Working with the AN758 AMP from CCI

I have the amp together and attempting to run it.

IMG_3208

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.

AN758 Bias Circuit

 

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.

AN758 Amp Circuit

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Unfortunately the CAN version of the MC1723 is obsolete and hard to come by.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

 

10 thoughts on “Working with the AN758 AMP from CCI

  1. Greetings,
    I’m building one in the same.

    Q3 shows as a 2N6388 versus a 2N5991.
    The 2N6388 pin out is Base – Collector – Emitter, however the 2N5991 is pined out Emitter – Collector – Base. Should be flipped up-side down (Kit supplied stand-off and screw / nut)

    Zener D2 measured on a curve trace is 30 Volts (as supplied from CC)

    Have just started to put together and play…
    Thanks for your experience and please keep up-dating your progress as well as I.

    KJ6WSM
    73
    Mike Auer

    • My bag of parts had the 2N6488. I did not make the connection in my mind about the pin-out reversal. That may be part of my problem! Thanks for the tip.
      Virgil K5OOR

  2. Hi Virgil, K5OOR

    Did some checking/testing this afternoon ~ my amp has the 1N5363B Zener = 30v device
    but R12 is only 1/2w rated (shall change to 1W tomorrow) Also has the 2N6488 mounted
    upside-down.
    BIAS voltage adjustment is between 0.83 and 0.96 only —> is the MC1723 defective ???
    Ic ‘idling’ is thus ~ 3A for the pair = too high !

    Comments appreciated

    Regards, Jan K1ND

    • Hello Jan, K1ND

      I abandoned that project after many hours trying to get it to work. Must be me or something. The only notes I have are what is posted here. I did get the bias to work but power out test was miserable. I reverted to my standby: http://site.hfprojectsyahoo.com/hf_superpacker_pro_ver_2
      Just completed a group build of 25.

      Good luck! If you have success, please let me know about it.

      73, K5OOR – Virgil

  3. Good morning.

    I am currently building an AN758 and running into similar problems. Did you resolve them. Do you have a more detailed picture available. My kit seems to be missing L1 and L2.

    Regards, Dan kb0qil

    • Hello Dan,
      I really spent some effort to make the AN758 work. As I dug into it, I discovered schematic errors. After correcting all the perceived errors, I was not able to get it to work. My efforts lay at the bottom of some forgotten junk box never to be seen again. I conclude that the recipe that the originator had was lost somewhere along the way. I do know of anyone who was able to make it work.
      On a positive note, I have had very good results with the AN762 design.
      73, Virgil – K5OOR

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