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MacColby/128k Screen Issue


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My current project from the Colby estate is to revive a MacColby (1st pic). When I first turned it on, I actually got a loud bong, and the usual floppy question mark, so I was elated (2nd pic). It took me a while to de-stuck the two floppy drives, and I could get everything going in record time. Alas, just as I put everything back together, two new problems popped up. One was an easy fix (one FDD stopped ejecting), but the other one puzzles me: the screen all the sudden turned black. Not the J1-solder joint-problem black, but more like a fuzzy bright zone at the top of the screen (3rd pic). Before I start doing the usual swaps and get the scope out, I thought I'd ask. Has anyone seen this ? 

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Thanks. The flyback crossed my mind, but I did have a pretty strong spark when I discharged the HV connecter to the CRT. Also, I do have a strong single point in the center of the screen after I disconnect J1. Anyhow, I'll start the detective work soon. It's most likely a problem in the vertical raster circuit.

 

Yes, the twin floppy is handy. Turns out that the 128k LB has capability for two FDDs 9internal and external). The was Colby did it was to daisy chain both with one ribbon cable, and simply connect the FDD2 enable signal from the pin 17 of the external FDD connector to the second floppy via line 14 in the ribbon cable.

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  • 1 month later...

Sorry it took a while. I actually banged my head hard on this one but I finally found the source of the trouble. The trimpot R55 was slightly bent, and the lead to one pole was broken. This trimpot adjusts the height of the video on the AB. This was not really visible to the naked eye, but only with a magnifier. I must have pressed on it with my fingers at some point, which led to the breakage.

 

So everything works now, and it's a cute little machine. I took some video shots that I will edit and post on youtube at some point. 

 

Thanks all.

 

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  • 4 weeks later...

Someone asked why MacColby would make such a computer? 

Well, having traded in my Mac512 for a MacColby in 1985 or so, I can tell you that the MacColby was a HUGE improvement over the original Mac in terms of portability! Not only was pretty much everything inside the case, but the keyboard clipped onto the front of it, and the handle made it very easy to pick up! I think I gave them my Mac 512 plus about $1000 to get the MacColby, and I had it until it was stolen from the luggage bay of the bus we were traveling on from Atlantic City to Washington DC, while I was working on the Paul Anka show. Thankfully the insurance company covered my loss and since MacColby computers were no longer available by the time mine was stolen, they got me a brand new Dynamac! 

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On 2/5/2021 at 5:32 PM, Ken Newman said:

Someone asked why MacColby would make such a computer? 

Well, having traded in my Mac512 for a MacColby in 1985 or so, I can tell you that the MacColby was a HUGE improvement over the original Mac in terms of portability! Not only was pretty much everything inside the case, but the keyboard clipped onto the front of it, and the handle made it very easy to pick up! I think I gave them my Mac 512 plus about $1000 to get the MacColby, and I had it until it was stolen from the luggage bay of the bus we were traveling on from Atlantic City to Washington DC, while I was working on the Paul Anka show. Thankfully the insurance company covered my loss and since MacColby computers were no longer available by the time mine was stolen, they got me a brand new Dynamac! 

Thanks Ken for this comment. And I saw the video on this that you recently posted on youtube. That's great !

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