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Scott 99C - No more sound

Discussion in 'Tube Audio' started by havols, Mar 31, 2018.

  1. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    Hi to all of Audiokarma

    Yesterday I was listening to My Scott 99C, 70% of the total power.
    At one point, I heard a small squeak, and the amplifier was muted.
    The amplifier remained on, with the filaments lit.
    I turned him off immediately. I felt a faint smell of burning ... i think it is bad ...
    Today I tested with Dim Bulb Test, and found no short-circuit signal.
    What could have happened? Did I lose the output transformer?
    He was using 2 6l6GC HP.

    Could someone help me to find the problem?
    Regards
     

     

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  2. trainbuftony

    trainbuftony Electron Herder Subscriber

    Probably didn't damage a transformer. A resistor or capacitor probably gave up and quit. Have you tried the other inputs?
     
  3. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    I have not tried the other inputs.
    I turned it off and decided to get some tips here in AK before turning it on again.
    I'll try.
    Thanks for reply trainbuftony
     
  4. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    I Take some measures.
    I have voltage on Pins 4, 6 by rectifier tube. But I have no voltages on B+ (around 0V) after rectifier tube (C1A, C1B), and points A, B, C on schematic.
    Here is the link by schematic

    Regards from Brazil
    Havols
     
  5. primosounds

    primosounds Powered with pure tube sounds. Subscriber

    Messages:
    2,793
    Location:
    Terra, 21st century CE
    Does not sound good. But maybe not transformer. Remove the B+ tap , going to the first filter cap after the rectifier. Remeasure the B+ with a rectifier back in the socket. Caution as the voltage will be much higher without any load. If you have a SSR, to replace the tube rectifier, and a variac, this is where you can use them and at much safer levels. With a variac you can put on 10vac and should see roughly 80vac at the B+ tap. If you do your PT is probably fine. If it is 0 vac your PT is toasted.
     
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  6. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    Thanks for reply Primosounds
    So. I have not SSR or Variac
    I just Have the buck transformer.
    To complement the post above, the voltages on PINs 4 - 6 by rectifier tube is around 330 VAC each one.
    Wish i have to remove B+ on PIN 8 from rectifier socket? or between 33ohm - 160ohm?
    Regards
     

     

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  7. RUmad

    RUmad AK Subscriber Subscriber

    Messages:
    942
    Location:
    New York
    Ok, silly question, but if you've got voltage from the trans at the rectifier and no b+ coming out; have you swapped in another rectifier tube?
     
  8. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    RUmad
    i swapped the rectifier tube by another ones.. that I think is fine. Not new rectifier tubes.. but i think it is good tubes. I have no tube tester...
    Can be a Bad Filter Caps?
    Regards
     
  9. RUmad

    RUmad AK Subscriber Subscriber

    Messages:
    942
    Location:
    New York
    Ok. I would think if you have a bad cap you’d still be getting a reading from the rectifier. Let’s wait for somebody smarter than me to chime in.
     
  10. BillWojo

    BillWojo AK Subscriber Subscriber

    Messages:
    2,788
    Location:
    Burlington, NJ
    Are you checking the B+ on DC mode? Yea, silly question but I don't know your expertise.

    BillWojo
     
  11. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    Thanks RUmad
    Bill. Yes, a checked on DC mode. The B+ on C1A is around 7 Volts (DC)
    I removed the rectifier tube, and measured voltages on secondaries of power TAP.
    The secondaries seems fine. 5VAC, 345VAC - 0 - 345VAC, 6.2VAC
    Can be a problem on socket? maybe i need to buy a new one rectifier tube.
    Regards
     
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  12. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
  13. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
    3,176
    Location:
    Fernandina Beach, FL, USA
    If you have swapped in another rectifier tube and still have no voltage, it may be that the first filter has shorted. Especially if it is original. A shorted filter capacitor, electrolytic capacitor will kill the DC coming from the 5U4.
     
  14. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    Thanks kvflyer
    The filter caps was replaced in the past, by someone.
    I will replace the 4 - 20uf caps from C1 by new ones.
    then i will post results here
    i just hope that my power tap and output tap are good... :no:
    Regards
     
  15. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    Hi kvflyer
    I replaced the 4 C1 main filter caps by new ones. The problem stills the same
    No B+
    How can i do a test on Power Transformer and Output Transformer?

    In the power TAP, I take out the Rectifier tube, and take some voltages: The 3 secondaries are OK (5VAC, 345VAC, 6.3VAC)

    But how can i test the output transformer?

    Any further help to find the problem with the scott 99C, which is with B + = 5VDC.

    regards
     
  16. RWood

    RWood Nobody goes there anymore. It's too crowded. Subscriber

    Messages:
    1,527
    Location:
    Richmond, VA
    If you are getting AC voltages on all of the PT secondaries, that is your test. It passed.

    OT initial test on the primary is to see if you have similar resistance from the CT to one plate connection as you do on the other. Use ohm meter for this, pull power tubes, ohm from the red CT to the blue, then to the brown (pins 3 on the power tubes)

    Try what Don was saying - put in a rectifier tube BUT disconnect the DC out. Your pins 2 and 8 should only have the 5V filament connection.

    Connect a DC voltmeter between pin 8 and ground. Should get 450 or so VDC. If you don't get DC try a different rectifier. If you do get DC then the problem is downstream from there. Start by replacing the first filter cap and make sure it is grounded in the same place where the PT CT is grounded.
     
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  17. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    Thanks for reply RWood.
    Thanks primosounds for the first suggestion.
    the output transformer are fine, i think.
    The resistance in both sides to center tap is: 160 ohm on blue and 183 ohm on brown.

    I disconnected the DC out.
    After swapped 3 times the rectifier tube, measured the secondaries voltages.
    With 114VAC on the wall, i found: 4.7 VAC, 6.0 VAC and 375 VAC - 0 - 375 VAC (too low??)
    Without rectifier tube, i found 335VAC on PINS 4 - 6 of rectifier tube. (114VAC on the wall)

    Well, so i reconnect the DC out, and power on
    the B+ was low, like 180V DC. And i listened a big noise on the output speaker. Turn off quickly.

    I think the both TAPS are fine, right?
    If yeas, what can be wrong?
    Regards
    Havols
     
  18. triode17

    triode17 Super Member

    Messages:
    1,355
    Good point. Check to see if the 33 ohms resistor gets hot when you power the amp.
     
  19. triode17

    triode17 Super Member

    Messages:
    1,355
    What about the resistance of the high voltage secondaries? Do they read 35 and 40 Ohms per the schematic? There could be a high resistance in the center tap transformer lead.
     
  20. havols

    havols Active Member

    Messages:
    242
    Location:
    Manaus, Brazil
    Thanks triode17

    I think yes
    With thermometer, like a 230 F (110 Celsius) after 10 seconds power on.
    After that I turned it off, because something is wrong on this amp to keep it on for a long time.
     

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