I have been restoring a President AX-144 lately, and that usually leads me to stare at schematics in order to gather ideas on possible upgrades or mods i might like to try out.
yes, there is more mod info out there for this board than most others thanks to our friend Handy Andy, and Andy just let me say that all your past work is still appreciated.
one of the main things Andy said that i picked up on was just how many different versions of this board there are, and with that how different each version can be from others.
There are SOO many differences! some versions have mike gains, some don't, and even the ones that don't have mike gains differ from each other in the mic circuits.
then you have parts in the radio that aren't the ones listed in the schematic for your particular radio.
for example, TR25 and TR26 are mic amp transistors.
every schematic i looked at showed TR25 to be a 2SC945 and TR26 to be a 2SC1312 (might have that number wrong).
in my AX144, they are both 2SC1312.
i know, not a huge difference but it does illustrate how difficult it can be to decipher just what you do and don't want to mess with inside one of these beasts.
SO!
I stated all that just to get it out of the way that just because something is being discussed in this thread as a mod to a UPD2824 SSB chassis, that it may or may not be something you want to do to your particular radio.
I also want to keep this very specific in order to keep the posts short and to the point.
again, these stories have mostly all been told already.
To start, i would like to look at R501, which is a 100 ohm resistor mounted on the mic jack PC board, OF SOME MODELS!
it doesn't seem to be there on the PC122 versions.
so, why is this resistor there?
is it just to guard against high volumes on power mikes?
is it there because it was cheaper than putting a coil there, and hey, it's a wire wound resistor!
does anyone see any benefit to replacing it with a jumper and a ferrite bead?
i can't imagine that 100 ohms is having much effect on mic audio level, but that again begs the question of why it's there.
maybe i am missing what it's purpose is.
LC
yes, there is more mod info out there for this board than most others thanks to our friend Handy Andy, and Andy just let me say that all your past work is still appreciated.
one of the main things Andy said that i picked up on was just how many different versions of this board there are, and with that how different each version can be from others.
There are SOO many differences! some versions have mike gains, some don't, and even the ones that don't have mike gains differ from each other in the mic circuits.
then you have parts in the radio that aren't the ones listed in the schematic for your particular radio.
for example, TR25 and TR26 are mic amp transistors.
every schematic i looked at showed TR25 to be a 2SC945 and TR26 to be a 2SC1312 (might have that number wrong).
in my AX144, they are both 2SC1312.
i know, not a huge difference but it does illustrate how difficult it can be to decipher just what you do and don't want to mess with inside one of these beasts.
SO!
I stated all that just to get it out of the way that just because something is being discussed in this thread as a mod to a UPD2824 SSB chassis, that it may or may not be something you want to do to your particular radio.
I also want to keep this very specific in order to keep the posts short and to the point.
again, these stories have mostly all been told already.
To start, i would like to look at R501, which is a 100 ohm resistor mounted on the mic jack PC board, OF SOME MODELS!
it doesn't seem to be there on the PC122 versions.
so, why is this resistor there?
is it just to guard against high volumes on power mikes?
is it there because it was cheaper than putting a coil there, and hey, it's a wire wound resistor!
does anyone see any benefit to replacing it with a jumper and a ferrite bead?
i can't imagine that 100 ohms is having much effect on mic audio level, but that again begs the question of why it's there.
maybe i am missing what it's purpose is.
LC