frying. Although, maybe the current is limited by the series 
> 470 ohms and the opamp clamps it to the supply rails.
>
>
>
>
>
>
>> I have redesigned the amps to include a relay to short out the antenna when 
>> not powered up, so I feel I am protected from any cause.
>>
>> Here is a link to a YouTube video that shows the test set-up and the actual 
>> measurements I made in the field.  I also talk about using gas discharge 
>> tubes as a remedy.
>>
>> https://youtu.be/TmM_YnLob68 <https://youtu.be/TmM_YnLob68>
>>
>> 73, de steve ve6wz
>> _______________________________________________