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Old 1st July 2013, 08:11 PM   #171
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Is your scope set to 20v max volt/div..
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Old 2nd July 2013, 01:33 AM   #172
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Quote:
Originally Posted by amptech View Post
Is your scope set to 20v max volt/div..
I don't know. I have all the switches all the way to the top.
This picture is basically how it was set except it was set to chop and of course the Volts/Div and Time/div were maybe a little different. I was using it to work on a pre-amp today so those got moved.

Click the image to open in full size.
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Old 2nd July 2013, 04:27 AM   #173
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Your scope settings...
CH1 is set at 1V/div

CH2 is also set at 1V/div but it is now in uncalibrated mode. Turn the red knob clockwise to the click end and the UNCAL indicator on its right should go off.

Same is the case with sweep setting. The knob on the right that sets the time/div has a red knob. Turn it clockwise to the click position and the UNCAL indicator on its left will go off.

Gajanan Phadte

Last edited by gmphadte; 2nd July 2013 at 04:28 AM. Reason: spelling
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Old 2nd July 2013, 08:56 AM   #174
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Hi Terry, looking at post number 167 your meter's appear to be giving different readings with one under 4 ac volts and the other 86v... If the amplifier was giving such high o/p your jbl's would be cooked at that power. your need a heavy duty test load to check power out put of amplifier.

regards A.
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Old 2nd July 2013, 12:30 PM   #175
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Quote:
Originally Posted by amptech View Post
Hi Terry, looking at post number 167 your meter's appear to be giving different readings with one under 4 ac volts and the other 86v... If the amplifier was giving such high o/p your jbl's would be cooked at that power. your need a heavy duty test load to check power out put of amplifier.

regards A.
Mabe I wasn't clear. I didn't have anything hooked up to the output except for the scope and DVM. The meter on the left is hooked up to the input with the sine generator and the meter on the right is hooked up to the speaker jack. I do have a dummy load coming should be here any day. Once I have it I will run the test again
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Old 2nd July 2013, 01:53 PM   #176
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Quote:
Originally Posted by gmphadte View Post
Your scope settings...
CH1 is set at 1V/div

CH2 is also set at 1V/div but it is now in uncalibrated mode. Turn the red knob clockwise to the click end and the UNCAL indicator on its right should go off.

Same is the case with sweep setting. The knob on the right that sets the time/div has a red knob. Turn it clockwise to the click position and the UNCAL indicator on its left will go off.

Gajanan Phadte
Hi Gajanan,

Thanks for the info. I have had no instruction in using a scope aside from what I have picked up here from little things like this. I should probably look up some tutorials or something. I was using the little red nobs to adjust the height of the wave to try and fit both waves on the screen. I didn't know they should be locked. This is probably a better picture of how it was set when I was testing the leach but I'm not sure it is clear enough to see any of the settings.
Click the image to open in full size.

Thanks again
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Old 3rd July 2013, 02:19 AM   #177
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The ghost is back. It is a very puzzling thing. I had the amp playing all morning without a hitch. When the mail came my 300w, 8R resistor was in it so I was pretty excited about hooking it up to do some tests. Well I checked the right channel with the resistor hooked up to the outlet and I used a preamp so I could get a higher voltage on the input. I was able to get 59VAC on the output before it clipped. That was with 2.7VAC on the input. I then went about hooking up the left channel but when I powered it up it blew the fuse on the +rail. I figured I might have shorted the output while hooking everything up so I replaced the fuse and fired it up again. I checked all the voltages and everything looked good so I hooked up the input and began running up the sine wave. When I get to about 20Vac on the output is see the clipping start again. I can't believe my eyes. To test, I disconnect the dummy load and the clipping goes away. I reconnect and it's back. So recheck all the voltages and all looks right. If I run it up without a load everything looks fine. Another weird thing was I connected the speaker once and It went all the way to 59VAC on the output without clipping. So I rehook the dummy load and it clips again. However when I tried hooking up the speaker again it clipped this time. The other thing is that the heatsink got really hot. I though maybe the bias has changed so I rechecked that but it was good.

Any ideas? Any suggestions why it is only clipping on the bottom side of the sinewave?
Click the image to open in full size.

Thanks, Terry
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Old 3rd July 2013, 02:32 AM   #178
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Looks like oscillation, not clipping. That also explains the extra heat and blowing of fuses. Something is definitely amiss and causing a stability issue.
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Old 3rd July 2013, 03:56 AM   #179
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I misspoke earlier. the 59VAC was with no load. It was only 47VAC with the load attached.
Click the image to open in full size.
Click the image to open in full size.

So you think the oscillation could be caused by Q1-Q4 not being matched. I have been meaning to change those out. I will do that and see how it goes.
Thanks
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Old 3rd July 2013, 04:04 AM   #180
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OK, based on the formula in post #170 I have 47V/8R = 5.875 * 47V = 276W

That is pretty close to what the Superamp is rated for.
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