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Old 17th October 2012, 11:31 AM   #11
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Location: Louisiana
Did you check the transistors?

Were any defective?
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Old 19th October 2012, 03:00 PM   #12
wrxer is offline wrxer  Australia
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Location: perth, west australia
I checked them in post 3, and thought i had a defective one in channel 3.

But checking them now that they are removed, shows they are OK. I dont understand. So was one of the outputs being driven on, resulting in the bias?

CH3:
TIP35,
legs 1-2. 0.553V. legs 1-3, 0.553V. legs 2-3, open.
TIP36,
legs 1-2. 0.566V. legs 1-3, 0.567V. legs 2-3, open.

CH4:
TIP35,
legs 1-2. 0.549V. legs 1-3, 0.549V. legs 2-3, open.
TIP36,
legs 1-2. 0.565V. legs 1-3, 0.566V. legs 2-3, open.
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Old 19th October 2012, 03:53 PM   #13
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You cannot rely on the readings in the board. Checking in the board is mainly to determine if any are shorted.

Those transistors appear to be OK.

Reinstall the outputs and re-read post #9.
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Old 21st October 2012, 11:21 AM   #14
wrxer is offline wrxer  Australia
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The problem is definately ch3. I replaced the outputs in ch4 first and it powers up fine without the outputs in ch3.
Then replaced outputs in ch3 and the amp returns to going into protect.
Also, it has a seperate input board that was removed in the pics. This board is the preamp board for ch3 and 4. The preamp section for ch1 and 2 is on the main board. With the board disconnected from the amp, it powers up fine and doesnt go into protect.

Also the emitter resistors for ch3 are the only ones that have voltage (around 7v) the others have around 0.05v

Yep, I have pulled R169, and replaced the outputs. It runs for longer than before, but still goes into protect.
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Old 21st October 2012, 12:02 PM   #15
wrxer is offline wrxer  Australia
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This is the board. the coloured wires are the rca wires for ch1 and 2, and the +16v and -16v
DSCN4386.JPG
If i disconnect the white wires the amp fires up and idles.
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Old 21st October 2012, 04:23 PM   #16
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If it remains on with the preamp board out of the circuit, remove the outputs for channel 3 so that you can power it up and it will remain on. Measure the DC voltage on the output pins (pins 1 and 7 for 8-pin op-amps, pins 2 and 8 for 9-pin op-amps). Which ones have more than a fraction of a volt. Check all of the op-amps.
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Old 22nd October 2012, 03:25 PM   #17
wrxer is offline wrxer  Australia
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Ok, op-amps hurt my head. I will reread the section on the disc another few times.

There are 5, 8pin njm4558l op-amps on the board. I measured them anticlockwise from top right. so U15, U14, U13, U4 then U2.

U15: pin1, 15.27v. pin7, 14.94v.
U14: pin1, 0.002v. pin7, 0.002v.
U13: pin1, 0.000v. pin7, 0.000v.
U4: pin1, 0.000v. pin7, 0.000v.
U2: pin1, 0.001v. pin7, 0.001v.

Rechecking U14 again.
pin1, 15.3v
pin4, -15.91v
pin7, 14.92v
pin8, 15.96v
Also, pin1 varies around 0.02v when changing the 'bass boost' switch from 0db to +6db to
+12db.
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Old 22nd October 2012, 03:28 PM   #18
wrxer is offline wrxer  Australia
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Attached is the corner of the preamp board. I have transposed the traces from the underside. It may not be very clear about the connections. I hope it is ok.
DSCN4386b copy.jpg
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Old 22nd October 2012, 04:57 PM   #19
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Post the DC voltage on all pins of U14 and U15.
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Old 24th October 2012, 02:36 PM   #20
wrxer is offline wrxer  Australia
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Location: perth, west australia
It is throwing into protect when I try to probe the other pins. I probably could get it to idle by removing ch4 outputs. But I have unplugged the output side of the preamp board. It seems to be stable and is receiving regulated power to the op-amps.
I hope that is ok.

U15:
1) 15.2v,
2) 11.55v,
3) 0.31v,
4) -15.88v,
5) 0.32v,
6) 12.14v,
7) 15.23v,
8) 15.92v.

U14:
1) 0v,
2) 0v,
3) 0v,
4) -15.90v,
5) 0v,
6) 0v,
7) 0v,
8) 15.93v.
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