From: kr-lund@nogarbage.online.no   
      
   solarfox@DON'TMESSWITHtexas.net wrote:   
   > On 31 May 2007 05:21:39 -0700, Kallikak  didst   
   > expound thusly:   
   >   
   >   
   >>this one has a fault with the video, so it may not attract much   
   >>interest.  Anyone here have any thoughts on what could cause the   
   >>particular fault described and how easy or difficult it would be to   
   >>fix?  I am used to playing around in side Apple IIs and Macintoshes,   
   >>but don't know the internals of a model 1 TRS-80 at all.  I do have a   
   >>copy of the technical manual though.   
   >   
   >   
   > 	If you can diagnose and fix Apple II problems, and you have the TRS-80   
   > technical manual with the schematics (which the seller is including as part   
   > of the auction, BTW), you shouldn't have too much trouble working out   
   > what's going on inside a TRS-80 -- the Technical Reference Manual gives   
   > pretty complete explanations as to how each part of the circuit works, and   
   > has a fairly comprehensive troubleshooting section.   
   >   
   > 	Judging from the screen shots, it's definitely a video problem...   
   > Normally, my first suspect would be the video RAMs, but in this case I'm   
   > more inclined to think either the character-generator ROM or (more likely)   
   > one of the TTL chips in the video processing chain are at fault.  Either   
   > the ROM is serving up garbage, or the pixel data isn't being clocked   
   > through the chain properly.   
   >   
   > 	According to the tech-ref manual (page 70), likely suspects are Z10   
   > (74LS166 shift register), Z26 (74LS20 dual 4-input NAND gate), Z27 (74LS175   
   > quad D flip-flop), and Z30 (74LS02 quad 2-input NOR gate).  The character   
   > generator at Z29 is also potentially suspect, but I'd rule out the other   
   > chips in the video system first, since 74LS chips are much easier to come   
   > by than the MCM6670 character generator.   
      
   To me it seems you see part of the correct characters, it is just that   
   the lines in the character are messed up.   
      
   My guess would be bad contact on the character generator ROM (it sees   
   one line as open which in TTL means high).   
      
   Z12 74LS93 outputs the line "addresses" L1 L2 L4 but it is in the   
   divider chain and must be counting correctly as you have the normal   
   number of lines (of characters) on the screen. The character generator   
   ROM might be broken but it is more likely it is only bad contact. L4? If   
   L4 was stuck out of Z12 there would be no video at all. Contact cleaner   
   spray in the ROM socket might do wonders.   
      
   --   
   Knut   
   (delete 'nogarbage.' for email)   
      
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