AN758 Amp Assembly continues

The development wheel continues to turn slowly. I finally received the LM723C replacement part. Through a blog post from a member, I was alerted that the series pass transistor, Q3, substitute provided by CCI has a reverse pin out from the circuit board. I replaced that part as well as I installed the replacement LM723C. A check on the bias now under RF load shows that it stays stable at about 0.7V bias. The RF output doubles with a doubling of the RF drive input (1W to 2W).

That being said, the RF output is still very low (only about 10W max). I can crank up the drive to draw about 4.5A from the transistors. I don’t know where the RF is going but it is not reaching the output. I reviewed the T1 connections and looked for balanced gate signals with the scope. I reviewed T2 to make sure that was correct. My next thought is to remake T3 with short leads but I think that will not be the answer. I am going to test the T3 circuit independently to make sure it is working and delivering RF power to the load before re-installing.

I ordered a used HP Model 428B clip-on current meter so that I can see the DC current for each transistor.

I really thought I was going to be up and running with the AN758 Amp without any problem before now. That just proves that you can’t get sure of yourself as if all this was easy. I appreciate your feedback and experience with the AN758 amp assembly and test.

The AN762 Amp was not a problem in the past although I did make about 5 prototype layouts before I was happy with it.

73, K5OOR –  Virgil



3 thoughts on “AN758 Amp Assembly continues

  1. The problem you’re having with the missing RF power sounds as though one or more of the transformers is saturating and not passing the power through to the secondary…. Larger cores may help.


    • Thanks Rob, I am using the parts supplied with the kit. It could be little else. The output transformer is made using 25 ohm coax (according to instructions). 25 ohm coax not available and I am unsure it is correct material. I am considering paralleling 50 ohm coax to make equivalent. It has been disappointing to say the least. I also have some large binocular cores to try.
      I appreciate your feedback!
      73, K5OOR – Virgil

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