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Little Dot MK8SE / MK6 Super Mods (All verified mods are on first page)

Discussion in 'Headphone Amps (full-size)' started by redge78, Sep 26, 2015.
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  1. SonicTrance
    Yes, there is! The 5998!
     
  2. baronbeehive
    Yes I know, I was keeping quiet about them, I know where I can get some. I dare say I will probably end up with those as a compromise..... with the bias mod suitably adjusted!

    I think you said you liked them but they were not quite as dynamic as the 421A's.
     
  3. SonicTrance
    Right! I still have a NOS quad (+ 1 spare) WE421A.

    If you're interested shoot me a pm.
     
  4. baronbeehive
    Thanks Sonic, but it's not the right time. I intend to live with these 421A's for a while, then decide between these and some 5998's. I think I can get the 5998's for one third the price of 421A's,that will probably clinch it for me!

    Another interesting tube is the TS 6080 and 6AS7 but they're impossible to get hold of.
     
  5. baronbeehive
    Still trying to find the problem and I have a couple of questions:

    - The amp meters read 60mA, left meter, and 30mA, right meter with 6AS7G's in
    - Power/driver anode/cathode readings all good
    - Both terminals on the meters read 100v
    - Both legs of the .68R in the middle of board going to left channel read 100v
    - The sound appears to be all good
    - the only discrepancies in voltage readings appear to be in the meter lighting circuit and so surely that is not important

    Bearing that in mind where could the problem be and where on the board are the 2 circuits for the right channel connected?

    Could the meter itself be a problem? This is not the meter that I changed.

    I'm wondering if the output of only one circuit is getting through. I'm at a bit of a loss here.

    Edit: I did notice that the resistance over the terminals of the right meter is twice that of the left meter, which could indicate a fault further down the line but I can't find any faults anywhere.
     
    Last edited: Aug 12, 2018
  6. baronbeehive
    I've drawn a conclusion about this:

    The voltages at the anodes/cathodes are good
    The voltages at the meter terminals are both the same
    The resistance across the meter terminals with the faulty reading is way higher than the other, new, meter
    I can't find any fault with anything else.. so...

    ... the meter must be faulty, probaly gone open due to a voltage surge either when the capacitor broke off or due to the mistake with the resistors

    I was probably right about a meter fault before but for the wrong reason - so the amp is off to the engineer again to get the other new meter soldered in place...... hope I'm right about this…..
     
  7. bloodhawk
    Did you check if the new meter has a resistor soldered in line?
     
  8. baronbeehive
    Do you mean the resistor on the end of the red lighting wire?

    That one is still there, haven't checked the value though. But I measured across the terminals above that resistor, not from the other end of the 2 wires going into the terminals.
     
    Last edited: Aug 15, 2018
  9. bloodhawk
    Nope there is a resistor in line with one of the terminals on the ammeter. If i remember correctly its a 68Ohm resistor in series on the positive terminal of the meter. Its pretty well heat shrunk, so hard to notice.
     
  10. baronbeehive
    upload_2018-8-15_21-29-41.png

    You mean this one?
     
  11. bloodhawk
    Looks like it yeap.
     
  12. baronbeehive
    upload_2018-8-16_9-48-46.png

    Yes, that's the low voltage backlight circuit, with the resistor to lower the voltage, which you found when you were working on your amp. The other circuit is the power circuit coming off those 2 x 0.68R resistors, which I mistakenly thought were 0.68k, which could have blown open the meters. The fact that the resistance across those terminals is different must result in the different current at the meter.... I hope!

    Edit: you were right the resistor under the heatshrink is 68R.
     
    Last edited: Aug 16, 2018
  13. SonicTrance
    The 68 ohm resistor is a current limiting resistor. So not to damage the backlights at startup.
     
  14. baronbeehive
    Well.... it was the meter, 2 new meters installed and everything's fine now thankfully!
     
  15. SonicTrance
    I’m glad you got it sorted Baron!
     
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