01-12-2022, 01:38 PM
The service manual is here: https://elektrotanya.com/toshiba_19dv501...nload.html
And it includes the schematic of the PSU. That's not something one can take for granted these days!
I've attached the start-up circuitry from what I believe is the board you have - note that there is more than one possibility.
As you can see, the unregulated supply from the main caps (labelled 400V, but more likely 325V or thereabouts in practice), goes via 3 100k resistors in series. At least, they'd better be 100k! The schematic says "100R", but that has to be typo..
This is how the unit starts up. The current - about 1mA - from these resistors charges C807, and that allows the supply to try to run. The supply labelled "VCC" goes to the controller IC.
But think of this as being like a starter motor. That 1mA might be enough to turn the engine over, but it's not enough to run the engine. So in the same way that the starter motor starts the process of getting sparks and bangs into the engine, this 1mA current is enough to try getting some flux into the switching transformer.
Note D808, which also supplies energy to the VCC rail. This energy comes from a separate winding on the transformer that is intended to run the supply in normal operation. The small current from those 3 100k resistors is only used initially; once running, the bulk of it comes from the transformer.
Unfortunately, high value spiral-cut film resistors with high voltages across them are prone to gradually failing high. Depending on the supply, this might not be noticed for ages - if your engine is easy to turn over, a tired starter motor (or battery) might still succeed in getting it going, but if your engine is diesel, that's far less likely. But eventually, it's possible to get to the point where there isn't enough current available via the startup resistor(s), and the supply won't run.
For stuff that is powered for years at a time - there's a lot of that where I work - this fault often develops months or even years before it's noticed. Should your starter motor mysteriously fail during a long journey, you won't know about it until you switch off the engine and try to restart it!
Anyway, by all means, check each of those 3 resistors. Also check the cap (C807) - that's like the engine flywheel, and needs to store energy as it attempts to start.
But I'm guessing that the resistors are SMT. It will be interesting to hear if these are OK or not. Generally, the start-up resistors I've replaced have all been conventional spiral film resistors.
BTW, I also assume that R947 also has a typo and should be 15k rather than 15R.
Alternatively, if you have 2 identical sets, you could just swap the PSU board if they are the same version? There's a chance the fault is something completely different, but I agree that this is a good place to start.
And it includes the schematic of the PSU. That's not something one can take for granted these days!
I've attached the start-up circuitry from what I believe is the board you have - note that there is more than one possibility.
As you can see, the unregulated supply from the main caps (labelled 400V, but more likely 325V or thereabouts in practice), goes via 3 100k resistors in series. At least, they'd better be 100k! The schematic says "100R", but that has to be typo..
This is how the unit starts up. The current - about 1mA - from these resistors charges C807, and that allows the supply to try to run. The supply labelled "VCC" goes to the controller IC.
But think of this as being like a starter motor. That 1mA might be enough to turn the engine over, but it's not enough to run the engine. So in the same way that the starter motor starts the process of getting sparks and bangs into the engine, this 1mA current is enough to try getting some flux into the switching transformer.
Note D808, which also supplies energy to the VCC rail. This energy comes from a separate winding on the transformer that is intended to run the supply in normal operation. The small current from those 3 100k resistors is only used initially; once running, the bulk of it comes from the transformer.
Unfortunately, high value spiral-cut film resistors with high voltages across them are prone to gradually failing high. Depending on the supply, this might not be noticed for ages - if your engine is easy to turn over, a tired starter motor (or battery) might still succeed in getting it going, but if your engine is diesel, that's far less likely. But eventually, it's possible to get to the point where there isn't enough current available via the startup resistor(s), and the supply won't run.
For stuff that is powered for years at a time - there's a lot of that where I work - this fault often develops months or even years before it's noticed. Should your starter motor mysteriously fail during a long journey, you won't know about it until you switch off the engine and try to restart it!
Anyway, by all means, check each of those 3 resistors. Also check the cap (C807) - that's like the engine flywheel, and needs to store energy as it attempts to start.
But I'm guessing that the resistors are SMT. It will be interesting to hear if these are OK or not. Generally, the start-up resistors I've replaced have all been conventional spiral film resistors.
BTW, I also assume that R947 also has a typo and should be 15k rather than 15R.
Alternatively, if you have 2 identical sets, you could just swap the PSU board if they are the same version? There's a chance the fault is something completely different, but I agree that this is a good place to start.
