|"U3S, the doctor will see you now..."|
Up to now I've used only one transistor and 5 volts. This gives me a maximum of 200 mW RF output on 14 MHz and below - slightly lower at higher frequencies.
A few weeks ago I added the other two transistors and a DC-DC converter that provides 9VDC to them. Upon powering the U3S back up after the modifications I quickly realized that I'd forgotten to lower the bias voltage to minimum in preparation to adjust it to a new value. I was pleased to see about 2-1/2 watts of RF output - for about 2 seconds.
The RF output then went to zero and I said a few words not meant for fragile ears.
Since I now had to replace three transistors I decided that it was also a good time to relocate the trimmer pot that adjusts the bias. With the optional relay board installed, accessing that particular (and only) adjustment is impossible. The always-faithful Mouser and UPS had the parts on my doorstep two days later.
|Toothpick inserted, clearing hole of solder|
- Cut the transistor leads as far up on the body of the transistor as possible. The idea is to leave as much of their leads still in the circuit board as possible.
- Hang the hemostat on each lead, one at a time, while heating up the opposite side (non-component side) while pulling the hemostat, and the lead along with it, away.
- The part and its leads are now removed but solder clogs the hole. Push a toothpick against the hole while heating up the opposite side with the soldering iron.
- The toothpick will insert itself into the hole as soon as the solder melts. Pull the toothpick out and the hole will remain clear.
I prefer this method of clearing the hole rather than with solder wick because much less heat is
I had to whittle mine down a bit for it to fit (cue Michael Scott) --->
Three brand-spanking new BS170's were then easily installed and then I used the same technique to remove the trimmer pot next to them. In its place I soldered three short wires and ran them to an unused hole on the front panel of the U3S where a new 4.7k pot was waiting patiently for them.
I can now easily adjust the pot without opening the case.
|Trimmer pot now relocated to unused hole upper-left of U3S front panel|