With the emitter 0.6v above the base, I'd expect the transistor to be conducting. It should be pulling its collector up.
I changed out the 1B transistor and now it seems to be working.
And yes, used donor parts off of another preamp board.
And yes, used donor parts off of another preamp board.
Post the DC voltage on all 3 terminals of both transistors if the amp is working, as a reference for others.
Those voltages aren't good.
Can you parallel a 10k resistor across the resistor marked 4702 that connects between the emitter of the 3B transistor and pin 7 of the LM324?
We're looking for a lower voltage on the collector of the 1B transistor.
Can you parallel a 10k resistor across the resistor marked 4702 that connects between the emitter of the 3B transistor and pin 7 of the LM324?
We're looking for a lower voltage on the collector of the 1B transistor.
That 1B tranistor looks like it's defective.
Do you have a new NPN transistor that you can install there?
Do you have a new NPN transistor that you can install there?
I used a 1B from a donor board.
Yes amp seems fully operational now.
Can you briefly describe what that circuit controls?
Thanks for your help.
Yes amp seems fully operational now.
Can you briefly describe what that circuit controls?
Thanks for your help.
This is the muting circuit. The muting for the 4 channels needs to have negative voltage to drive the jfets off (stop clamping audio to ground).
Initially, there was no negative voltage. Post 17 showed that the 3B transistor was pulling the 1B base up but the collector wasn't being pulled down. That meant that the transistor was open.
When you replaced the transistor, there was some negative voltage but not enough to drive the jfets off. In post 28, there was some negative voltage on the collector of the 1B transistor but the Vbe was too great. That meant that the base junction was damaged.
In post 33, the voltages were right. The Vbe was about 0.6v and the collector was being pulled down to near the emitter voltage.
If the 10k resistor is still in the circuit, you can remove it to see if the 1B collector is still very near its emitter voltage.
Initially, there was no negative voltage. Post 17 showed that the 3B transistor was pulling the 1B base up but the collector wasn't being pulled down. That meant that the transistor was open.
When you replaced the transistor, there was some negative voltage but not enough to drive the jfets off. In post 28, there was some negative voltage on the collector of the 1B transistor but the Vbe was too great. That meant that the base junction was damaged.
In post 33, the voltages were right. The Vbe was about 0.6v and the collector was being pulled down to near the emitter voltage.
If the 10k resistor is still in the circuit, you can remove it to see if the 1B collector is still very near its emitter voltage.