1. Forum
  2. FidoNet
  3. sci.electronics.repair

  • Fun with amplifier distortion!

    From Cursitor Doom@3:633/10 to All on Thursday, May 14, 2026 12:15:10
    I finally found some time to set aside to look into the distortion
    issue I've been experiencing with the Uher 4000 Report Monitor AV I
    bought a while ago. I've narrowed it down to one particular stage at
    or around the BJT labeled T14 in the attached schematic snippet. The
    1000Hz sinewave test signal's coming from the playback head at the far
    left and is nice and pure right up to the base of T14, but is
    noticeably distorted thereafter. No clues from the DC bias points:
    they all check out as per the diagram give or take a tenth or two at
    most. So I thought it might be a bit of fun (anyone remember that?) to
    let the Panel here have a guess at which component(s) might have gone
    rogue here. I hope to be able to complete my investigations on Sunday
    afternoon if time permits and we'll find out who was right when I
    report back. So: your guesses invited!


    https://disk.yandex.com/i/9hIxKidwWlcfJQ

    --- PyGate Linux v1.5.14
    * Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)
  • From Cursitor Doom@3:633/10 to All on Friday, May 15, 2026 18:05:15
    I meant to also mention that since this is effectively the pre-amp for
    the replay head, it's dealing with extremely weak and delicate
    signals. By the time those signals hit T14, they're still too small to
    be seen on a scope even at the 5mV lowest range and are buried well
    and truly in the noise. So I've been unable to glean any clues by
    probing T14 or probing the components around T14 either. I was only
    able to spot this stage as the culprit thanks to Liz Tuddenham's high
    impedance probe+ audio amp suggestion (HT, Liz).

    --- PyGate Linux v1.5.14
    * Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)
  • From piglet@3:633/10 to All on Sunday, May 24, 2026 20:12:53
    On 15/05/2026 18:05, Cursitor Doom wrote:
    I meant to also mention that since this is effectively the pre-amp for
    the replay head, it's dealing with extremely weak and delicate
    signals. By the time those signals hit T14, they're still too small to
    be seen on a scope even at the 5mV lowest range and are buried well
    and truly in the noise. So I've been unable to glean any clues by
    probing T14 or probing the components around T14 either. I was only
    able to spot this stage as the culprit thanks to Liz Tuddenham's high impedance probe+ audio amp suggestion (HT, Liz).

    If DC voltages check out OK and AC signal is good at T14 base but bad at
    the collector maybe check the emitter cap C22 and also that only 1 of
    the 4 equalizer transistors T9-T13 is on? Another possibility is
    something downstream of C25, not shewn in your diagram, is overloading
    or distorting? Finally T14 itself could be damaged?

    piglet


    --- PyGate Linux v1.5.14
    * Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)
  • From pitter_smith@3:633/10 to All on Friday, June 12, 2026 21:30:02
    It?s always surprising how a tiny amount of crossover or clipping distortion can sound much worse than the numbers alone would suggest, especially on vocals and higher frequencies.

    --
    For full context, visit https://www.electrondepot.com/repair/fun-with-amplifier-distortion-4402913-.htm


    --- PyGate Linux v1.5.16
    * Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)