[TenTec] Hercules II LPF board issues
Carlos PECO BERROCAL
carlos.peco at gmail.com
Sun May 9 04:58:15 EDT 2021
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.
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
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.
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