8b1f24c6   
   From: joe@user.com   
      
   "Keith"  wrote in message   
   news:fede4afe-b4ac-4e65-bac7-f33737c605d2@u26g2000yqu.googlegroups.com...   
   > Hi All,   
   >   
   > I am a collector of antique computers, and have recently acquired a   
   > TRS-80. It's just the base model, with a tape drive and monitor.   
   >   
   > I've noticed that the video displays incorrect characters; letters are   
   > switched, that sort of thing. It looks like bits are getting flipped   
   > in the video RAM, but unfortunately, I'm a little ignorant when it   
   > comes to TRS-80 hardware. I was wondering if anyone here has ever   
   > experienced this before, or if someone knows where I can go for   
   > replacement video RAM chips! I'm not decided whether or not it is the   
   > video RAM specifically; I suppose even a bad capacitor would be   
   > capable of causing such a bug. I'm a tad lost!   
   >   
   > The bug is pretty consistent; about 2/3 of the screen is covered in #   
   > marks, which leads me to believe the space character is getting   
   > flipped to something else in video RAM, but also leaves me thinking   
   > that some of the video RAM is still working (there are places where   
   > the space character shows up as a real empty space).   
   >   
   > I've also confirmed that the characters are being stored in system RAM   
   > correctly; I was able to write a simple BASIC program that caused an   
   > infinite loop that printed "HELLO" forever (although "HELLO" looked   
   > more like "HEVV5", or something to that effect).   
   >   
   > I appreciate your time. Thanks for your help!   
   >   
   > Keith   
      
   Ok, first thing to do is open the unit up and see if your characer generator   
   chip is in a socket.  I forget whiich one it is, unfortunately, and my old   
   TRS-80 schematic is nowhere to be found.  Early units had the chip soldered   
   in.  Later units had it socketed to make it easy to upgrade to 'lower case'.   
   Still later units had the lower case chip installed anyway when they ran out   
   of the original character generator chip.  If it's in a socket, press it   
   down so as to make sure it's firmly seated.  If that doesn't work.,   
   carefully pull it out, and plug it back in after checking to make sure there   
   are no bent pins.   
      
   Next step...  Look at the display to figure out what 'bit' is bad.  You say   
   characters are swapped?  Are they really?  Or is it 'one way'?  That is, you   
   imply A and C are swapped, but do you every see both A and C?  Or is it that   
   you see an C where there's supposed to be an A, but you still see a C where   
   there's supposed to be a C?  If it's swapped, I've never seen that.  But   
   when it's 'stuck' on direction, that's pretty common.   
      
   Since the failure is  over 2/3 of the screen, that means it's probably a   
   failure to the chip itself.  The whole screen might mean memory addressing   
   in the video.  The video memory consists of 2102 parts if I remember right,   
   and there are 7 of them in a row.  Unless you have the lower case mod to the   
   machine in which case there will be an 8th chip 'piggy backed' on one of the   
   others with a couple of jumper wires.   
      
   In what you describe, you say spaces are #.  In ascii, a space is 32, and #   
   is a 35.  Also, L is a 76 and V is a 86 in HEX.  O is a 79 and 5 is a 53.   
   There's not a consistant error here, like also off by 8 or something.   
      
   When I see errors like this, it usually points to an address line somehow   
   shorting to a data line.  Not simple to find, but sometime simple to fix.  A   
   lot of times it might be a solder splash, or even a small 'solder ball' that   
   got wedged under a chip lead in the video memory.  Get yourself some wooden   
   toothpicks and start poking around.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)