Jcm 2000 Tsl 122 Bias Help

Discussion in 'The Workbench' started by Sapphire Sounds, Apr 13, 2019.

  1. Sapphire Sounds

    Sapphire Sounds New Member

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    I am 100% going to update with those readings, last night I couldnt get any readings and there was a battery symbol on the meter. I dont have any replacements handy this size.

    20190414_093929.jpg

    20190414_095111.jpg

    The 200 ohm reading for the tester is 00.4 when firmly connected otherwise it is all over the place connected and no load when disconnected.

    The mA wasnt showing me any numbers so I just played the amp last night and it sounded tight and amazing but I think the reverb tank spring is chattering so I'll troubleshoot that after all this is sorted.

    Today I will pull the tubes turn the power on and set the standby to warm up before trying to find the pin 5 readings.

    Where exactly am I going to find pin 5 of an output tube socket to start?

    EL34-Octal-socket.jpg

    Should I use this picture or is that the opposite side than what I will see when I pull the EL34 out?
     
    Last edited: Apr 14, 2019
  2. Sapphire Sounds

    Sapphire Sounds New Member

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    The tubes are apparently apex matched EL34s and I have no idea what the little letters are on them so I took pics of the lettering, are they actually not a matched set like advertised and sold?

    To test if the bias pots are working properly is important, from what I read these amps can have resistors, capacitors and bias pots go bad. Maybe there has been some work done but I do not know, the tubes are still in at the moment but ready to come out for those pin 5 500 DC readings.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    "The mA wasnt showing me any numbers so I just played the amp last night..."

    You shouldn't have put the tubes in....

    "Where exactly am I going to find pin 5 of an output tube socket to start?"

    There are numbers printed on the socket. Pin 5 should show a negative voltage an all sockets.
    You should not play this amp until you make sure it's working right.
     
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  4. ampmadscientist

    ampmadscientist Well-Known Member

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    "...To test if the bias pots are working properly is important,..."

    Then why did you skip the tests?
     
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  5. Sapphire Sounds

    Sapphire Sounds New Member

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    Sorry lets do the tests correctly I appreciate the guidance more than it comes across, tubes are going back into their boxes now. I am learning what to do for the first time please bear with me, your right I dont want anything to bake and sound like dog ass!
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Do the tests, or buy a box of fuses and a fire extinguisher.

    [​IMG]
     
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  7. Sapphire Sounds

    Sapphire Sounds New Member

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    Hahaha! I will report the findings either later today or tomorrow night I've got to leave the house but I put the tubes in their boxes and can identify the pin 5 from where the power tubes were, these are the output sockets?

    20190414_113039~2.jpg
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    The output sockets are 8 pins.
    The preamp sockets are 9 pins.
     
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  9. Sapphire Sounds

    Sapphire Sounds New Member

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    All 4 read -36 in warm up now switching to operate

    15553738083218285701923114824396.jpg

    The two on the preamp side are -36 and the others are both -37 so ( -37 -37 -36 -36)

    Edit: the one closest to the preamp is -37 the other three -38 after 20 minutes. (-38 -38 -38 -37)

    Edit: After 20 more minutes -40 -40 -39 -39

    Edit: All are now showing -41 and its been ten more minutes of standby operate looks like soon -42 on the left side...

    Edit: almost two hours now -44 -44 -43 -43 stable

    Ps. Speakers have always been connected in the 16 ohm dedicated jack
     
    Last edited: Apr 16, 2019
  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    What a thread. I bet New Religion is popping excedrin like pez if following this one.
     
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  11. Sapphire Sounds

    Sapphire Sounds New Member

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    downloadfile-1.jpg

    Well I am picking up that the dc current I'm measuring might supposed to be -39 according to the tube labeling.. And guessing that the 7830 number would be the millivoltage of the bias? Baby steps... To be continued
     
    Last edited: Apr 15, 2019
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    No the tube label is not related to where the bias should be set.

    You started out -36 and went to -40 on one side? (standby warm-up)
    And you went from -37 to -39 on the other side (standby warm-up)
    That was over period of 40 minutes?

    Then you had -36 in warm-up and -44 in operate on one side over 2 hours. ( 8 volt change)
    The other side was -37 in warm up to -43 in operate. (6 volt change) over 2 hours.

    And you had the power tubes out when you did this? Yes?

    Plug a speaker cable into the cabinet and set meter for 200 ohms.
    Measure the end of the speaker cable with the meter.
    What is the ohms reading of the cabinet?

