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Old 24th October 2012, 01:38 PM   #21
wrxer is offline wrxer  Australia
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Join Date: Nov 2011
Location: perth, west australia
Quote:
Originally Posted by wrxer View Post
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.
Sorry guys, there is a typo here. I rechecked U15 not U14. Sorry for any inconvenience.
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Old 24th October 2012, 09:12 PM   #22
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Location: Louisiana
Post the voltages again. Copy and paste the following for each IC. Look up the datasheet for the IC for the pin numbering scheme or look at the audio troubleshooting page of the tutorial, items 12-15.

IC#
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
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Old 25th October 2012, 12:24 AM   #23
wrxer is offline wrxer  Australia
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Join Date: Nov 2011
Location: perth, west australia
Quote:
Originally Posted by wrxer View Post
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.
Sorry, like this?
IC# U15
Pin 1: 15.2v
Pin 2: 11.55v
Pin 3: 0.31v
Pin 4: -15.88v
Pin 5: 0.32v
Pin 6: 12.14v
Pin 7: 15.23v
Pin 8: 15.92v

IC# U14
Pin 1: 0v
Pin 2: 0v
Pin 3: 0v
Pin 4: -15.90v
Pin 5: 0v
Pin 6: 0v
Pin 7: 0v
Pin 8: 15.93v

I have found the part you refer to in 12-15, also in the beyond the basics. I just need to read it another few times for it to sink in.
Cheers
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Old 25th October 2012, 01:56 AM   #24
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U15 appears to be defective.
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Old 25th October 2012, 11:58 AM   #25
wrxer is offline wrxer  Australia
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Join Date: Nov 2011
Location: perth, west australia
Thankyou, I have ordered a couple of the op-amps.
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Old 13th November 2012, 01:26 PM   #26
wrxer is offline wrxer  Australia
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Join Date: Nov 2011
Location: perth, west australia
op amp has arrived. From a company called littlediode.com
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Old 15th November 2012, 12:53 PM   #27
wrxer is offline wrxer  Australia
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Location: perth, west australia
I have now replaced U15. The amp is better, but still a bit unstable. Looking at the bias on the outputs with the scope, It varies up and down like it is trying to get back to 0v, and sometimes it throws into protect.
this is on ch4, so is the same side of the amp. The ch1-2 side bias is dead flat on the speaker terminals.
I will retest U15
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Old 16th November 2012, 04:48 AM   #28
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Post the DC voltage on all pins of U14 and 15 if you can't find the problem.
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Old 18th November 2012, 12:02 PM   #29
wrxer is offline wrxer  Australia
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There is some issue related to the Bassboost area of the preamp. U15 is the opamp involved.
All the other opamps on the Ch3-4 preamp board show -15v and +15v on pins 4 and 8, and 0v on all other pins. This includes U14.
U15 has different voltages when the switch is changed from 0db to +6 and +12db
IC# U15 @0db
Pin 1: 1.5-1.8v
Pin 2: 1.3-2.2v
Pin 3: 0.42v
Pin 4: -15.88v
Pin 5: 0.42v
Pin 6: 0.4-2v
Pin 7: 1.8-2v
Pin 8: 15.92v

With the 12db Bassboost selected, the voltage varies on the pins. I have taken a pic of the scope from pin4 as a example:
DSCN4405.JPG
Its a bit hard to read, but it switches every 0.45 secs (0.1 sec vertical divisions), with voltage around 7v and -7v (10v horizontal divisions)

With the 6db Bassboost selected, the voltage varies on the pins. I have taken a pic of the scope from pin4 as a example:
DSCN4406.JPG
Its a bit hard to read, but it switches every 0.5 secs (0.1 sec vertical divisions), with voltage around 9v and -9v (10v horizontal divisions)

No other settings seem to affects the opamp
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Old 18th November 2012, 12:59 PM   #30
wrxer is offline wrxer  Australia
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The scope captures are labelled wrong in the post above, the top pic is 12db but it switches every 0.5 sec and voltage is +-9v.
The lower pic above shows 6db and switches every 0.45sec and voltage is +-7v.
Sorry about that...

From post 18 this thread:
Attachment 307693
and a newer clearer pic, most of the parts in this area seem to have a oscillating voltage.The switch at bottom right is the bassbost
DSCN4408a.JPG

Thanks for looking.

Last edited by wrxer; 18th November 2012 at 01:06 PM.
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