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Old 9th November 2007, 10:00 PM   #11
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Go to eBay and search for:
10x lighted magnifier

For inspecting solder connections on SMD components and reading the numbers or color codes on resistors (particularly the 1% resistors with the dark blue bodies), these work very well. For viewing anything larger than ~1/2", you'll need something else.
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Old 10th November 2007, 02:11 AM   #12
jol50 is offline jol50  United States
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I looked at it for a while and have not found anything yet. I did find a SMD resistor that was replaced way over by the power section, right under the large transformer you can see the solders for. Thinking I will have to test and try to trace back voltage differences, maybe check some things first.

I took a cruddy pic of things. The "spot" SMD says ALY on it, then sideways 03. Very hard to see, that magnifier looks like a great idea! Hope nobody has a dialup...this pic is the board:
Click the image to open in full size.

Here is half with hot sink at left, pot for it highlighted, upper right is a box with the spot in it near RCAs.
Click the image to open in full size.

And this is close up of spot. It was covering the solder too until I hit it with my tester. It could just be ink, it is that deep purple color.
Click the image to open in full size.
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Old 12th November 2007, 03:17 PM   #13
jol50 is offline jol50  United States
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I got some batteries for a better tester I have, a BK Precision. Should be easier to test stuff, I guess I'll start checking things on the board. I found two resistors replaced under the coil there, or at least the solder is not shiny like the rest but they did a good job. I don't yet know what that little section of components is for, unless it's some kind of mod but seems unlikely with this type of amp. Naturally I'd hoped the mosfets would have fixed it but I guess not. Also don't have a scope to set the bias if I do figure it out.
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Old 12th November 2007, 06:22 PM   #14
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You don't need a scope to set the biasing. You use the voltage drop across the large white source resistors. Each resistor has two resistive elements. The center leg is common. To determine if the current flow is equal in all of the transistors, you need to measure the voltage across each of the 6 resistors for the channel.

On the good channel, what's the lowest voltage you can get across the source resistors with the pot set at the lowest bias level?

What's the lowest you can get on the channel that's overheating?

You'll have to set you meter to the lowest range if it's not auto-ranging. The voltage will be well under 1 volt.
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Old 12th November 2007, 06:33 PM   #15
jol50 is offline jol50  United States
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I will check it out. Did test the resistors across and they were all the same ohms if that matters (end to end), I see they have three legs. This meter is auto, much better than the clark I was using. Had to find those little 44 batteries.

Once fixed I could try to match it with the good channel since it should not have been moved, but I don't know the proper setting via meter unless I buy the manual...might have to. And here I just bought another amp cheap that has damage...
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Old 13th November 2007, 03:22 AM   #16
jol50 is offline jol50  United States
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Ok, I checked and the hot side was from 6.5-9mv and the cool side 3-5mv. Re-reading the post I let it cool a minute (not that warm) and set both bias all the way down...and they are all 0mv and it does not get hot The rails were at 48v +/-. I then checked the gate voltage (to chassis -) and the P were 2.7 and N was 2.25 on both channels, all very close to those numbers.

When I had moved the hot side bias down before, I was looking at speaker terminal voltage and the block takes a little to warm/cool.

Did I assume nobody had touched the gain pots or might there still be an issue? Both pots were near the same place, but they are not the same now full minimum. Do I set bias and try it...but don't know what to set it at since what I thought was the good channel may have been fiddled with too, if that is all that was/is wrong.
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Old 13th November 2007, 03:28 AM   #17
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Slowly increase the bias while watching the voltage across any one of the source resistors. Increase the biasing until the voltage is ~0.001 volts across the resistor. After checking one, check the rest to confirm that they're all the same (or close) for that channel.

Do the same for the other channel.

The amp will run cool at idle set this way. This should be enough bias current to eliminate crossover/notch distortion.
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Old 13th November 2007, 03:39 AM   #18
jol50 is offline jol50  United States
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I will try it, though its getting late here. You don't think replacing the power mosfets would affect the bias do you?
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Old 13th November 2007, 03:41 AM   #19
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The replacement outputs would probably have a slightly different threshold voltage so the bias current would probably have to be adjusted after they were replaced.
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Old 13th November 2007, 03:56 AM   #20
jol50 is offline jol50  United States
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Thanks again, I will try that out next session and let everyone know what it does. If it works, well I never said I was very good at this...but that is how you learn.

I could use this for a sub amp, but actually am trying to get a class D that has less power draw. It would need obvious distortion to notice on subs.
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