Found another one:
https://i.postimg.cc/HnWtqr0Z/signal-2021-05-07-230643-1.jpg
Sorry for the mess !
Carlos VK1EA
On Tue, May 11, 2021 at 11:18 PM Carlos PECO BERROCAL <carlos.peco@gmail.com>
wrote:
> Hi,
>
> I received a very nice email from Lee Jones (former Ten-Tec CTO) where he
> explained me that the capacitive nature of the filters under 4 MHz was
> introduced deliberately to compensate for some shunt inductance in the RF
> decks. So I'll have to undo my "non-improvements" during the weekend !
>
> I uploaded some screenshots and photos to a image host site so they can be
> seen clicking in the link, if interested.
>
>
> - Original response of the 160 m filter (it is a simulation from SimSmith
> using the values found in my filter as I did not make a proper capture with
> the antenna analyzer)
>
> https://i.postimg.cc/Jn5LTX0s/160m-Original.jpg
>
>
> - Modified response after shuffling some capacitors around:
>
> https://i.postimg.cc/4NnH76W4/160m-Modified.jpg
>
>
>
> - and then some photos using my small antenna analyzer (an EU1KY analyzer,
> best thing since sliced bread)
>
> https://i.postimg.cc/G2kVjTzH/signal-2021-05-08-152802-1.jpg
>
> https://i.postimg.cc/T1N9jR8K/signal-2021-05-08-212715-1.jpg
>
> https://i.postimg.cc/qvNkxQKq/signal-2021-05-08-132842-1.jpg
>
> https://i.postimg.cc/wBhYwrjq/signal-2021-05-08-160232-1.jpg
>
> https://i.postimg.cc/15G9C47h/signal-2021-05-08-215128-1.jpg
>
> I hope that the links work.
>
>
> Great thanks to Lee Jones for taking the time to send me a kind email
> explaining me some details about his design.
>
> 73,
> Carlos VK1EA
>
>
>
>
>
> 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
>>> > _______________________________________________
>>> > TenTec mailing list
>>> > TenTec@contesting.com
>>> > http://lists.contesting.com/mailman/listinfo/tentec
>>> >
>>> _______________________________________________
>>> TenTec mailing list
>>> TenTec@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/tentec
>>>
>>
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