A15mk2 ch 2 not working, bias voltage out

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    • A15mk2 ch 2 not working, bias voltage out

      Channel one seems to work fine (testing through a cheap full range speaker, so can’t determine difference with fat switch but not too concerned at this stage).

      Channel two is silent until volume is pushed. When at max there is significant buzzing and only an extremely feint guitar signal can be detected when putting my ear to the cone.
      Treble and mid seem to affect the buzz tone in the correct way.
      Bass affects tone for first 25% of rotation.
      Gain makes no difference at all.
      What bit of guitar signal that exists is not distorted at all.
      There’s a bit of a “clunk” when the ++ gain switch is first engaged but not if repeated.

      All voltages seem fine when compared to the schematic EXCEPT:

      Heater wires are 6.9v AC (which is a bit high but not serious, I think).
      Bias voltage range is between -10 and -17 (should be -21 to -27)...
      ...and the pins 8 on V5 and V6 are 49.7mv and 49.3mv instead of <35mv.

      I don’t know how the bias would affect Ch2 as Ch1 seems to sound decent, maybe it’s an unconnected problem.
      I have checked, double and triple checked connections under magnification and can’t find any errors or potential short circuits. All electrolytic caps are oriented correctly.
      I’ve changed the channel switch, makes no difference.
      All tubes glow as normal.

      Help is greatly appreciated!

      Thanks, Paul.
    • Hi Paul,
      sorry for the late reply but I completely overlooked your post. Are your issues still existing? If yes,
      1) what is the supply voltage at your home (230V? 240V? - please measure)
      2)The cathode voltages at the cathodes of your 6V6 are too high because the bias voltage is too low. So either your bias circuit is wrong (likely) or the power transformer is broken (not likely). What is the AC voltage from the transformer at the input of the bias circuit? Is there something else connected to that supply (e.g. footswitch supply)?
      3) the second channel problem is most likely disconnected so let's focus first on the bias issue
      can you post a picture of the power supply board please.

      Regards,
      Martin
      ========== MAD = Martin Adams / Digital ist nicht alles / madamp.de ==========
      ========= Meine Alben: musikding.rocks/index.php/AlbumList/6475/ ========
    • New

      Hi Martin,

      Many thanks for getting back to me. Must admit, I'd stopped looking after a while!

      The AC mains voltage is 235v.
      The AC voltage at lug 15 of the power transformer which connects to the diode on the power board is 2.2v with output tubes inserted. It measures 24v when tubes are not inserted.

      Picture of power board is attached. I hope you find a glaring error!!!

      Many thanks again,

      Paul.
      Images
      • IMG_20200522_135742.jpg

        710.16 kB, 4,000×2,248, viewed 8 times

      The post was edited 1 time, last by Burlo1973 ().

    • New

      There should be no change of AC voltage between lug 15 and ground with or w/o tubes. If this is really the case, please measure the DC voltage across R45 and R46 respectively. I'm happy to take a look at the photo but cannot open it to full size (only half of the photo opens up, the rest remains grey), can you upload it one more time?
      ========== MAD = Martin Adams / Digital ist nicht alles / madamp.de ==========
      ========= Meine Alben: musikding.rocks/index.php/AlbumList/6475/ ========