Welcome, Guest. Please login or register.

July 16, 2020, 01:08:53 PM

Login with username, password and session length

Recent Posts

 

Author Topic: Backline 600 no output  (Read 308 times)

gbono

  • Elite
  • ****
  • Posts: 277
  • Chip Points: 7
    • View Profile
Backline 600 no output
« on: May 25, 2020, 05:52:49 PM »
G&K backline 600 which initially required new output transistors (drivers also changed). Amp powers up, supply voltages look good +-50V, +-14V, etc., and has strong signal into power amp board (base of Q7a) but no signal at output or at D6 anode.

I believe the issue is the mute/fault circuit. Voltage across R4 is 0V. Checked all transistors in fault/mute circuits but nothing is damaged. Amp powers up and preamp stage works so power on and protect is working but no output voltage on mute node?? How does the mute/fault work on this design?

Enzo

  • Legendary
  • ******
  • Posts: 1953
  • Chip Points: 209
    • View Profile
Re: Backline 600 no output
« Reply #1 on: May 26, 2020, 05:11:48 AM »
No signal at D6 anode.  OK, how about at its cathode?  If it is there, then Q7 is working, which means the mute it working.

Look top left of power amp drawing section.  I see Q5,9,17, but more important I see DC voltages.  R17, 15, 27, 31.  Note those voltages are ACROSS the resistors, NOT to ground.  Got those?

Prolly not, if R4 is at zero.

First things first I guess.  Zero across R4?  What voltage is ON R4?  Should see V-.  An open R33 would remove V- from the lower left corner.

How does it work?  Signal MUTE has to go positive to turn on the amp.  A positive there will turn on both Q3,4, with completes current path for Q7s.

Upper right is where MUTE signal comes from.  Is it some positive voltage or zero?  Q13 controls MUTE.  To get it turned on, we need teh base to get dragged down more negative than the emitter.  IS it?

In MUTE section upper right, look lower right of that section Q10,12.  If either of those turns on, it shuts down the amp.  Q12 is turned on by excess current in the powr amp.  Q10 is interesting.  S1 in teh base circuit is a thermostat on the heat sink.  Normally it is closed and grounds the base so the Q10 is not on.  If the amp overheats, S1 opens allowing R24 to turn on Q10 and shuts down teh amp.

gbono

  • Elite
  • ****
  • Posts: 277
  • Chip Points: 7
    • View Profile
Re: Backline 600 no output
« Reply #2 on: May 27, 2020, 04:41:32 PM »
Thanks Enzo ... There was a 13V trigger coming out of the mute circuit to R8 but zero volts across R4. As you suggested, R33 was open (R34 is good). Once I get a few 10 ohm fusible resistors from Mouser I will see what happens next.