Tom,
Back when I was resorting to the full bag of tricks the goal was not more power!
The goal was even power from 80 through 10 and improved distortion levels.
Alas, the were other issues found that superseded that work.??
For the moment it was and is possible to improve the amp.? The problem is?
All the spurious product presented to the amp would be amplified as well.
That and the basic modulator/filter/if had issues as well.? The result was a
good amp would be largely wasted.? Then a collective bunch of us noted
other things like compromised output filters that allowed all that excess to
escape.? I like Warren reached the point that salvage was not possible.
I did literally cut the board up to investigate root cause issues and the
conclusions are harsh.
The amp is a disaster.? The radio is 80-10 the amp never made it to 20m.
The choice of devices and layout were big issues and the IRF510 was
blamed?for most of it and it was already a mess before we got to them.?
Plain english, the 2n3904 was a weak choice and poorly implemented
as asking it for more than 10DB at at 30mhz was impossible.? ?Layout
made for further issues with stability.
Output filters due to layout.? Again layout at HF still counts.
Lack of band pass filters before the amp.? I'll repeat what I've said?
garbage in nets garbage out, minimal filtering is wishful thinking.
IF gain issues.? Asking for 18db from 2n3904s is not engineering its wishful.
There are a few that will and many that will not.? IF one asked for 10db it
would not be an issue.? On that same boat , the high gain of the transmit
path leads to spurious signal creation.? To go from the modulator to the last
mixer no more than 16db of gain is needed across the 12 and 45 mhz if
for transmit.? We have on average 32 to 28db of gain.? Too much gain
causes problems those include poor carrier to signal and overload.
On top of that? The "12mhz" IF and filter layout made it susceptible to
the 11.997mhz carrier further reducing the carrier to modulation.?
Board layout is the major issue not circuit failure.
The above applies to transmit only but echos of that are in the
receiver as birdies and spurious responses.
The whole 12V thing was a result of the TDA2822.? My board I put in a
regulated voltage?for the TDA and end of issue. By doing that 13.8 was
the standard.? However that does not fix all the broken.? We or at least
I was not?tied to 12V.? If you think fixing that by running 20V or more?
is a solution its clearly a CB solution.
The bottom line is mod like mad for power?and your amping up a
lot of spurious trash and putting it on the band.
Allison
