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Old 2nd December 2012, 05:33 PM   #351
pdul is offline pdul  Denmark
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A lot of guys out there with instruments worth the GDP of a small country
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Old 2nd December 2012, 05:44 PM   #352
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by analog_sa View Post
Fixed the issue with the insufficient voltage across the output 2SK170. Indeed Joachim was correct - top FET needed replacement.

As for the randomly moving dc levels i still have no idea. And it is very consistent between channels.
I had similar issues...How did you choose the new top fet ? Different idss ?

Quote:
Originally Posted by Joachim Gerhard View Post
I had initially an offset of around 25 to 40mV. That went down after a while. I assume that is forming of the electrolytics in the input stage ( 2 x 6800uF ). They have some leakage current that goes down with time in use.
If after several days in use the DC offset at the output does not go down there are instructions in the building manual how to raise the "authority" of the servo.
Yes, i ment searching for J310 that leave enough Vds ( Uds ) over the 2SK170.
In my build the output offset was inittially around 100mV but after some hours it came down to around 10mV, so I agree it might have something to do with the input emitter caps.
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Old 2nd December 2012, 06:37 PM   #353
beetle is offline beetle  Germany
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Ok... hunting tha oscilations. Perfect band for this job: THE OSCILLATION 'Future Echo' - YouTube

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Old 2nd December 2012, 06:43 PM   #354
FdW is online now FdW  Netherlands
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Originally Posted by beetle View Post
It seems not to be. Still the same issue. I bet there is something oscillating at the negative side. Will fire up the scope..
O.k, show us a picture, also show the voltages at the different test points.
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Old 2nd December 2012, 06:45 PM   #355
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We will attack that DC offset problem and we will find a solution. I mean, it can not be,
My Paradise R3 worked right away after eroding two stuffing mistakes.
Ether you have made a stuffing mistake, or a part is faulty. When it is something else we have to find out. There must be a reason.
Hesener mentioned some time ago that the hunt for ultra high Hfe COULD create some of the oscialltion problem we had on the Paradise 1.
I repeat is now again : an Hfe of 400 is more then abundant, in ALL positions, also the input.
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Old 2nd December 2012, 06:49 PM   #356
beetle is offline beetle  Germany
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Scope draws a flat line at the output.

Shunt neg: -0.558V
Shunt pos: +0.544V

Pre Reg neg: 23.24V
Pre Reg Pos: 23.9V
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Old 2nd December 2012, 06:51 PM   #357
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Another approach :
Disconnect the cartridge, terminate the input with a 10 Ohm resistor, trim the input offset DC to zero ( more or less ), look what happens at the output.
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Old 2nd December 2012, 06:53 PM   #358
beetle is offline beetle  Germany
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The output stage is not looped in. "Bridge after testing" is still open.
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Old 2nd December 2012, 06:56 PM   #359
beetle is offline beetle  Germany
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Unfortunately Alfred didn't send me the proper qty. of heatsink clips. So I can not test the other one. Need to obtain one from reichelt later.
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Old 2nd December 2012, 06:56 PM   #360
hesener is offline hesener  Germany
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Originally Posted by beetle View Post
How hot should the headsink become? Mine is getting boiling hot. Can't leave my fingers on it for more then a second. Output voltages are ok. Input voltage 30V@206mA positive and 30V/152mA negative rail.
The supply current should be 110mA ...130mA, not more. This is irrespective of whether the amplifier is connected or not (the big power jumpers in the middle), because the shunt will reduce its current accordingly. But I read the problem is solved already.....
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