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OLD THREAD DAC End by Andrea Ciuffoli

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#901
adamus said:
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?
Click to expand...

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''.
 
#904
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??
 
#907
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.
 
#915
Look on its correctly functioning (?) twin +5V for parts orientation, correct types and values on parts, connections and copper lines underneath, measurements. There must be a bug somewhere in your build. Vgs (gate to +V) of the Mosfet must be around 4V and drop across Q14 must be existing. Is R15 actually 47R?
 
#917
Its not my design, I don't recommend. I just thought aloud. This must come officially from the maker and GB holder, or any other successful tweaks from the builders. As for the part that they adopted, namely the 5V SSV1.0 reg array, no you don't need to match leds, since you got trimmers.

Regards
 
#918
the gate of q16 is sat at the output voltage. so surely it points the finger at q14.... trouble is i have already replaced it.

I'll try replacing again.

edit, replaced and no joy. I'll try q15 again..... this has me stumped.
 
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