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Osborne Executive CRT problem

If there's no light on the CRT, you could check the voltage to the heater on the neck of the tube. Actually, for all "Tube Dead" type scenarios, the first thing is to check that the voltages are all present. The Shunt on the Osborne 1 / Executive takes the power from the PSU to the Screen ( if there's no shunt, there's no screen, power or otherwise ) so make sure your shunt is in place and in good condition... Then check that you have the appropriate voltages at the screen.

The shunt carries Voltage, Signal and some other adjustments to the screen, and when you unplug the shunt to install an external monitor, all of this disappears and the video is disconnected.

Once you've done that, you can check for things like the heater voltage on the neck, which doesn't go through much of the monitor circuitry, since that should heat up even if your deflection and other circuits are not running.

Here's the Osborne Service Manual which contains the Executive Monitor service manual elements.


As you can see, from the block diagram, it's a very simple monitor, and your primary checks should be that the heater is working - which from your earlier posts, appears to be the first issue. This should be a simple circuit to trace from power to the tube neck.
 
the label on the shield says Zenith which means there is no schematic. As cj7hawk said you need to find the source for the 12V to the tube filament. If you carefully pull off the tube socket the printing on the socket might give
some clues, Filament pins are usually labeled H or F. I'm guessing the red wire is 12V. You could measure red to black and see if there's 12V. Following the red wire back to the main board and then circuit traces might lead to the shunt
resistor he mentioned which would be a higher wattage low ohms so physically bigger. If you find it measure ohms across it with power off. That's assuming no 12V to the tube. I'm just guessing, I've never seen the insides of an Osborne.
If two pins are labeled H or F measure ohms across those pins on the tube. The filament should measure low ohms. It's also possible the filament is open, I hope not.
 
cj7hawk said:
If there's no light on the CRT, you could check the voltage to the heater on the neck of the tube. Actually, for all "Tube Dead" type scenarios, the first thing is to check that the voltages are all present. The Shunt on the Osborne 1 / Executive takes the power from the PSU to the Screen ( if there's no shunt, there's no screen, power or otherwise ) so make sure your shunt is in place and in good condition... Then check that you have the appropriate voltages at the screen.

The shunt carries Voltage, Signal and some other adjustments to the screen, and when you unplug the shunt to install an external monitor, all of this disappears and the video is disconnected.

Once you've done that, you can check for things like the heater voltage on the neck, which doesn't go through much of the monitor circuitry, since that should heat up even if your deflection and other circuits are not running.

Here's the Osborne Service Manual which contains the Executive Monitor service manual elements.


As you can see, from the block diagram, it's a very simple monitor, and your primary checks should be that the heater is working - which from your earlier posts, appears to be the first issue. This should be a simple circuit to trace from power to the tube neck.
Click to expand...
Thank you, I did have a manual but it was in German and not as complete. what is a shunt? I look up "crt shunt" and get pictures of implants, I have thought about checking voltage on the neck but I'm hesitant just to put my probes on any wire as I don't want to get electrocuted/ blow components as it would be game over. When i get free time this week i will dive into the heater circuit to the neck tube. I forgot to mention, I do have one of those small handheld oscilloscopes but have never used it/know how to.
 
retrogear said:
the label on the shield says Zenith which means there is no schematic. As cj7hawk said you need to find the source for the 12V to the tube filament. If you carefully pull off the tube socket the printing on the socket might give
some clues, Filament pins are usually labeled H or F. I'm guessing the red wire is 12V. You could measure red to black and see if there's 12V. Following the red wire back to the main board and then circuit traces might lead to the shunt
resistor he mentioned which would be a higher wattage low ohms so physically bigger. If you find it measure ohms across it with power off. That's assuming no 12V to the tube. I'm just guessing, I've never seen the insides of an Osborne.
If two pins are labeled H or F measure ohms across those pins on the tube. The filament should measure low ohms. It's also possible the filament is open, I hope not.
Click to expand...
I haven't discharged the crt itself, I will be super safe and considerate locating the 12v, the crt itself if you look at the pics, has 8 or so colored wires, my previous problem was not having the right schematic to see which was the 12v.
 