    That seems like a pretty big voltage drift.
    Maybe the bias pots are bad.

    Power tubes out.
    Set meter for DC 500 volts again.
    Test pin 5 of both sockets again.
    Turn on power and set standby to warm-up.
    Try to adjust bias pots so that all sockets read -45 to -46 volts.

    When you adjust pin 5 voltage (turn the bias pot),
    the voltage should hold steady on pin 5.
    Did the voltage hold steady or does it drift all around when you adjust the pots?
    Are you able to adjust it to -45-46 volts DC?
     
    Last edited: Apr 16, 2019
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  13. Adieu

    Adieu Well-Known Member

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    It's a '99 jcm2k tsl100 head repackaged in a premium combo shell with great speakers... but still a tsl100 inside.

    Just with upside down labels, otherwise identical

    ...so isn't this just early board revision bias drift?
     
    Last edited: Apr 16, 2019
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  14. ampmadscientist

    ampmadscientist Well-Known Member

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    We are trying to document what readings will appear when the bias drifts.
    One of the symptoms is that the bias won't adjust, or drifts all over when (you try to) it's adjusted.
    So far that's been fixed on 4 out of 6 amps by replacing the bias pots.

    Rather that go by dates we are trying to get some hard information.

    But yes that's the bias defect board...I am going through tests to see if the board is good or bad. Or rather create a test to see if the board is good or bad.

    Utilizing the amps own power supplies to check if there is insulation breakdown.
    - 36 volts on start up was one of the symptoms of bad bias pots. Normally it's about -45 and holds steady.

    Also on several of these amps the speaker load was wrong that's why I asked him to test the cabinet w/ an ohm meter.

    If the speaker load is wrong: the current reading of the output tubes will also be wrong.
    Example amp at 16 ohms, speaker is 8 ohms = double the current reading, which throws the whole bias adjustment way off.
     
    Last edited: Apr 16, 2019
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  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Where are you dude?
    Did you give up?
     
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  16. Sapphire Sounds

    Sapphire Sounds New Member

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    I have a little time now and tomorrow evening to do this should I buy new bias pots? Heres a picture of the cabinet speaker wiring and some chinese food

    Speakers are stock mismatched 8 ohm celestions

    20190416_223348.jpg
    20190416_222746.jpg
     
    Last edited: Apr 16, 2019
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    1. Measure the end of the speaker cable and confirm that it reads 12.6 ohms. (meaning a 16 ohm load)

    Amp power on, standby in warm-up, tubes out.
    2. Measure pin 5 on all sockets.
    Turn the bias adjust pots and see if you can adjust the voltage to -46 VDC on all sockets.
    See if the voltage holds steady after you adjust it.
    Or if it keeps drifting around.

    Trying to decide if this is the bias pots gone bad or if the board is bad.
    Tell us what you get when you adjust it.
    Need to do more testing to see if the bias will hold steady...at the moment the bias voltage is drifting all around.
    Is this caused by bad pots? or is this a bad board?
     
  18. ampmadscientist

    ampmadscientist Well-Known Member

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    OK do us a favor here?
    Unplug power from amp.
    Take out all tubes.
    Set meter for high ohms. Set meter for: "1 MEG" or higher range.
    Measure between pin 5 and pin 4 on all output tube sockets.
    What is the ohm readings you get on all the output tube sockets?

    Measure ohms between pin 4 and pin 5 on the output tube sockets.
    1: Ohms reading climbs higher and higher : normal
    2. Ohms reading stays low and does not climb higher and higher: Board may be bad. (example reading 800 ohms steady between pins 4 and 5)

    Advisory:
    If you replace the bias pots, do not put in the power tubes yet.
    Do not turn standby to "operate" yet.

    Turn power on, set standby to warm-up. (do not turn standby to "operate.")
    Set meter for DC volts.
    Measure pin 5 on all output tube sockets.
    Adjust bias pots so that there is -46 volts on all output tube sockets pin 5.

    If the voltage is LOW on pin 5 (less than -46 volts after replacing / adjusting pots) the board might be junk.
    If the voltage on pin 5 will not adjust to -46 (turn bias pot), do not turn standby to operate.

    For example: you replaced the bias adjust pots, but the voltage on pin 5 is 30 volts (or even less than that), the board may be bad. So don't put in tubes or turn the standby to operate position.
     
    Last edited: Apr 20, 2019 at 9:44 AM

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