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Old 10th April 2008, 09:54 PM   #21
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Location: Louisiana
Please post the voltages on the two op-amps near the 3120s.

Op-amp closest to 3120s
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

Other op-amp
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:


Is the large copper area connected to R414 connected directly to the + speaker output?
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Old 10th April 2008, 10:27 PM   #22
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Quote:
Originally posted by Perry Babin
Please post the voltages on the two op-amps near the 3120s.

Op-amp closest to 3120s
Pin 1: 0.688v
Pin 2: 1.599v
Pin 3: 1.024v
Pin 4: -15.14v
Pin 5: 0.665v
Pin 6: 0.121v
Pin 7: 0.688v
Pin 8: 15.8v

Other op-amp
Pin 1: 0.827v
Pin 2: 0.41v
Pin 3: 0.271v
Pin 4: -15.8v
Pin 5: 0.274v
Pin 6: 0.412v
Pin 7: 0.829v
Pin 8: 15.8v


Is the large copper area connected to R414 connected directly to the + speaker output?

Yes, it is electronically connected.
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Old 10th April 2008, 11:22 PM   #23
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Location: Louisiana
Those are not what I'd expect. Even taking into account that the outputs have been removed, the voltages should be significantly different. For example, the two inputs to each op-amp should match. If they don't match within ~0.01v, the output should swing as far as it can (within the limits of the power supply) to try to make them match.

Since the last post, someone dropped off an MTX 81001D. It's close enough to use as a guide to help. I'm going to go through it to see if I can find anything that could produce the 8v on the output.

The following voltages are from the 810001D.

Op-amp closest to 3120s
Pin 1: -6.15
Pin 2: 0.034
Pin 3: 0.035
Pin 4: -15.32
Pin 5: -5.91
Pin 6: -5.86
Pin 7: -0.07
Pin 8: 15.07

Other op-amp
Pin 1: 0.018
Pin 2: 0.004
Pin 3: 0.003
Pin 4: -15.32
Pin 5: 0.005
Pin 6: 0.005
Pin 7: 0.011
Pin 8: 15.08

Does the 7801 have an LM311 near the 15v reg?
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Old 10th April 2008, 11:36 PM   #24
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Yes, there is a LM311, you can see it in picture P5020013 in the pictures of the 7801 in your repair tutorial.
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Old 11th April 2008, 01:00 AM   #25
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Location: Louisiana
I can't find anything that would cause 8v to appear on the outputs. It's a low current source so it's not a significant problem. When you power up the amp, connect a speaker or dummy load to the amp to keep the DC at a minimum. If you use a speaker, make sure that the DC is less than 1v when the amp is powered up.

At this point, I'd suggest that you reinstall the outputs and replace the two op-amps (NE5532s) near the 3120s.

Have you ever replaced any SMD ICs?

Do you have any kapton tape?

Has the red LED near the 3120s (LED400) ever lit up?
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Old 11th April 2008, 01:19 AM   #26
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I haven't replaced and SMD IC's yet, this will be my first. Is there a section I can look into in the tutorial?

I don't have any Kapton tape, I have been wanting to get some though, why do you ask?

And yes, LED 400 lights up every time about 3 seconds after remote power is applied.
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Old 11th April 2008, 01:45 AM   #27
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#12 on the video clips page. Read the notes under the image.

The area has VERY tight spacing between components. If you use the Kapton, you can cover the other parts so that the IC can't stick to them as you move the op-amp off of the pads. If you don't want to buy the tape, you can try lifting the IC after the solder is hot enough on ALL of the legs. You must be careful not to lift before the solder is hot enough. If you aren't patient, you could lift one or more of the pads.

