Sorry, I meant to say "at frequencies lower than 4 MHz"
my fault
Carlos
On Mon, May 10, 2021 at 11:02 PM Carlos PECO BERROCAL <carlos.peco@gmail.com>
wrote:
> Hi Ken,
>
> As far as I know, filters have been designed for 50 Ohms at both ports. At
> the beginning, I terminated the filter with the help of two 100 Ohm
> resistors (with the combiner in place). After finding a bad VSWR response,
> I opted for testing the filter in isolation (which is easily achieved
> soldering a 50 Ohm resistor in one end and a short piece of coax for the
> antenna analyzer in the other end), getting almost the same (bad) results.
>
> You get an slightly different result when testing the filter in isolation,
> as the parasitic effects of the PCB tracks (the common RF tracks where the
> relays are connected) as well as the broadband combiner are no longer
> there. By "slightly different results" I mean reading VSWR of 2.5:1 instead
> of 2.6:1, for example. Interestingly, at some frequencies the parasitic
> effects of the PCB tracks "help a bit" while in other frequencies "make
> things a bit worse".
>
> Anyway: 160 m and 80 m filters had the right shape (the cut-off frequency
> of the filter was about right) but VSWR was bad. I think that the lists at
> contesting.com do not allow for attachments, so I'll send you an email
> directly with some captures and photos.
>
> So after the modifications, I have all seven filters behaving beautifully
> when terminated with 50 Ohms (directly soldering a single 50 Ohms load or
> by using the relays and terminating the two amplifier ports with a 100 Ohm
> resistor each).
>
>
> However, I still don't know if that response was there to compensate
> something at the RF decks. Maybe during the development somebody found
> that, at frequencies lower than 40 MHz, the filter's input impedance had to
> be increasingly capacitive to maximize power transfer into a high-power
> dummy load... I don't now.
>
> So what I'm going to do next (time permitting) is to build a directional
> coupler for 100 Ohms, a 100 Ohm load (there are inexpensive RF loads on
> ebay, they just need a heatsink) and then, with a manual tuner in between
> the directional coupler and the load, move the knobs around to see what
> settings allow for maximum RF voltage developed in the 100 Ohms load. That
> should clarify (I hope) if what I observed in the 160 m and 80 m was a
> design issue or a design feature.
>
> The last thing I want is to modify the filters thinking that I'm improving
> the life of the transistors when in reality I'm making things worse.
>
>
> I'll send you that email in a minute, it is a bit late here in VK.
>
> 73,
> Carlos VK1EA
>
>
>
>
> On Mon, May 10, 2021 at 12:21 AM Ken Brown <kenradiobrown@gmail.com>
> wrote:
>
>> 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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>> >
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