Yes, the drivers and predrivers are soldered in (in a line ready for tapping into a heatsink) but they are unpowered.
It is easy to connect the NFB pad to C1 since both are above board, whereas the collectors of Q15 & Q16 + extra resistors would be below board.
I'm away to remove the FC resistors and move the NFB tapping point.
It is easy to connect the NFB pad to C1 since both are above board, whereas the collectors of Q15 & Q16 + extra resistors would be below board.
I'm away to remove the FC resistors and move the NFB tapping point.
changed R15 & R18.
1227 & 1225mV across indicating ~8.3mA.
The mirror has reduced current to ~1.6mA, leaving the same ~6.7mA flowing into the cascode. As expected the R19 current has not changed.
Vr12=1.3mV
Vr13=0.3mV
Vr14=0.2mV
These seem to confirm that the transistor have sensible hFE.
Not moved the NFB to the Q15 & Q16 collectors yet.
Total current draw down to ~19.5mA
1227 & 1225mV across indicating ~8.3mA.
The mirror has reduced current to ~1.6mA, leaving the same ~6.7mA flowing into the cascode. As expected the R19 current has not changed.
Vr12=1.3mV
Vr13=0.3mV
Vr14=0.2mV
These seem to confirm that the transistor have sensible hFE.
Not moved the NFB to the Q15 & Q16 collectors yet.
Total current draw down to ~19.5mA
It's running off my lab supply set to maximum of +-31.6Vdc.
Not far below +-39Vdc specified for the front end.
What would the symptoms be, if either the Cascode transistor/s or the input jFet/s were damaged?
I've been poring over the schematic for days to try to find the missing current. Checked the resistor values for R20=1k2 and R29=9k96
Not far below +-39Vdc specified for the front end.
What would the symptoms be, if either the Cascode transistor/s or the input jFet/s were damaged?
I've been poring over the schematic for days to try to find the missing current. Checked the resistor values for R20=1k2 and R29=9k96
Hi,
I fitted the 470r resistors last night and moved the NFB to the connection. No change.
I will measure Vbe and Vce of all the transistors.
I shall double check the current in the collector leg of Q17. It's unmatched and easy to replace if I overheat it.
I'm loathe to take out the cascodes and jFETs on a hunch, until I'm sure one or other are at fault.
I fitted the 470r resistors last night and moved the NFB to the connection. No change.
I will measure Vbe and Vce of all the transistors.
I shall double check the current in the collector leg of Q17. It's unmatched and easy to replace if I overheat it.
I'm loathe to take out the cascodes and jFETs on a hunch, until I'm sure one or other are at fault.
replaced Q17, no change.
shorted D1, now 850mV across the 9k96.
Vds of the jFETs is +5.6mV.
Vsg = +717mV
Vdg = +722mV
It appears that the Vbe of the cascode and the Vds of the Fets is setting the voltage drop across D1+R
Cascode sits with Vce=2.4V, Vbe ~700mV.
Looks like my next step is pull out the FETs.
FETs out and Idss are still matched.
Where to now?
shorted D1, now 850mV across the 9k96.
Vds of the jFETs is +5.6mV.
Vsg = +717mV
Vdg = +722mV
It appears that the Vbe of the cascode and the Vds of the Fets is setting the voltage drop across D1+R
Cascode sits with Vce=2.4V, Vbe ~700mV.
Looks like my next step is pull out the FETs.
FETs out and Idss are still matched.
Where to now?
Did you measure the Idss for k170 used? What values did you found?
I will suggest you to measure all components ... also, please verify all the links between components to be in conformity with schematic. It is hard work, but it is the only method which can give results is short time.
If everything turn to be just fine, then jfets are toasted.
I will suggest you to measure all components ... also, please verify all the links between components to be in conformity with schematic. It is hard work, but it is the only method which can give results is short time.
If everything turn to be just fine, then jfets are toasted.
On the datasheet of 1n4148 the curves shows about .45forward voltage at .1mA. So when connected in parrallele with a Vbe of about .65V (Q18,Q21) it's normal that the diode conduct a little, and your numbers seems to be normal for me.
If this diode have forward leak above this the problem may be amplified. So i think this is normal that a small amout of current 'leak' a bit by D1, even if the Jfets have a 0V VDS.
Denis
If this diode have forward leak above this the problem may be amplified. So i think this is normal that a small amout of current 'leak' a bit by D1, even if the Jfets have a 0V VDS.
Denis