As an alternative, you can desolder one side at a time. Apply additional solder to one row of legs and get the entire row hot by running the iron back and forth across the legs. When it's hot enough, use a small screwdriver to lift the chip to a 20-30 degree angle. Hold it there until the solder cools. Use desoldering braid to remove the solder from the side that you lifted. When you go to the other side, you'll lift the IC as soon as the solder is hot enough.
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Old 16th April 2008, 09:39 PM   #28
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Quote:
Originally posted by Perry Babin


Op-amp closest to 3120s
Pin 1: -13.84
Pin 2: 0.27
Pin 3: 0.008
Pin 4: -15.09
Pin 5: 0.001
Pin 6: 0.012
Pin 7: 0.152
Pin 8: 15.77

Other op-amp
Pin 1: 0.45
Pin 2: 0.45
Pin 3: 0.46
Pin 4: -14.71
Pin 5: 0.435
Pin 6: 0.009
Pin 7: 0.425
Pin 8: 16.22


Those are the new measurements after replacing the op-amps.

Now on the 3120 closest to the output fets I get:
Between pin 5-8 = -15.5V
Between pin 2-3 = 0.11V
Between pin 5-6 = -0.199V

Other 3120
Between pin 5-8 = -14.68V
Between pin 2-3 = 0.111V
Between pin 5-6 = -4.88V

I still get DC across the outputs, at first I got 1.5V DC, then after powering off the amp and powering it back on because the OP-Amp furthest from the 3120's was getting hot, the DC output now is 4.11V and 3.96V with a 220 ohm resistor across it.

Still no output. I tried to test the pins individually on the 3120's and this is what I got below:

3120 closest to output fets:

Pin 1:0.011
Pin 2:0.426
Pin 3:0.453
Pin 4:0.003
Pin 5:-84.2
Pin 6:-84.2
Pin 7:-84.2
Pin 8:-69

The other one:

Pin 1:0.009
Pin 2:0.269
Pin 3:0.376
Pin 4:0.014
And on pins 5 to 8 what ever it was caused my meter to go crazy, couldn't get any ready in DC, AC, or frequency on my DMM, it just made the screen go nuts. ?
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Old 16th April 2008, 11:25 PM   #29
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The op-amps appear to be working normally now but there's a problem with the voltage readings. When troubleshooting, you need accurate numbers.

If you look at the voltages on pin 4 of both op-amps and on pin 8 of both op-amps, you can see a big difference. If you check the board, you'll find that there's a direct connection from 4-4 and from 8-8. You shoud read precisely the same voltage on pin 8 of both op-amps and the same voltage on pin 4 of both op-amps. If there's more than a 0.01v difference, you need to determine why.

It appears that pin 1 and pin 2 of the op-amp (the one with 0.45v on those pins - let's call it op-amp A) may have the pins shorted together. If they are shorted together, fix that and then check for DC on the output.

When you measure the voltage between pins 5-6, 5-8 on the 3120s, place the black meter lead on pin 5.

The meter probably had trouble reading the voltage on pins 5 to 8 because they're floating and have a rail to rail square wave on them.
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Old 17th April 2008, 06:13 PM   #30
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Well, I replaced op-amp A, the one that looked shorted, and I checked for continuity between all the combinations on each op amp and theres nothing shorted, but the numbers did change, however it still gets pretty warm compared to the other op-amp closer to the 3120's

The new numbers are below.

Quote:
Originally posted by Perry Babin
Please post the voltages on the two op-amps near the 3120s.

Op-amp closest to 3120s
Pin 1: -5.76
Pin 2: 0.347
Pin 3: 0.337
Pin 4: -14.79
Pin 5: -5.29
Pin 6: -5.21
Pin 7: 0.245
Pin 8: 16.10

Other op-amp
Pin 1: -1.4
Pin 2: -0.38
Pin 3: -0.36
Pin 4: -14.80
Pin 5: 0.278
Pin 6: 0.278
Pin 7: 0.274
Pin 8: 16.11
And now the 3120 closest to the output fets
Black leads on 5
5-8 =+15.55V
5-6 =+0.42V

Other 3120
5-8 =+14.82V
5-6 =+5.59V

DC on the output is still present , now 5.82V

Could there be a faulty 3120 causing this trouble, if im not mistaken, shouldn't pin 5/6 and 2/3 have opposite polarities from each other?

The op-amps are NE5532DX's from Fairchild that I ordered, the originals were NE5532's.
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