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Old 14th November 2012, 01:10 AM   #11
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Sorry Perry, they were of bad quality. I have tried again. I could not get a good focus on the screen when trying to capture the knob setting in the photo. The volts are all set at 5 volts per division on each shot and the time division is set to 5 us. one of the transistors on the heatsink gets real hot while it is running, it is all I can do to keep it held tight to the heat sink, the other 3 feel good. The transistor is good, I just replaced that same one that fried the other day. Thanks for any help and I hope the pics are better this time.

Kelvin
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File Type: jpg IMG_0668.jpg (611.2 KB, 49 views)
File Type: jpg IMG_0669.jpg (598.4 KB, 50 views)
File Type: jpg IMG_0667.jpg (632.9 KB, 38 views)
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Old 14th November 2012, 01:14 AM   #12
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Sorry they were rotated wrong again, my wife will be the first to tell you that I am horrible at following instructions, LOL. The one pic is of the power supply at almost idle, then next is the power supply while it is producing that distorted sin wave in the last pic
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Old 14th November 2012, 01:20 AM   #13
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Is the scope locking on the waveform or does it look precisely as captured in the photo?
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Old 14th November 2012, 01:32 AM   #14
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Sorry, just verified. The scope was set to lock
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Old 14th November 2012, 07:22 AM   #15
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I don't know what you mean.

By 'locked', I mean that the waveform was perfectly still on the display and there was only one instance of the waveform, not multiples like you have. The attached photo shows a locked waveform.
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Old 14th November 2012, 08:17 AM   #16
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It almost looks like the ground for the scope is not making good contact with the amplifiers source ground???
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Old 14th November 2012, 01:15 PM   #17
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I agree about the grounding issue with the last pic of the sine wave output. Since this is a mono amp and does not have the non bridged speaker terminals that I typically use for ground when taking measurements in the audio section I was just relying on the ground between the pyramid power supply. I need to find a good grounding point off the high side of the transformer and try again. The first pic of the power supply was locked in nice (this was what the power supply looked like right before I played the sine wave). The middle pic was jittering from left to right as the sine wave played, pretty much at the peaks and valleys of the wave form where it looks fuzzy. The last pic jittered up and down and was not properly grounded to the step up side of the transformer. I tore back into the amp last night and I think I may have found what might have been causing this particular problem. There are 3 resistors on each side of the pre driver section, a 75 ohm a 240 ohm and a 470 ohm. The 75 and 240 resistors where way out I tolerance. I noticed that they appeared to be in series and I had wired an equivalent 320 at what I thought were their end points. As I went through trying to jot out a circuit diagram, I found a trace that was connect to where these two resistors appeared to just be ties together in series which pretty much left one of the pre drivers without a gate resistor. It's ashame we don't have a handy store nearby that sells surface mount parts and has a good supply of transistors. I guess I will go by the mall and pick up some potentiometers to temporary wire in to make sure this takes care of the problem before I order the correct parts online. Also, I notice this amp appears to have a trimmer pot in the audio section, I didn't think class D amps had the bias pots, I assume you set it the same way as the class AB pots?
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Old 14th November 2012, 01:18 PM   #18
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Don't turn any pots in the amp.

The amp's main ground is the ground you need to use when probing the various points.
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Old 14th November 2012, 01:28 PM   #19
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Thanks Perry, lets say that I have hypothetically already tweaked the said pot mentioned above. How would one go about getting it set back to where it should be. I believe it was pretty close to being turned completely down.
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Old 14th November 2012, 01:36 PM   #20
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If it was the DC offset pot, you can readjust it. If it's the protection circuit pot, it may not be easy to get right without specs from JL. If it has only one blue trimmer pot. That's likely DC offset and can be set after the amp is fully repaired.
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