1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
    Dismiss Notice
Dismiss Notice
This forum does not allow a single user to have more than one username. If anyone wants to change their username contact an admin and it will be done. Multiple accounts belonging to the same member will be deleted without warning.

CB amplifier troubleshooting

Discussion in 'CB and Export Equipment and Accessories' started by tomcat671, Jan 5, 2021.

  1. tomcat671

    tomcat671 Member

    Joined:
    Apr 8, 2011
    Messages:
    11
    Likes Received:
    3
    I'm trying to figure out why my positive peaks are higher than my negative peaks (see photo) and how to fix this. It's a Black Cat HD300 with 2X SD1446. Appears to be the same schematic as a Palomar Blue Lightning. Thanks in advance for the help.


     

    Attached Files:

    #1 tomcat671, Jan 5, 2021
    Last edited: Jan 5, 2021

  2. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

    Joined:
    Apr 23, 2018
    Messages:
    4,551
    Likes Received:
    5,588
    upload_2021-1-4_21-41-16.png

    Is your amplifier biased with 15 ohm resistors?

    I've seen this before, but the one I fixed was ... well, not of a reason for what many people would think would cause it.

    The above uses this layout
    upload_2021-1-4_21-45-13.png

    Note where L1 is, and Note where T2's "B" wire is.

    Deals with the "trombone" and the winds of T1 and T2 along with R5 and R4 - which is why I asked about those 15 ohm resistors.

    The Trombone is the "twin tube" - need to verify that it's soldered and CENTERED in their "spots". Winds also, they need to go to specific spots - not just taken for granted.

    So you may need to remove the tubing assembly from both sides to verify that the PCB board the tubes are supported by is soldered fully to their "tube" sections for Power to get to where it needs.

    The other issue is L1 - is it even there?

    This is a board to sink clearance problem inclusive - so if L1 exists - make sure it's not "spot welded" before it is supposed to be grounded - past the ferrite.

    This Amp doesn't use Negative Feedback like T/Star does, It's non-existent - which tends to leave these at the mercy of ground loops and the inductive issues of high-gain Open loop - that Push Pulls' have as one of many problems fighting you.

    The Techs I've dealt with me handling this - simply said to change the 15 ohm back to ten and leave it. But to me that didn't answer or fix the problem.

    You running this thru a NPC radio over there? This never should come up because what comes out of the back of the Radio should be symmetrical - so wondering if you are seeing something of a BIAS problem in the scope than a NPC gone wrong display.

    So I've got questions because how I fixed the previous one - required a blowtorch and a lot of patience to get the ferrite/trombones right as well as their coils - which required others to help from their memory because of the rework needed to be done to the amp, had from a previous owner, took much more than just a simple repair - it was a rebuild.
     
    Shadetree Mechanic likes this.
  3. ExitThirteen

    ExitThirteen Grumpy and Cranky

    Joined:
    Apr 18, 2008
    Messages:
    1,960
    Likes Received:
    1,549
    That scope reading does not look right at all, the amp is biased class C, but that's a non-issue at this point, I am curious as to what the radio is, what kind of wattage is being driven into the amp, and if there are any modulation modifications present in the radio. That's the kind of info needed to help this along. :) I'd also like to see a scope picture of JUST the radio to see how it looks before it goes into the amp.


    ~Cheers~
     
    Robalo likes this.
  4. tomcat671

    tomcat671 Member

    Joined:
    Apr 8, 2011
    Messages:
    11
    Likes Received:
    3
     
  5. tomcat671

    tomcat671 Member

    Joined:
    Apr 8, 2011
    Messages:
    11
    Likes Received:
    3
    The radio is a General AP Hill keying 3 Watts. I removed a mod to the limiter that was causing it to overmodulate. Attached is the barefoot signal. If I modulate it to 100% then the output of the amp will be overmodulated.
     

    Attached Files:

    ExitThirteen likes this.
  6. tomcat671

    tomcat671 Member

    Joined:
    Apr 8, 2011
    Messages:
    11
    Likes Received:
    3
    One other tidbit: the 27 Ohm resistor before T1 has been non-existent. I tried adding it back in but the amp would not work at all so I took it back out.
     
    Handy Andy likes this.
  7. ExitThirteen

    ExitThirteen Grumpy and Cranky

    Joined:
    Apr 18, 2008
    Messages:
    1,960
    Likes Received:
    1,549
    Sent you a PM!


    ~Cheers~
     
    Handy Andy likes this.

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!