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G&K Backline 600

Started by gbono, February 28, 2015, 03:39:55 PM

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gbono

The initial fault was traced to Q24-26 which were shorted. Replaced three output transistors and found Q20 (+15 rail series regulator) getting hot. It appears there is an issue in the mute/fault circuit since I can disconnect J2 and still have Q20 getting hot.

Can S1 (thermal switch?) fail? I assume it should be normally closed.
I haven't measured voltages on Q10,12,13 but Q10 EB measures short with DMM check though I'm assuming that S1 closed?

How does this mute/fault circuit work?  I assume that pins 2 and 6 in the 555 must be held in some "state" to get output at pin 3???


https://docs.google.com/folderview?id=0BwnWRG81HCWEYmJjMDFkYzUtOGZiYy00OTEwLThmM2QtYWMxMjllZTk5ODcx&usp=drive_web

g1

Quote from: gbono on February 28, 2015, 03:39:55 PM
found Q20 (+15 rail series regulator) getting hot. It appears there is an issue in the mute/fault circuit since I can disconnect J2 and still have Q20 getting hot.
Are Q13 or U1 getting hot?  With J2 disconnected, what kind of resistance do you measure from +15 to ground?
Everything else in the Q20 circuit is ok?
The protect circuit sends the mute signal to R8 (near Q4).  And turns on the protect light.

gbono

only Q20get hot not Q13 or the timer and the zener diode, caps also were okay. Something is loading down the +15V rail but what?

gbono

#3
Attached the schematic.

Briefly checked to output waveform and only getting conduction during negative half of input sine wave. Multiple problems?

Roly

1) are you using a limiting lamp?

This is a must.

2) did you remove and check out-of-circuit a) the driver transistors Q14 & Q18, and b) the other main output transistors, when doing the original repair?

3) what is the idle voltage on the output (at speaker sockets or inductor L1)?

4) this circuit is helpfully splattered with test voltages; have you measured any/all and confirmed?


Quote from: gbonoBriefly checked to output waveform and only getting conduction during negative half of input sine wave.

Did you check what was going in?  Was that a full sinewave, or only a half wave?
If you say theory and practice don't agree you haven't applied enough theory.

g1

  And is the +15V holding up or not?  What does it measure?

gbono

#6
I always connect damaged or suspect gear to a LBL connected to a VARIAC.

I found R34 open and will replace. This may explain the issue with the output only conducting on negative cycle of an applied sine wave.

The +15 rail measures 15volts so it isn't sagging.

Once I replace R34 I will start testing again.

Roly

Quote from: gbonoI always connect damaged or suspect gear to a LBL connected to a VARIAC.

Wise, particularly with a larger amp like this.


Quote from: gbonoI found R34 open

Ah ha.  That's only one of half a dozen fusible resistors in the vicinity.  Easy to overlook if they don't show any signs of distress.
If you say theory and practice don't agree you haven't applied enough theory.