mutetourettes
Active member
Hello all, I've had two very different (but both late 80s early 90s tape) machines recently whose 12v psu lines have given up, and in both cases it looks like a very similar fault. I just want to run this by your good selves to discuss and see:
a) have I understood this right
b) have you all found this to be common too
One circuit is a series-pass transistor, one is a darlington, but they're both taking a larger DC input into a resistor/13vzener voltage divider to put 13v into a transistor (for the 12v line after the transistor's drop). They also have a capacitor parallel to the zener.
The issue is that the transistor is getting 0v, rather than the 13v the resistor/zener should control.
I'm thinking four possible cases for both machines
a) the resistor has failed open
b) the zener has failed short circuit
c) the capacitor has failed short circuit
d) some strange fault with the transistor where the base is going to ground
I have not unsoldered anything yet.
I suspect the capacitors (for no good reason), though they do not look bad. are they common to fail short-circuit? What is the function of the capacitor (smoothing power supply ripple?) if I take the capacitor out and test the circuit briefly would that commonly work ok enough to tell me "yes it was the capacitor"?
Attached to this post the Rotel RD865 12v line schematic
a) have I understood this right
b) have you all found this to be common too
One circuit is a series-pass transistor, one is a darlington, but they're both taking a larger DC input into a resistor/13vzener voltage divider to put 13v into a transistor (for the 12v line after the transistor's drop). They also have a capacitor parallel to the zener.
The issue is that the transistor is getting 0v, rather than the 13v the resistor/zener should control.
I'm thinking four possible cases for both machines
a) the resistor has failed open
b) the zener has failed short circuit
c) the capacitor has failed short circuit
d) some strange fault with the transistor where the base is going to ground
I have not unsoldered anything yet.
I suspect the capacitors (for no good reason), though they do not look bad. are they common to fail short-circuit? What is the function of the capacitor (smoothing power supply ripple?) if I take the capacitor out and test the circuit briefly would that commonly work ok enough to tell me "yes it was the capacitor"?
Attached to this post the Rotel RD865 12v line schematic