Really sorry for the post deletion. It was an accident.
I would suggest following.
Remove BJT and FET. Measure the drop at base of the BJT, R25/27 junction.
Put in the BJT. Put a 10k resistor in place of the JFET and measure the voltage.
Now put the bias pot to middle resistance. I am assuming your power devices are not in circuit at this time. This should not be done with them connected.
Put in the JFETs and take a measurement.
For each of the above, take a measurement at the base node of the BJT, the top of the cascode (BJT collector) and the bottom of the cascode (BJT emitter). Try it with pot at minimum resistance and at middle resistance. You should measure the resistance from rail to the BJT collector, set it to about 200 ohms using the pot. More is not necessary.
Post these three for each of the above. We should be able to guess at a solution then.
I would suggest following.
Remove BJT and FET. Measure the drop at base of the BJT, R25/27 junction.
Put in the BJT. Put a 10k resistor in place of the JFET and measure the voltage.
Now put the bias pot to middle resistance. I am assuming your power devices are not in circuit at this time. This should not be done with them connected.
Put in the JFETs and take a measurement.
For each of the above, take a measurement at the base node of the BJT, the top of the cascode (BJT collector) and the bottom of the cascode (BJT emitter). Try it with pot at minimum resistance and at middle resistance. You should measure the resistance from rail to the BJT collector, set it to about 200 ohms using the pot. More is not necessary.
Post these three for each of the above. We should be able to guess at a solution then.
OK to try and answer Sangram, Bob and Buzz and AndrewT's inquiries. Here is a summary.
AndrewT : no I didn't measure Idss. I don't have a tester
Buzz : yes I'm pretty sure I installed the JFET correctly, unless the board is marked incorrectly Q1 or sk170 on the board is marked from top to bottom SGD so flat would be facing the other flat of Q2(J74) which is marked from top to bottom DGS #3102
With BJT and JFET out I get :
Q5 --> base to collector 40V
Across R27/28 -> 16V
nothing at Q1 and Q2
Just BJT in :
Q1 -> gate to drain 15.87V
-> drain to ground 15.87V
Q2 same as Q1
BJT and Jfet in :
Q1-> drain to ground 0.7V
-> gate to drain 0.7V
-> gate to source 50mV
Q2 -> drain to ground 0.5V
-> gate to drain 0.5V
-> gate to source 50mV
All these were done with with the P1 and P2 at close to min resistance.
If I remember correctly (I'm not infront of it ) TP3/TP4 tested at 1.8V and TP1/TP2 at 1.3V.
Bob, when I did the measurements I believe I did try testing G and S to ground but it didn't give me any higher reading so I never wrote it down. sorry I can try it tonight.
Sangram when I get back I can turn up the pots and see what happens. would this actually do anything though?
Oh one other perhaps tidbit of info that shouldn't affect the readings but I might as well report just in case. I did change the feedback resistors (R8 and R9) to 200ohms instead of 120ohms I was hoping to get a gain of 27dB instead of only 22dB. However I don't think this would make a difference in the issue here.
AndrewT : no I didn't measure Idss. I don't have a tester
Buzz : yes I'm pretty sure I installed the JFET correctly, unless the board is marked incorrectly Q1 or sk170 on the board is marked from top to bottom SGD so flat would be facing the other flat of Q2(J74) which is marked from top to bottom DGS #3102
With BJT and JFET out I get :
Q5 --> base to collector 40V
Across R27/28 -> 16V
nothing at Q1 and Q2
Just BJT in :
Q1 -> gate to drain 15.87V
-> drain to ground 15.87V
Q2 same as Q1
BJT and Jfet in :
Q1-> drain to ground 0.7V
-> gate to drain 0.7V
-> gate to source 50mV
Q2 -> drain to ground 0.5V
-> gate to drain 0.5V
-> gate to source 50mV
All these were done with with the P1 and P2 at close to min resistance.
If I remember correctly (I'm not infront of it ) TP3/TP4 tested at 1.8V and TP1/TP2 at 1.3V.
Bob, when I did the measurements I believe I did try testing G and S to ground but it didn't give me any higher reading so I never wrote it down. sorry I can try it tonight.
Sangram when I get back I can turn up the pots and see what happens. would this actually do anything though?
Oh one other perhaps tidbit of info that shouldn't affect the readings but I might as well report just in case. I did change the feedback resistors (R8 and R9) to 200ohms instead of 120ohms I was hoping to get a gain of 27dB instead of only 22dB. However I don't think this would make a difference in the issue here.
something is not logical
without Jfets and cascode bjts , you confirmed proper voltage at voltage dividing network
if everything is OK , you'll have that voltage (minus Vbe drop ) at emiters of cascode transistors , whatever load below is ( speaking about positive ones , so "below" .... take that as "above" for lower one)
so - you can put 1 or 50 Jfets below , voltage at emiter can't change much , if everything is put correctly on pcb
without Jfets and cascode bjts , you confirmed proper voltage at voltage dividing network
if everything is OK , you'll have that voltage (minus Vbe drop ) at emiters of cascode transistors , whatever load below is ( speaking about positive ones , so "below" .... take that as "above" for lower one)
so - you can put 1 or 50 Jfets below , voltage at emiter can't change much , if everything is put correctly on pcb
You don't need a tester.
The resources you do need are:
A source of current, a 9V pp3 battery is good enough.
A resistor from 100r to 1k0
A multimeter that can read 1mV easily, a DMM reading to 1999mVdc is good enough.
Some bits of wire to make connections.
Yes Zen, thats what I thought too. after the BJT's I should get 15.87 V no matter how many jfets I put in, thats what I'm stumped.
Oh I should note that :
Across R27/28 -> 16V should actually read 16.44V so there is a little over half V drop after the BJT's are in.
Bob, yes I will stick a 10K resistor in tonight in place of the jfet to see what I get.
Oh I should note that :
Across R27/28 -> 16V should actually read 16.44V so there is a little over half V drop after the BJT's are in.
Bob, yes I will stick a 10K resistor in tonight in place of the jfet to see what I get.
I think he means 2sc4793, BJT.
I agree that this seems extremely odd. It is unusual to have the VDD disappear when the JFET is inserted and only indicates a fully short circuited device. It is also unusual to have two sets of devices behave in the exact same way.
You have to test all devices out of circuit. This is probably the only way. The BJT seems to be OK as the emitter voltage is fine when open circuit.
As Andrew says, a jfet test is not difficult as it requires minimum components. Maybe you need to run one for all those devices, remember to reverse polarity for the j74.
I agree that this seems extremely odd. It is unusual to have the VDD disappear when the JFET is inserted and only indicates a fully short circuited device. It is also unusual to have two sets of devices behave in the exact same way.
You have to test all devices out of circuit. This is probably the only way. The BJT seems to be OK as the emitter voltage is fine when open circuit.
As Andrew says, a jfet test is not difficult as it requires minimum components. Maybe you need to run one for all those devices, remember to reverse polarity for the j74.
Yeah I guess I'm going to have to rig something up and read up on how to test a jfet.
These are the BJT's I'm using
http://www.customworks.cz/wp-content/uploads/2012/09/2sc4793.pdf
and
http://www.customworks.cz/wp-content/uploads/2012/09/2SA1837.pdf
These are the BJT's I'm using
http://www.customworks.cz/wp-content/uploads/2012/09/2sc4793.pdf
and
http://www.customworks.cz/wp-content/uploads/2012/09/2SA1837.pdf