Culprit said:
I haven't discharged the crt itself, I will be super safe and considerate locating the 12v, the crt itself if you look at the pics, has 8 or so colored wires, my previous problem was not having the right schematic to see which was the 12v.
Click to expand...
I would also like to mention that I am pretty experienced with soldering, coming from modding consoles, but I am just not good at using an oscilloscope and tracing with a diagram.
 
A shunt simply connects two points of an electrical circuit to "bypass" a point on the PCB. In this case, there's a small edge connector on the Osborne Executive near the reset button that moves signals from the bottom of the PCB to the top.

Osborne Executive Shunt.PNG
I've marked the shunt with the red circle in the picture above.

The black IDC plug fits over the edge connector and can be easily removed. It's a ribbon cable that has been shifted angularly by 1 pin so the wipes on the bottom are connected to the wipes on the top, and they then cut off the remainder of the cable and put some silicon there to make it look nice.

Anyway, all of the video signals are on the bottom of the shunt, and the top of the shunt goes to the internal connector to the monitor. If this shunt plug isn't in place, then the signals from the video circuit AND the power to the monitor are cut off.

It's there so you can remove the internal video ( and shut down the internal CRT ) and run this to an external monitor. If it's missing? Then no power power to the monitor. It's a common cause to think an Osborne is faulty when working with an Osborne for the first time. So if the monitor isn't working, step 1 is to make sure this is there.

If it's not, you can make one pretty easily. Just get an edge plug and solder from the bottom to the top of the connector, or do like they did, and put in a ribbon cable with 1 less wire, and twist it sideways as it goes between the contacts to short the top to the bottom.
 
cj7hawk said:
A shunt simply connects two points of an electrical circuit to "bypass" a point on the PCB. In this case, there's a small edge connector on the Osborne Executive near the reset button that moves signals from the bottom of the PCB to the top.

View attachment 1289205
I've marked the shunt with the red circle in the picture above.

The black IDC plug fits over the edge connector and can be easily removed. It's a ribbon cable that has been shifted angularly by 1 pin so the wipes on the bottom are connected to the wipes on the top, and they then cut off the remainder of the cable and put some silicon there to make it look nice.

Anyway, all of the video signals are on the bottom of the shunt, and the top of the shunt goes to the internal connector to the monitor. If this shunt plug isn't in place, then the signals from the video circuit AND the power to the monitor are cut off.

It's there so you can remove the internal video ( and shut down the internal CRT ) and run this to an external monitor. If it's missing? Then no power power to the monitor. It's a common cause to think an Osborne is faulty when working with an Osborne for the first time. So if the monitor isn't working, step 1 is to make sure this is there.

If it's not, you can make one pretty easily. Just get an edge plug and solder from the bottom to the top of the connector, or do like they did, and put in a ribbon cable with 1 less wire, and twist it sideways as it goes between the contacts to short the top to the bottom.
Click to expand...
Oh yes, the shunt is present on my machine and I cleaned the contacts. I tried the shunt connected and disconnected as a sanity check. Once I see if there is (or isn't) 12v going to the CRT, this could be the culprit if there is. Also beautiful machine if that is your picture! Is the output always active? I had the shunt in place and had output from the RCA jack.
 
Wow Adam Osborne made the first docking station, same concept of re-routing power and video externally.

Larry G
 
Culprit said:
Oh yes, the shunt is present on my machine and I cleaned the contacts. I tried the shunt connected and disconnected as a sanity check. Once I see if there is (or isn't) 12v going to the CRT, this could be the culprit if there is. Also beautiful machine if that is your picture! Is the output always active? I had the shunt in place and had output from the RCA jack.
Click to expand...

Not my machine. I just googled for images to find a clean image to highlight the shunt. My machine is nice, but not that nice. Yes, check 12v is making it through the shunt first, and reaching the internal monitor.
 
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