JustinDePolis said:Galaxy 88 / 99 modification.
Here we go, C187 and C235 go to a .2 monolythic, the input coupleing capacitor goes to a 2.2uf tamtalum, and the low end "rolloff" capacitor goes to a 4.7uf tantalum. R187 to 10K. R 235 to 10K, this improves AM modulation depth.(this increases the highs and the lows) Remove incorrect bypass capacitors on several version of the echo boards. (sorry, do not have component values off hand, I go off memory, but typically there are about anywhere from 2 to 4 depending upon the echo board version. They are incorrect values, and roll off the highs and lows too soon.
Add .047 mylar across speaker terminals as RF bypass. Add .047 from left side front screw trace to circuit board ground by mic amp as RF bypass.
Change 820pf capacitor to metal mica near finals, change 560pf and 68pf (by finals) to 680pf silver mica.
Remove J30 add 250pf silver mica capacitor. (this blocks DC, but passes R.F. in this circuit just fine) Looking at where J30 was, on the side AWAY from the 1st R.F. amp, solder side up (to the left) add two 1N4148 diodes back to back, in other words, one in one direction, and another in the other direction. This provides clamping for receive, should a strong station get too close. (prevents damage to the front end)
From middle pin of the 2SB754, take a piece of teflon wire, and run it from where the final connects to the mirror board on the solder side to the 2SB754 solder side. This make up for any voltage drop. (several small jumpers supply VCC to the final output section.)
Next increase the gain of the TX mixer, by removing the 1K ohm resistor fron pin 12 to ground, and adding a 270 ohm resistor from pin 12 to ground (pin14). Don't go below 270 ohms. Remove C235.
Add about 68pf to 100pf across the driver to final transistor coupling capacitor.
Align the TX bandpass filters.
You should see 50 watts EASY on a Bird 43P @ 14.1 volts supplied to the radio.
There are of course more things that we do than just this, but this is the basic modification.
Maybe next month we'll get into changing the 1st R.F. amp to a dual gate mosfet, and the mixer to a dual gate mosfet, and how to control the gain.
Hmmm :?