[TowerTalk] YCCC 9 circle preamp failure test

[jimlux]

On 9/24/19 1:14 PM, Wes wrote:
> Why all of the complications?  The preamps are unity gain. 

Aha.. I was hunting around for a circuit description. Yeah, it looks 
there's a noninverting buffer followed by a 1:1 transformer.

So it's more of an impedance buffer.  What Z does it present to the 
element?

(I do a lot of work with HF receivers that use short (2-3 meter) 
antennas feeding high Z buffers, so they're more "E-field" probes)







  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
>>> _______________________________________________ 
> 
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