UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Horrid intermittent on Transistor Black Box 1004

frsimen · 21st Dec 2011, 9:38 am

Andy,

VT2's base potential will be less than would be expected if its base current is higher than it should be.

In this circuit, the emitter voltage is mainly set by the potential divider consisting of R14+R15 and R27, dividing down what should be the mid rail voltage from VT7's collector. That would give an emitter voltage around 0.8V. The base voltage could be expected to be around 0.2V higher, at 1V or so.

With the fault, the mid rail voltage is very low, so the emitter voltage is now determined mainly by the emitter current flowing through R14+R15. The design emitter current seems to be about 650uA, which would only give an emitter voltage of 0.1 volts and a base voltage of around 0.3V. This isn't too far off what you're measuring.

Paula

21st Dec 2011, 9:38 am	#16
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Horrid intermittent on Transistor Black Box 1004 Andy, VT2's base potential will be less than would be expected if its base current is higher than it should be. In this circuit, the emitter voltage is mainly set by the potential divider consisting of R14+R15 and R27, dividing down what should be the mid rail voltage from VT7's collector. That would give an emitter voltage around 0.8V. The base voltage could be expected to be around 0.2V higher, at 1V or so. With the fault, the mid rail voltage is very low, so the emitter voltage is now determined mainly by the emitter current flowing through R14+R15. The design emitter current seems to be about 650uA, which would only give an emitter voltage of 0.1 volts and a base voltage of around 0.3V. This isn't too far off what you're measuring. Paula