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Old 4th June 2008, 12:21 PM   #41
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Default Clc Psu

Hi Karl,

I removed the earth from the PSU last night. Then I took a stab at it with the probe connected between the rectifier and the first set of capacitor banks. It was difficult to make out the exact value. I just adjust the Volt/Div and Sec/Div until I saw a couple of /\/\/ not sine wave the upward curve is steep but the downward curve is not steep at all. (Will take picture tonight it was 12:45pm late and felt very tired). But when I move the probe to after the inductor coil and the second bank of capacitors it was even more difficult to get the display properly no matter how I adjust the dials. May be I need to use the 10X probe? I am still waiting for it in the post.

I may try different 2N3055 one at a time to see if it get any better first before trying different emitter resistors.

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Old 4th June 2008, 06:03 PM   #42
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Hi Chris,
What do mean you "removed the earth"
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Old 4th June 2008, 06:20 PM   #43
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O.K. bit more time now. One of those days
Connect your 'scope ground lead on the probe to the PSU zero volt line, that's the negative end of C6 and the positive end C7 on your circuit or just to the negative speaker socket if you prefer. If you set your scope to say 5 volts/ div and AC coupled so that the trace stays central and is not offset by the rail voltage you should be able to read the ripple. Set the timebase to around 5 milliseconds per div so you can get a few complete cycles on screen and make sure the triggering is on internal and set for Channel one if that is what you are using. By adjusting the trig controls you should get a stable display. The ripple will be in the range of, what, 2 to 3 volts peak to peak, says he taking a wild guess. Depends how much current the amp is drawing.
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Old 4th June 2008, 07:54 PM   #44
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Hi Karl,

Remove earth, meant lifted the E not neutral and live would be the green wire in UK for the mains.


With the above settings and measurements

I thought the amp is drawing

(0.135Volt X 2)/ 0.1R = 2.7Amp

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Old 4th June 2008, 11:44 PM   #45
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This is the PSU
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Old 4th June 2008, 11:50 PM   #46
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The AC measured before the smoothing caps and right after the rectifier.

Volts/Div is set at 0.1
Sec/Div is set at 1 ms


The cycle takes 8.2 division

8.2 X 1ms= 8.2ms

Convert ms to second 8.2ms / 1000 = 0.0082

1 / Time in second = Frequency

1 / 0.0082 = 122 Hz
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Old 4th June 2008, 11:53 PM   #47
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This is the best I can do with the probes at + C6 and 0V ground


Volts/Div is set at 2mV
Sec/Div is set at 10 ms

Can not stable the display no matter what
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Old 5th June 2008, 08:04 AM   #48
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Hi Chris,
That looks O.K. You have worked the frequency out to 122 Hz. I take it in your part of the world it's 60Hz mains. So what you are seeing is the effect of the bridge (full wave rectifier) topping up the caps each half cycle-- thats why you are seeing 122 (It will be 120Hz, but measuring that accurately on a 'scope is not easy).
Your first pic shows the voltage on the cap rising as the bridge starts to conduct, the voltage keeps rising untill the mains is at its "peak" and then the caps maintain the rail voltage as the mains falls away. The falling part of the waveform is the cap discharging as the amp draws current, when the next half cycle comes along the process repeats. The bridge only conducts when the voltage on the rising part of the incoming half cycle exceeds the DC voltage on the cap ( +0.7 volts diode drop ). If you vary the quiescent current of your amp you will see the ripple voltage vary. This ripple is sat "on top" of the D.C. voltage of the rail. So at 0.1 V/Div you only have 0.6 volts peak to peak-- that sounds very good to me.
Second shot, if you are on 2mv/div ( 2 MILLIVOLTS ), that really is a half of a quarter of nothing .
Must be those coils , they look like something out of a particle acccelerator !
I am sure you are right but can you just confirm, pic two looks as though there is a low frequency component lurking there. Try and keep the timebase the same as in shot one and just alter the volts/div setting-- and make sure any variable gain pots are in "cal"
Try measuring directly across the speaker outputs for interest, with the inputs to the amp shorted. If the 'scope trace seems to "thicken" but you can't make anything out turn the timebase to much higher speeds, this will show if there is any high frequency oscillation of any sort.
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Old 6th June 2008, 02:17 AM   #49
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Default At the speaker outputs

OK,

Volts/Div is set at 2mV
Sec/Div is set at 1ms

Input is short---the whole band(line/trace) is like the picture below very steady


Input NOT short--- the whole band wave a little bit and not steady


EDIT: But this is the same even after the power is turn off. The same band(trace) appeared anyway

EDIT#2: I connected/shorted the probes together and get the same trace.
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Old 6th June 2008, 07:48 AM   #50
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Hi,
Nothing strange going on--- you are discovering the mysteries and limitations of doing simple but sensitive measurements with simple test gear and wiring arrangements.
The 'scope trace is ( looks like from here-- let's put it that way ) just white noise. You say with the inputs to the amp NOT shorted you get a on the trace. That's just the noise being modulated by the stray 60 Hz pickup.
Just to be absolutely sure, with it as in your picture turn the 'scope gain up as high as it goes, and expand the timebase to much higher speeds-- is it still just noise or is there any kind of "sinewave" lurking there. Almost certainly not, it's just a check to make sure it really is just noise.
Now your last comment-- with the scope set to 2mv/Div and timebase back at 2m/s Div and the scope leads connected together but NOT touching the amp is the trace still "thick" with noise. It should not be. If it is disconnect the probe from the front and see it it goes back to normal.
Did that info arrive by the way ?
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