Jump to content

MacColby/128k Screen Issue

Recommended Posts

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 ? 


Link to post
Share on other sites

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.

Link to post
Share on other sites
  • 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.



Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...