Hi Lee…
You are absolutely right, a failed coupling cap should not change the basic amp
performance. All that we will get is some of the op-amp voltage leaking onto
the element.
However, my initial indication that something was wrong was in the east
direction I had what sounded like BC inter-mod rectification products. Most of
the noise, crud and carriers were centred around 1820 kHz. On the waterfall it
was really obvious. Its that typical BC junk that shows up with a poor
connection somewhere. I was sure it was a poor connection somewhere on the
vertical or at the connections. After I found nothing, I switched the amp out
with a spare I had and the noise junk was gone.
I was sure it was a bad Op-Amp. I switched it out, but also noticed that the
coupling cap was leaky, so I switched that out too. Amp was restored.
A few weeks later, the same noise showed up again, but this time in all
directions. In this case I realized it was the central vertical. I tested the
cap, and it was leaky so I replaced it, and the amp tested good with the
original op-amp. Later on, I tested all the pre-amps and found a third bad
cap. However, in this case the pre-amp was still performing ok with no noise,
so as you suggest, not in all cases do the failed coupling caps cause a problem.
The “bad” caps do not fail completely short, but are leaky…around 20k-50k ohms.
Is it possible that the dielectric is compromised enough that we are getting
some diode rectification action that is generating the crap?
Either way, whatever the failure cause, it could be putting stress on the
op-amps which could cause them to fail eventually too.
Steve
> On Sep 24, 2019, at 1:08 PM, Lee STRAHAN <k7tjr@msn.com> wrote:
>
> Hi Guys,
> Assuming your verticals have a very good insulator there is NOTHING to stop
> that input capacitor from charging to very high like kilovolt static voltage
> levels. What I don’t understand is why leakage in that capacitor would cause
> any trouble or differences if it were not even there. All that would happen
> is the vertical would assume about 5 volts DC on the elements. This problem
> rearing its head would indicate the vertical insulators or something in that
> path must be compromised as well. Or is the 8055 possibly going in to
> oscillation with a leaky connection to the element.
> Something does not add up here.??? What is the clue that tells you the
> capacitor has failed. Lack of performance/directivity or noise. Noise might
> indicate oscillation.
> Lee K7TJR
> Hi-Z Antennas
>
>
>
> -----Original Message-----
> From: Topband <topband-bounces@contesting.com> On Behalf Of VE6WZ_Steve
> Sent: Tuesday, September 24, 2019 11:09 AM
> To: topband <topband@contesting.com>
> Subject: Topband: YCCC 9 circle preamp failure test
>
> 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.
> 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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