samgraysound
Well-known member
Hi,
I'm trying to fix an Ashdown MK500 bass amplifier head. When I got it, the speaker protect relay was stuck open. I cannot figure out why. The rest of the amp works fine. At this point I have completely disconnected everything else from the power supply and protect circuit and the relay still wont close under normal conditions. Attached is a schematic.
I have replaced the JFET, the relay, and all diodes. I have checked the other two transistors with a transistor tester, and checked the electrolytics with a ESR meter. I have checked all resistor values.
When I bring it up on my variac to a mains voltage of about 130 the relay closes.
My understanding of the circuit is the JFET gets reversed biased, causing a positive bias to appear at the base of TR17, which causes a positive bias to appear at the base of TR15 allowing current to flow through the relay to ground. It seems like the JFET is not cutting off. But the voltage at the gate of the JFET is -2.5vdc, and it should cutoff below -1vdc according to the datasheet. When I raise the mains voltage and the relay closes the gate voltage only changes to -2.6vdc
TR17 voltages are:
B - 110mv
C - 42.5v
E - 640mv
TR18 voltages are
B - 640mv
C - 42.5v
E - 0v (grnd)
Tell me the obvious thing I'm missing people, I've been fiddling with this thing for months.
I'm trying to fix an Ashdown MK500 bass amplifier head. When I got it, the speaker protect relay was stuck open. I cannot figure out why. The rest of the amp works fine. At this point I have completely disconnected everything else from the power supply and protect circuit and the relay still wont close under normal conditions. Attached is a schematic.
I have replaced the JFET, the relay, and all diodes. I have checked the other two transistors with a transistor tester, and checked the electrolytics with a ESR meter. I have checked all resistor values.
When I bring it up on my variac to a mains voltage of about 130 the relay closes.
My understanding of the circuit is the JFET gets reversed biased, causing a positive bias to appear at the base of TR17, which causes a positive bias to appear at the base of TR15 allowing current to flow through the relay to ground. It seems like the JFET is not cutting off. But the voltage at the gate of the JFET is -2.5vdc, and it should cutoff below -1vdc according to the datasheet. When I raise the mains voltage and the relay closes the gate voltage only changes to -2.6vdc
TR17 voltages are:
B - 110mv
C - 42.5v
E - 640mv
TR18 voltages are
B - 640mv
C - 42.5v
E - 0v (grnd)
Tell me the obvious thing I'm missing people, I've been fiddling with this thing for months.