Will try. Anything is better than desolering and stripping boards.
Just had a brain wave from from the 42nd dimension.
I think I know your problem.
Post a schematic of your circuit and I will elaborate.
Are you using exactly all the same component values?
Eg is R3 and R4, 22Ohms etc etc
Is anything different?
Just want to confirm first.
Eg is R3 and R4, 22Ohms etc etc
Is anything different?
Just want to confirm first.
Yes, changed R9 from 10k to 5k to bias properly.
There are 3 other channels working with this change.
Should I try 10k? Like I said it does bias up, but then drops.
There are 3 other channels working with this change.
Should I try 10k? Like I said it does bias up, but then drops.
Just leave R9 the way it is for now.
When you first turn it on when cold, does the weird channel bias properly?
At least for a little while?
When you first turn it on when cold, does the weird channel bias properly?
At least for a little while?
Now when powering on, the channel stays low biased.
The last changes cause it to drop to zero volts across the source resistor.
Should I attach the resistors across r6 and r7?
The last changes cause it to drop to zero volts across the source resistor.
Should I attach the resistors across r6 and r7?
Before you turn it on again reset biasing trimpot, to minimum bias position (max resistance).
Just a reminder.
Just a reminder.
I'm never sure which direction to set the pot.
Assume setting to max resistance impedes current, but I guess it depends on the circuit.
Hence the variac.
Assume setting to max resistance impedes current, but I guess it depends on the circuit.
Hence the variac.
Yes, biased up to 160mV and let it sit.
Both channels got up to 190mV after some time.
Adjusted for 200mV.
The offset fluctuates a little in the millivolts range, but thats occurring in both channels.
Both channels got up to 190mV after some time.
Adjusted for 200mV.
The offset fluctuates a little in the millivolts range, but thats occurring in both channels.
It still needs to come back up biased correctly and stay biased to correct level when connected to the rest of the system.
This circuit is highly sensitive to Idss of Jfets used.
If you have 8mA running through the jfets you will be starving the TL431A shunt
As the amp heats up the Jfets will draw even more current then TL431 turn off
If you have 8mA running through the jfets you will be starving the TL431A shunt
As the amp heats up the Jfets will draw even more current then TL431 turn off
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Most TL431 devices require 1mA to operate but you also have around 0.3mA going through the program resistors so you need at least 1.3mA
I was checking. There is another device ATL431. It can run on 100uA so this would be the better device to use for this circuit.
Or drop the resistor values feeding the shunt
I was checking. There is another device ATL431. It can run on 100uA so this would be the better device to use for this circuit.
Or drop the resistor values feeding the shunt
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