I've had the U3s on the left in the photo below for quite a while (at one time it had the relay board installed). It was my first one and I bought the OCXO version. The first time that I put the assembly in a case I wired the little power supply board (I swapped it out today) so that if power was applied the board was hot and switched the 5V to the U3s. Oops. I also used a single BS170 and jumped 5V to the PA. When I built the second U3s I used some eBay power supplies, the PS for the controller is set at 5V. 13.8 VDC is switched BEFORE the two supplies. I also installed (3) BS170 transistors.?
The overhauls were to (1) convert the left side switch to high and low PA power (5V or 10V approximate), (2) rewire the old U3s so that power to the supplies was switched, (3) separate the PA supply from the control board supply (old U3s), and (4) add two more BS170 transistors to the old U3s. Essentially, other than the synth and low pass filter (one is 20m and one is 30m) they are functionally the same now.??
I have to set the transistor bias for the old U3s and the OCXO was never really set right. The synth only version is more stable than the OCXO. I saw this??on Hans site. I have a fairly decent bench power supply with 1/10mA current display so I'll probably just use it
Question- Should I set the bias with the 5V PA supply and adjust for 250mW output into 50Ω using an oscilloscope and then adjust the high voltage to the PA to yield 500mW? How high can I run the PA voltage? Where should I limit the output of the (3) BS170??
