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Old 26th February 2010, 02:40 PM   #901
Salas is offline Salas  Greece
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Quote:
Originally Posted by adamus View Post
Salas.

I am testing the shut reg for the +5 dig that wont go above 5.16.

Leds are dropping 3.92v and the trimmer is dropping 0.75v at 161 ohms. combined vref of 4.67

I through trimmer = 0.75/161 = 4.6ama, which seems low for the jfet (originally matched at 7ma idss @10v.

Strnage thing is, the trimmer has more turns left, but it doesnt affect the voltage.

any thoughts from the wizards?
4.67+Vbe is marginal when you load it. Has to drop a bit on small resistances on everything in layout when the current loops to the load. Just exchange with a 9 to 10mA at 9V battery test Idss 2SK170 under the leds, so to know if its the Vref only. It will lose maybe 30% to 35% of that reading under only Vbe margin that it works here. Check your heat on your related small Mosfet sinks. If such small sinks don't feel hot enough, it shows that you can run more ICCS for the elements to stabilize better and drop Zo. Maybe down to circa 20R with some calculated paralleling value on the 27R for a quick test. Not everywhere, only to that +dig branch. I would set to 5.6V for the 431's balast to ''breath''.
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Old 26th February 2010, 02:41 PM   #902
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The Salas shunt is good!
So in the DAC board

you need to check and measure all voltage after the 10 ohm resistor, I think will find the wrong place.
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Old 26th February 2010, 03:15 PM   #903
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all tl431 2.5v check fine. all 2.49v to 2.5v

from the right, the voltage across the 10r in mv is 141, 113, 147, 127, 111. but this is without the chips in.
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Old 26th February 2010, 04:43 PM   #904
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further tests.

With no chips (all IC taken out of the dac board) the 5v digital is pulling 78ma

all +5 points are reasonable, but one is 4.98v the rest are 5. I cut that part of the circuit out (just removed the 10ohm resistor) and it made no difference.

changed the k170 to a 10ma idss and no change.

changed the 27ohm to 20 and no change, solid as a rock at 5.16 - no further.

checked all parts, all are correct.

reflowed joints, no change.

any more idea's??
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Old 26th February 2010, 04:55 PM   #905
Salas is offline Salas  Greece
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What is the Vgs of the shunt mosfet on that branch? What is the voltage across the 2sk?
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Old 26th February 2010, 05:00 PM   #906
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we may be getting somewhere. r18 is dropping no voltage.

vgs is output voltage.

k170 dropping no voltage.....

Last edited by adamus; 26th February 2010 at 05:04 PM.
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Old 26th February 2010, 05:19 PM   #907
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Vgs must be around 4V normally. R18 is your gate stopper? No real drop there, but you must have 0.6-0.7V across the 2SK under the Vref if the error amp BC550 is OK, and a reading across the 10R beneath the 2SK. Maybe you have a dead Mosfet by static or a dead little semi bjt or not somewhere.
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Old 26th February 2010, 05:28 PM   #908
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another check 10r read 73mv
q18 314mv across

Last edited by adamus; 26th February 2010 at 05:32 PM.
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Old 26th February 2010, 05:32 PM   #909
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Q15, Q14 check first. That 314mV does not tie with expected Vbe, should read near 0.66V from Q15's base to gnd. Maybe your R15 is 470R instead of 47R by mistake?
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Old 26th February 2010, 05:38 PM   #910
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q14 0v
q15 5.16v
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