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AC4TV Attenuator - Convert to 4 Ohm?

Discussion in 'Amp Tech Center' started by nash076, Oct 18, 2020.

  1. nash076

    nash076 TDPRI Member

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    Hey folks,

    I have an old AC4TV I'm monkeying around with. I've got a load of spare parts and I'm seeing what I can do as a "junk build." I really don't want to sink any money into the unit if I don't have to.

    The original speaker is garbage, but all I have to replace it right now is a moderately okay Jensen MOD 10. Trouble is, it's 4 Ohm. Now, I have a 4 ohm Champ-style transformer that should be just fine in this build, but the issue is the AC4TV's rotary switch attenuator.

    Here's the schematic. My question is this: if I switch to a 4 ohm speaker and 4 ohm OT, what changes do I have to make to the attenuator? Do I need to alter the resistor values? Do I need resistors that can handle higher wattage? (I believe the included resistors are 3 watt.)

    If you comprehend how this would work, your input is appreciated. Thanks! upload_2020-10-18_22-9-11.png
     
  2. nash076

    nash076 TDPRI Member

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    Here's the schematic from the service manual, if it helps. Denotes the wattage levels: upload_2020-10-18_22-33-43.png
     
  3. elpico

    elpico Tele-Afflicted

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    Yeah if you wanted to keep the attenuator function you would need to build a new one. This one is designed to be a 16ohm load (roughly)

    Probably a lot easier to get a different 16ohm speaker.
     
  4. nash076

    nash076 TDPRI Member

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    That's kind of what I'm aiming to do.

    I got some feedback on Twitter to replace R13 and R7 with 45R resistors and was told it would be fine from there. Was that incorrect, and why?

    If that's not right, why can't I just replace the resistors in this circuit?
     
  5. elpico

    elpico Tele-Afflicted

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    That twitter advice is wildly incorrect. To change this from a 16ohm attenuator to a 4ohm one the resistor values need to go down not up.

    I never said you couldn't, I only said a different speaker would be a lot easier. Sure you can replace the resistors. The caps need to be changed too if you want to the frequency response to stay the same so every component needs to be changed. You need to build a new attenuator. The resistors would be 4ohm and 1.5 or 2 ohm and the caps 40u
     
  6. nash076

    nash076 TDPRI Member

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    Okay, that's a huge help.

    Rebuilding it actually is kind of easier because I have tons of components and re-assembling it isn't a huge problem.

    Thank you so much for this.
     
  7. nash076

    nash076 TDPRI Member

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    Made the modifications, tested with the multimeter and both attenuator poles show around a 4 ohm resistance. It's not connected to the transformer yet. Does that sound correct?
     
  8. elpico

    elpico Tele-Afflicted

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    Are you measuring the attenuator by itself? or the combined resistance of the attenuator and speaker?

    By itself you'd expect the 1w mode to read 8ohms and the 1/4w to be 5 or 6.

    In combination with the speaker you should see 5 to 6 ohms on either setting
     
  9. nash076

    nash076 TDPRI Member

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    I'm reading it by itself, measuring the two leads coming off the switch that places the attenuator between the speaker and the OT.

    The 4w setting reads about 0.5 ohms, and both the 1 watt and the 1/4 watt read 4.2ish.

    I replaced the resistors and capacitors as you described: R7, R13 and R25 are now 4 ohms, and R14 is around 1.7 ohms.
     
  10. elpico

    elpico Tele-Afflicted

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    To be clear, you're measuring the pins marked 4, 5, and 6 on the schematic right?

    If you get 0.5 ohm when measuring pin 4 to ground then either the OT is connected to the switch or you've got a short.

    It sounds like you've got the OT connected and you're measuring pins 4, 5, and 6 without moving the switch. If you did that you would indeed expect to read less than an ohm on pin 4 and 4ohms on the other pins. Pin 4 would be telling you the resistance of the OT secondary and the other two pins would be measuring that + R7.
     
  11. nash076

    nash076 TDPRI Member

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    I'm measuring across Tag 3 to Tag 5. The attenuator is not in the circuit. It's not connected to anything yet. It's by itself.
     
  12. elpico

    elpico Tele-Afflicted

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    Ahhh okay. Well hopefully the 0.5ohm measurement is inaccurate because you're measuring the switch's on resistance then. It can be hard for meters to read <1ohm things accurately so it's probably lower than that.

    Okay that test isn't telling you much. You're measuring the switch and then r7 alone, twice.

    Try measuring from pin 4 to pin 7 and move the switch to each setting.
     
  13. nash076

    nash076 TDPRI Member

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    Okay, measuring from pin 4 at Tag 3 to pin 7 at Tag 9:

    4W setting: 0 ohms
    1W setting: 4.3 ohms
    1/4W setting: 5.8 ohms.
     
  14. elpico

    elpico Tele-Afflicted

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    4w is supposed to read open.

    R7 and R14 must be good because the 1/4w setting reads right.

    1w mode reads R7 and R25 in series, that should give 8 ohms. Check R25 for a short or incorrect value.

    edit: oh you read a short on 4w? That's not supposed to happen.
     
    Last edited: Oct 20, 2020
  15. elpico

    elpico Tele-Afflicted

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    Wait, are those speaker jacks possibly still connected to this?
     
  16. nash076

    nash076 TDPRI Member

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    Nope. Speaker jacks aren't connected. It's one of these, it's all by itself, independent circuit. Currently it's not connected to anything, it just has leads coming from the tag points.

    I'll double check for shorts, hold on ...
     

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  17. nash076

    nash076 TDPRI Member

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    Double checked. You must have been right about a short, because when I pulled the PCB from the chassis and tested again, I got:

    4W: 0 ohms
    1W: 8 ohms
    1/4 W: 5.7 ohms

    I don't see the problem with the 4W setting being 0 ohms, though; shouldn't that just be a straight connection from the OT to the speaker, taking the attenuator out of the circuit?
     
  18. elpico

    elpico Tele-Afflicted

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    Tag 3 and tag 9 connect to opposite ends of the OT secondary. A short between those two points is a short across the OT secondary. That would mute the amp, zero voltage output.

    Some pictures of the solder side of the board might needed here

    edit: Just to double check, the meter isn't reading "0L" rather than "0Ω" right? What model are you using? What does it read when the probes aren't attached to anything?
     
    Last edited: Oct 20, 2020
  19. nash076

    nash076 TDPRI Member

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    Derp. Yup. 4W setting is 0L, not 0 ohms.
     
  20. elpico

    elpico Tele-Afflicted

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    okay cool that's good news. So open, 8ohms, and 5.7ohms then? You're good to go. This doesn't test R13 or the caps, DC won't go through there, but as far as what you can check with the meter that sounds right.
     
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