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Old 6th February 2009, 07:21 PM   #651
roender is offline roender  Romania
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Andrew, you could measure all BJT transistors mounted on PCB.
There is a small possibility to find some PCB vias without metal deposit. Take a closer look around 10K bias resistor
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Old 6th February 2009, 07:40 PM   #652
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Lift Q17 collector and insert 1k resistor. Measure the voltage drop and calculate the current through resistor. If you find something very different than 1mA then Q17 is damaged ... otherwise something is wrong with Q21 or/and Q18 or j-fet transistors
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Old 7th February 2009, 07:48 AM   #653
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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.
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Old 7th February 2009, 04:11 PM   #654
DRZ1 is offline DRZ1  Canada
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D1 is leaking ?
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Old 7th February 2009, 04:23 PM   #655
AndrewT is offline AndrewT  Scotland
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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?
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Old 7th February 2009, 04:33 PM   #656
DRZ1 is offline DRZ1  Canada
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Quote:
Originally posted by AndrewT

It appears that the Vbe of the cascode and the Vds of the Fets is setting the voltage drop across D1+R

I think that's exact. And it is the reason i think D1 is leaking. My first step would be to change this diode with a eally good one, mean no measured leakage in this order on the bench.

I will do measurments in mine when i will have time.
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Old 7th February 2009, 04:36 PM   #657
AndrewT is offline AndrewT  Scotland
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but D1 is forward biased.
Why should leakage value be a problem?
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Old 7th February 2009, 05:16 PM   #658
roender is offline roender  Romania
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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.
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Old 7th February 2009, 06:10 PM   #659
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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
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Old 7th February 2009, 06:14 PM   #660
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Denis,

Andrew already did the test with D1 in short-circuit, without solving the problem. D1 may be the cause only if is reverse mounted
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