Osborne 1A help fixing

[agentb]

I picked up an Osborne 1A today in excellent condition. But of course not working yet!

Some issues:

1. Missing the video plug/shunt -- anyone have one? hah -- or idea on how to source a 2x10 PCB edge connector with what looks like 2.5mm pitch. I couldn't find anything.
2. Turn on and just get a speaker blip, and LED floppy blip, but nothing after that. I tested the +5 and +12 and looks to have proper voltage. I'll need to take a closer look at the power supply for any signs of trouble. I know those RIFA caps are supposed to be a problem, but I didn't see anything terrible (yet).
3. CRT doesn't sound like it's powered up. Cap issue? Or possibly +12V on the video shunt is required to have the CRT turn on? as no video shunt is used for hooking up an external monitor, maybe internal CRT power is disabled when using an external monitor?

Any other "must fix" or high priority issues would be appreciated. I tested some of the little blue caps, but didn't notice any shorted. Would love to get this back up and running! I never used CP/M or an Osborne before.

 

[Chuck(G)]

To answer your first question, the video shunt is simple a "straight across" connection. Upper pins are connected to their corresponding lower pins.

 

[agentb]

To answer your first question, the video shunt is simple a "straight across" connection. Upper pins are connected to their corresponding lower pins.

Thanks -- I found this on digi-key which looks suitable: https://www.digikey.com/en/products/detail/te-connectivity-amp-connectors/5-5530843-0/770548

 

[agentb]

Good news, I just hooked up the keyboard and hit Return after turning on and the A drive turned on! So looks like the logic board and drive are potentially working. To attempt to boot from the B drive, it's SHIFT+" ? I'll try that next. Seems like I need to focus on the display, hah

 

[agentb]

Tried SHIFT+” and B drive attempted to boot! I’ll need to get or make a video shunt and dig into the CRT

 

[agentb]

Magically picked up *another* Osborne 1 today locally, and this one had the shunt. I tried the video shunt on my current system and no dice. Gotta be a problem with the CRT. I'll have to research the CRT further, I found a SAM PDF that has some info on the CRT.

 

[agentb]

As far as I can tell, the CRT is powered only by 12 volts -- is this correct? I thought for sure there would be AC involved.

 

[tingo]

There must be a high voltage to the anode of the CRT, but it is possible that it could be generated from a 12V power supply. I haven't any insight in the Osborne, so I can't talk about specifics for that machine.

 

[Chuck(G)]

As far as I can tell, the CRT is powered only by 12 volts -- is this correct? I thought for sure there would be AC involved.

Yes, the O1 uses an OEM "kit" monitor, powered by 12V. Not sure which OEM was used, possibly Ball Bros. TV-50 series. OCC did not manufacture the video display hardware.

 

[agentb]

Thanks for the responses. I did confirm it’s only the 12V which (now that I have the video shunt) is being routed to the monitor. Next I’m going to try the CRT from my dead OCC1 in the machine that appears to be booting. (It didn’t work when trying it in the dead logic board machine, which did also seem to have +5 and +12)

Any known common failures on these 5” CRTs? I suppose I could just start replacing caps. I had found a message mentioning a specific part number that often fails but damn if I can find that message again!

 

[agentb]

boo ya, monitor from the dead logic board works! I'll swap this over to the working machine. And probably remove those RIFA caps from the power supply before they explode :wow: