18-05-2015, 10:15 PM
OK, so if the dots can't be moved horizontally, that suggest a problem with the X output stage. The fact they are in the centre of the screen tells us that the X plates have the same voltage across them. Good...
The fact the dots are of normal brightness is good - that means the timebase bistable has told the Z-amp to brighten up the trace. You might find (with a DVM) that the Gate output is high. I would have thought that by fiddling with the timebase and trigger controls you should be able to make the dots disappear and/or flash...
It's perhaps safe to assume that only one component is at fault (famous last words, but you have to start somewhere!). So, looking at the schematic, is there a single component that could cause this?
Before going further, it would help to know what the voltages are on the X plates. If they are high, for example, then that tells us that the output transistors aren't conducting at all, and that might point us to TR82, Or perhaps R137 and/or R152, which supplies base current. In my experience, any resistor that has more than about 50V across it is immediately suspect - even though the resistor might not be running obviously hot, film resistors have a failure mode that sends them high and ultimately O/C.
The fact the X-shift controls do nothing is a big clue here. Despite the apparent complexity, the X-amp is pretty simple when you break it down. TR76 is a common-emitter amplifier with shunt feedback via R122. The base of the transistor is a summing point, taking current from R121 (from the X-shift controls) and R103 (from the sweep generator or Ext X). The output is fed into the long-tail pair made of TR78 and TR79. The output transistors TR77 and TR81 are cascoded from the collectors of the LTP. There is some complexity caused by all those diodes, but those are switches used to tell the long-tail pair to take their input from a Y amplifier when in X-Y mode.
Anyway, a few voltage checks would be useful. I appreciate what you say about the difficulty of doing this, but you should be able to reach the nodes from the component-side of the PCB? You might need to go to a resistor lead rather than a transistor, of course. At the risk of stating the obvious, it's useful to remember that the metal cans of transistors are usually connected to collector - if that helps?
If nothing else, what's the voltage across R136?
The fact the dots are of normal brightness is good - that means the timebase bistable has told the Z-amp to brighten up the trace. You might find (with a DVM) that the Gate output is high. I would have thought that by fiddling with the timebase and trigger controls you should be able to make the dots disappear and/or flash...
It's perhaps safe to assume that only one component is at fault (famous last words, but you have to start somewhere!). So, looking at the schematic, is there a single component that could cause this?
Before going further, it would help to know what the voltages are on the X plates. If they are high, for example, then that tells us that the output transistors aren't conducting at all, and that might point us to TR82, Or perhaps R137 and/or R152, which supplies base current. In my experience, any resistor that has more than about 50V across it is immediately suspect - even though the resistor might not be running obviously hot, film resistors have a failure mode that sends them high and ultimately O/C.
The fact the X-shift controls do nothing is a big clue here. Despite the apparent complexity, the X-amp is pretty simple when you break it down. TR76 is a common-emitter amplifier with shunt feedback via R122. The base of the transistor is a summing point, taking current from R121 (from the X-shift controls) and R103 (from the sweep generator or Ext X). The output is fed into the long-tail pair made of TR78 and TR79. The output transistors TR77 and TR81 are cascoded from the collectors of the LTP. There is some complexity caused by all those diodes, but those are switches used to tell the long-tail pair to take their input from a Y amplifier when in X-Y mode.
Anyway, a few voltage checks would be useful. I appreciate what you say about the difficulty of doing this, but you should be able to reach the nodes from the component-side of the PCB? You might need to go to a resistor lead rather than a transistor, of course. At the risk of stating the obvious, it's useful to remember that the metal cans of transistors are usually connected to collector - if that helps?
If nothing else, what's the voltage across R136?
