Hi Carlos,
Thanks for the interesting information. I'm curious about the 160 and 80
meter filters. If their bandpasses were not at the right frequencies, where
were they before you changed the capacitor values? And what was the VSWR at
100 Ohms input impedance and 50 Ohms output at the frequencies they
previously worked best at?
I have a Hercules II and I like it a lot. Mine was not performing up to par
and I replaced all of the PA transistors. That brought the power back up to
specifications. Now after reading your post I wonder if it would work even
better and longer if I made the filter modifications you have made.
By the way, there are some very small PCB traces on the board that has the
band switch on it, and one of those traces needed to be repaired on mine. I
don't recall which band would not work because of that open PCB trace.
Thanks, Ken N6KB
On Sat, May 8, 2021 at 10:59 PM Carlos PECO BERROCAL <carlos.peco@gmail.com>
wrote:
> Hi all,
>
> Someone from the list kindly offered me a "tech special" Hercules II a
> couple of years ago. I finally found the time to start working on it.
>
> This amplifier has two separate RF decks with an output impedance of 100
> Ohms each (not 50) that get combined into a single 50 Ohm line and then
> filtered. To test the LPF board (in isolation), I placed two small 100 Ohm
> resistors in lieu of the amplifiers, and connected an antenna analyzer at
> the output port. Then, with a small 12 VDC power supply, I was energizing
> the relays corresponding to the filter under test.
>
> I found that the 20 m filter presented a VSWR of 7:1 (that was probably the
> cause of the amplifier's failure in the first place !) and traced the root
> cause to the rivets that connect the top and the bottom PCB ground tracks.
> So I renewed those connections (and added some more) and the filter showed
> a VSWR better than 1.3:1
>
> HOWEVER, the 160 m and 80 m filters still showed a really bad response,
> with a VSWR around 2.5:1
> This is where the mystery begins:
> - I grabbed the schematics and simulated the circuits on SimSmith. For the
> inductors, I just measured the core size and the number of turns, that gave
> me an initial ballpark figure to play with (that ended being a very
> accurate estimate, by the way).
> - Leaving the capacitor values fixed, no amount of inductor tweaking on
> SimSmith fixed the off frequency response.
> - I ended up unsoldering ALL the 160 m and 80 m components, measuring them,
> and simulating the filter with its actual values on SimSmith. It matched
> perfectly my measurements with the antenna analyzer.
>
> So,
>
> How is that five of the seven filters are perfectly fine, while the two
> bottom ones (160 m and 80 m) present a VSWR of 2.5:1 at the ham frequencies
> ?
>
> I could not restrain myself and ended up modifying the capacitor values
> (while keeping the inductors constant) and now I have filters presenting a
> VSWR better than 1:3:1 on both 160 m and 80 m. However... I'm a bit
> curious about this. I wonder if there is something going on at frequencies
> under 4 MHz with the RF pallets (like departing a lot from 100 Ohms) and
> the designer decided to compensate those issues in the LPF block ???
> The broadband power combiner is actually broadband... instead of two 100
> Ohm resistors I also used a single 50 Ohm resistor connected to one filter
> port and the antenna analyzer on the other.
>
> If someone wants to simulate the filters, the values (not present in the
> schematics) are:
> L1: 4.8 uH
> L2: 2.46 uH
> L3: 2.3 uH
> L4: 1.5 uH
>
> Sorry for the long post. In any case, it is worth looking at the ground
> rivets in the LPF filter block, as the built-in VSWR detector is located
> AFTER the filter, unaware of what is going on at the input.
>
>
> 73,
> Carlos VK1EA
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