Is that 16V across R25 or 16V referenced to ground at the junction (node) of R25 and R27? You need to annotate more voltages to make it meaningful. Rail voltage, all 3 pins of Q1,2,7 and 8 for starters.
Refer all voltages to ground or clearly show the endpoints.
fine gray print is hard to read. Use MSPaint and mark the numbers in red, using a larger font.
Refer all voltages to ground or clearly show the endpoints.
fine gray print is hard to read. Use MSPaint and mark the numbers in red, using a larger font.
Yes its 16V across R25 and R27 not to ground. Oh another note I was trying to put in but wasn't successful. both R25 and R27 are 12K not 10K
Rail voltage as said in previous post is 58V
Source to ground for both Q1 and Q2 is 50mV
I wasn't able to reach easily to measure Q7 and Q8. so I didn't
I was fiddling with msPaint trying to change the color to red.... gave up
Currently the two jfets are out. I wonder can I power it on without blowing anything without the jfets installed? Perhaps then I may be able to get to Q7 and Q8 pins.
Rail voltage as said in previous post is 58V
Source to ground for both Q1 and Q2 is 50mV
I wasn't able to reach easily to measure Q7 and Q8. so I didn't
I was fiddling with msPaint trying to change the color to red.... gave up
Currently the two jfets are out. I wonder can I power it on without blowing anything without the jfets installed? Perhaps then I may be able to get to Q7 and Q8 pins.
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58V rail less 16V across the top of the cascade means the JFET was seeing 42V at its drain. I'm not sure it would survive that. You needed to drop a lot more voltage in the cascode. I'm guessing your inputs are toast.
I wonder why the schematic uses a different cascade ratio for the p and n channel inputs? I don't remember that being discussed anywhere.
I wonder why the schematic uses a different cascade ratio for the p and n channel inputs? I don't remember that being discussed anywhere.
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Yes its 16V across R25 and R27 not to ground. Oh another note I was trying to put in but wasn't successful. both R25 and R27 are 12K not 10K
Rail voltage as said in previous post is 58V
Source to ground for both Q1 and Q2 is 50mV
I wasn't able to reach easily to measure Q7 and Q8. so I didn't
I was fiddling with msPaint trying to change the color to red.... gave up
Currently the two jfets are out. I wonder can I power it on without blowing anything without the jfets installed? Perhaps then I may be able to get to Q7 and Q8 pins.
Is it possible that you have R25 and R27 reversed or switched.
if rails are +/-58 V , then R25 , R26 need to be 47K , R27 and R28 need to be 12K
then bases of bjts will be at 56V( 12K/47K)=14V8 and drains of Jfets will be at 0V65 less
for R27 and R28 as 10K , that will be 12V34 (and 0V65 less)
and yes - R26 , marked as 47K5 is nothing else then typo ........ whatever it is , R25=R26 , and R27=R28
you can power it without bjts and jfets , and confirm that you have proper voltage at bases nodes
then put bjts and jfets in and fire it up again
whatever you set - jfets doesn't want to see more then 12 to 15V at drains
then bases of bjts will be at 56V( 12K/47K)=14V8 and drains of Jfets will be at 0V65 less
for R27 and R28 as 10K , that will be 12V34 (and 0V65 less)
and yes - R26 , marked as 47K5 is nothing else then typo ........ whatever it is , R25=R26 , and R27=R28
you can power it without bjts and jfets , and confirm that you have proper voltage at bases nodes
then put bjts and jfets in and fire it up again
whatever you set - jfets doesn't want to see more then 12 to 15V at drains
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I'm not following you with that ?
R25 & R27 are simple voltage divider net , for establishing base potential for Q7 , which is (nothing more and nothing less then) cascode for Q1
when establishing those values , you're (by rule of thumb) putting 0.5-1mA through divider and shooting for 12 to 15V potential at base , ref. gnd
base will pull what it needs (miniscule currents anyway) , so no need for limiting anything , except unnecessary dissipation in voltage divider itself
R25 & R27 are simple voltage divider net , for establishing base potential for Q7 , which is (nothing more and nothing less then) cascode for Q1
when establishing those values , you're (by rule of thumb) putting 0.5-1mA through divider and shooting for 12 to 15V potential at base , ref. gnd
base will pull what it needs (miniscule currents anyway) , so no need for limiting anything , except unnecessary dissipation in voltage divider itself
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Buzz , I don't think I have them switched.
If you see my post
http://www.diyaudio.com/forums/pass-labs/207103-f5-turbo-builders-thread-310.html#post3896918
at the top if you look closely 12k is the resistor in the middle as per the label R26 on the unpopulated board I posted here
http://www.diyaudio.com/forums/pass-labs/207103-f5-turbo-builders-thread-311.html#post3896978
Zen, are you sure thats not 4.75K? as per Audiosan's post?
What voltage should I get at the BJT nodes?
I'll desolder the bjt's tonight and try firing it up.
If you see my post
http://www.diyaudio.com/forums/pass-labs/207103-f5-turbo-builders-thread-310.html#post3896918
at the top if you look closely 12k is the resistor in the middle as per the label R26 on the unpopulated board I posted here
http://www.diyaudio.com/forums/pass-labs/207103-f5-turbo-builders-thread-311.html#post3896978
Zen, are you sure thats not 4.75K? as per Audiosan's post?
Note: I'm expecting only 52V or so after I connect the mosfets with 6A total draw. so I'm hopping it will come down to 14.75V when everything is connected. Right now I would consider the PSU and trafo as "unloaded"
What voltage should I get at the BJT nodes?
I'll desolder the bjt's tonight and try firing it up.
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You want ~15V across r=R27/28. That's how the cascode will protect the jfets. The actual voltage across R25/26 doesn't matter, as long as R27/28 are in that range. I agree with Sangram that since you hit the jfets with 40V they are probably destroyed.
Remember, that you don't have to measure on the BJT pins themselves. Look where they connect. You can get readings at the proper point on R5, R29 and Q1 if they are more convenient.
Remember, that you don't have to measure on the BJT pins themselves. Look where they connect. You can get readings at the proper point on R5, R29 and Q1 if they are more convenient.
Hitting it with 40V doesnt make sense. If he has the resisotrs in the proper place, why would the base of the bjt see anything other than the proper voltage. I assume the greatest possiblity is that he fried the bjt with too much bas current, but I dont know. It doesnt make sense that he is dropping 16V across the larger of the two resistors in the divider network.
OK I will retest tonight. I'm thinking I must have measured something incorrectly. From the math 12K at R25/26 and 4.75k at R27/28 should give me about 16.44V coming out.
The voltage divider calculator :
Voltage Divider Calculator
corresponds to Audiosan's math. where R1 is R25/26 and R2 is R27/28.
Or am I reading the voltage divider backwards on NP's schematic?
Dazed, I am not sure you are reporting the voltages correctly. Put one probe of your multimeter on one end of R27 and the other probe on the other end of R27 and report the resulting Volts. Do the same for R28.
You have 12k for R25 and R26 and 4.75K for R27 and for R28. Is this correct?