I have the amp together and attempting to run it.
The kit did not provide any hold down washers or any mention about how to mount. In the AN758 document picture it looks like fiberglass oversize washers. No matter, it is just for testing as a bench mark. The assembly took longer than expected because the documents were wrong and incomplete. See the markup corrections I made. Rather than loose this information, I wanted it to be available for other builders.
I changed the 1–turn pot to a 12–turn pot (same foot print). The zener supplied was a 45V device which is over-voltage for the MC1723C part. I changed to a 1N5365 36V zener. I added a 100 ohm pull down resistor on the bias supply so that the bias voltage would not float when the 48V was not present. I recorded the voltage range I obtained with adjustment. I added a 0.01uF cap across C13 for high frequency suppression. More about operational testing after showing you the amp schematic as built.
This schematic had a comedy of errors everywhere you look. I think I have it straightened out with the values I used to build this bench mark amp.
TESTING – I was able to easily set the bias. I used the recommended 300mA as a starting point. I have a 4dB resistive pi network on the input of T1 just to make sure the amp was seeing 50 ohms drive impedance. I keyed with 2W, 3W and 4W. My test was on 80M. I only measured about 10W with 2W drive. I expected it to double with 4W drive. The output only increased about 2W! So, I started trouble shooting and found that the bias supply is not stiff. It sags big time and has RF on the output. I know from past experience that the bias supply is very critical to the amplifier performance and it must be very stiff and transient responsive. This particular part obviously is not.
The only explanation I have, at this point, is that I caused a failure of the part when the wrong specified zener was in circuit and the Vc (pin 7) exceeded the 40V maximum input.
Unfortunately the CAN version of the MC1723 is obsolete and hard to come by. I may have to get it from CCI. I will check around. In my HF SuperPacker Pro V2, I used a dip part and it operated very well. While I am waiting on a replacement part, I may hack saw one my spare AMP boards for the HFSPP V2 and outboard the bias supply.
If you have any first hand knowledge working with the CCI AN758 AMP board, I would like to hear your experiences and whether you can validate my findings.
73, K5OOR – Virgil