I have got a Sony HCD-BX7700dbi that isn't worth a lot but I'm keen to learn how to repair and would appreciate some help with my train of thought. The protection circuit is kicking in and the standby light is blinking. It goes into protection mode within a second of powering it on.
So far the relay on the power board that enables the main transformer clicks on and then off when powering on the unit. Looking at the schematics, the permanently enabled power rail called 'unreg sub' provides about 13.5VDC. Where the power comes into the main board on connector CN303, there is a diode D331 and I see a 13.5VDC across that component. Am I correct in thinking that this indicates a short to ground the other side of D331. With the unit powered off there is no short to ground from D331. With my DMM in diode mode looking at npn transistor Q333 which if failed short would provide a path to ground, I am seeing a voltage drop on that transisotr base-collector of 1.6v and base-emitter of 1.1v. Is that good? Can this be measured in circuit?
Also I wanted to verify that the power board wasn't causing any problem and I have measured the voltages supplied by the power board with the relay pin grounded but without any load. Is this a good test? The voltages seem to be stable.
The electrolytic capacitors that I've tested on the board all seem good. There was one that I needed to take out of circuit but otherwise all good including ESR readings. Some of the resistors R425 and R461 are roughly half what they should be but I don't think that is causing the protection mode to kick in.
Any pointers much appreciated. Service manual is attached. Main board is on page 27-29.
So far the relay on the power board that enables the main transformer clicks on and then off when powering on the unit. Looking at the schematics, the permanently enabled power rail called 'unreg sub' provides about 13.5VDC. Where the power comes into the main board on connector CN303, there is a diode D331 and I see a 13.5VDC across that component. Am I correct in thinking that this indicates a short to ground the other side of D331. With the unit powered off there is no short to ground from D331. With my DMM in diode mode looking at npn transistor Q333 which if failed short would provide a path to ground, I am seeing a voltage drop on that transisotr base-collector of 1.6v and base-emitter of 1.1v. Is that good? Can this be measured in circuit?
Also I wanted to verify that the power board wasn't causing any problem and I have measured the voltages supplied by the power board with the relay pin grounded but without any load. Is this a good test? The voltages seem to be stable.
The electrolytic capacitors that I've tested on the board all seem good. There was one that I needed to take out of circuit but otherwise all good including ESR readings. Some of the resistors R425 and R461 are roughly half what they should be but I don't think that is causing the protection mode to kick in.
Any pointers much appreciated. Service manual is attached. Main board is on page 27-29.