Re: boosting the power on 28 MHz #ubitx

Lawrence Galea


On Mon, May 7, 2018 at 11:32 AM, Ashhar Farhan <farhanbox@...> wrote:
The DC currents are the same. I can't see why it will get any warmer

On Mon, 7 May 2018, 15:00 Lawrence Galea, <9h1avlaw@...> wrote:
did you check whether Q90 gets warm/hot after mods?
Last time I did some tx checking on CW and Q 911,912, 92, 93, 96 and 97 were getting warm after about 1 minute.
Supply was 11.64V, no supply to PA.
Haven't had time to check it on the air.

On Sat, May 5, 2018 at 2:13 PM, Ashhar Farhan <farhanbox@...> wrote:

28 Mhz is unusually open for the last one week. I realized that our ubitx output is woefully low. Here is a fix that I made today, if some of you can try it out, let me know if it works as well.

Step #1 Increase the predriver gain towards the higher frequencies
The predriver Q90 has a emitter degeneration capacitor C81 (0.1uf) and and R83 (10 ohms). I replaced the C81 with a 470 pf and the R83 with 5 ohms. With this, the emitter reactance decreases with increasing frequency, yielding higher gain beyond 14 MHz.

Step #2 Take off the feedback from the IRF510s. 
Just remove the R261, R262 (both are 220 ohms) to open up the gain.

Let me know if this mod is replicable?

- f

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