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Weird video issue on Model 1

R

rvdbijl

Experienced Member
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May 8, 2017
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I just got another Model 1 (the other one I have has an intermittent fault when pushing on the circuit board, but no matter how much reflowing I've done, I haven't been able to locate it). This one has an interesting video issue. See link to the YouTube video.

I would guess it's one of the static RAMs in the video area, but the fact that typing one character erases/changes another makes me pause that maybe it's something else? Or is this failure something that happens with a bad memory chip?
And if so, how would I locate said chip? I've seen something about piggybacking another 21L02 on top of each chip to see when the problem disappears .. Is that valid? Of course I don't have any, but I did order some 91L02's off eBay that should arrive this weekend that I could try that with.

Any other suggestions on how to debug this would be much appreciated! I did already check (and adjust) the +/-5V and 12V power rails (the +5V was a bit out of spec at 5.54V, the +12V was within the margins), but that adjustment didn't change anything (not that I thought it would ..).
 
The first two lines on your display should be:
MEMORY SIZE?
RADIO SHACK LEVEL II BASIC

But, you are getting something such as the following:
M4MCRQ SAZE?
RIDIG SHICK(DEVEL(II BASIC(

Will you Please post exactly what is displayed on your monitor? With the exact characters
you can decode exactly what Video RAM (BIT) is fiddling with the characters.

One that is easy to spot is the SPACE = 0x20 which is displayed as ( = 0x28
SPACE = 0x20 = 0010 0000B
( = 0x28 = 0010 1000B So Bit 3 is the problem

The word "SIZE" is also displayed as SAZE, making the I into an A
I = 0x49 = 0100 1001B
A = 0x41 = 0100 0001B So Bit 3 is the problem here also.

The Y in Memory also looks like a Q.
Y = 0x59 = 0101 1001B
Q = 0x51 = 0101 0001B So Bit three is the problem here also.

Larry
 
ldkraemer said:
The first two lines on your display should be:
MEMORY SIZE?
RADIO SHACK LEVEL II BASIC

But, you are getting something such as the following:
M4MCRQ SAZE?
RIDIG SHICK(DEVEL(II BASIC(

Will you Please post exactly what is displayed on your monitor? With the exact characters
you can decode exactly what Video RAM (BIT) is fiddling with the characters.

One that is easy to spot is the SPACE = 0x20 which is displayed as ( = 0x28
SPACE = 0x20 = 0010 0000B
( = 0x28 = 0010 1000B So Bit 3 is the problem

The word "SIZE" is also displayed as SAZE, making the I into an A
I = 0x49 = 0100 1001B
A = 0x41 = 0100 0001B So Bit 3 is the problem here also.

The Y in Memory also looks like a Q.
Y = 0x59 = 0101 1001B
Q = 0x51 = 0101 0001B So Bit three is the problem here also.

Larry
Click to expand...
Larry,

Thanks for the insight!

I see this when the system starts:
MMMORQ SAZE?
RIDIG(SHICK DEVEL(II BASIC(
READY (
>_((

Which IC holds Bit3 by the way? I haven't been able to find a clear diagram that shows it.

Robbert
 
Robert,
Bit 3 is Z45 of the Video RAM. See attached PNG file

The E & Y characters are also incorrect.
M = 0x4D = 0100 1101B versus the correct Character E = 0x45 = 0100 0101B ALSO Bit 3
Q = 0x51 = 0101 0001B versus the correct Character Y = 0x59 = 0101 1001B ALSO Bit 3

Replacing Bit 3 (Data 3) should fix your problem. If you haven't removed an IC from a
Motherboard just ask for the best way to prevent ripping up a PCB Trace.

Larry
 

Attachments

  • M1-Z45.png
    M1-Z45.png
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ldkraemer said:
Robert,
Bit 3 is Z45 of the Video RAM. See attached PNG file

The E & Y characters are also incorrect.
M = 0x4D = 0100 1101B versus the correct Character E = 0x45 = 0100 0101B ALSO Bit 3
Q = 0x51 = 0101 0001B versus the correct Character Y = 0x59 = 0101 1001B ALSO Bit 3

Replacing Bit 3 (Data 3) should fix your problem. If you haven't removed an IC from a
Motherboard just ask for the best way to prevent ripping up a PCB Trace.

Larry
Click to expand...

Larry,

Awesome! That one is suspiciously next to another one in a DIP socket. ;) Looks like this has been an issue before, but with another bit.
I haven't pulled IC's from the Tandy circuit board yet, but I was going to try a solder wick and gently remove the solder, or try my solder "sucker". And if that didn't work, go pin by pin after cutting the IC off.

I'm more than happy to hear other suggestions on how to proceed or not to proceed. This is very old stuff, and the last thing I want is to have to put in patch wires since the traces are gone. :)

Thanks!
Robbert
 
Robert,
I use Kester 63/37 Rosin Core solder in .050" size. First thing to do is apply fresh solder to each pin of the IC
you are going to remove (bottom side). Next I'd cut each leg off the IC next to it's body, and remove the IC.
Then use a Solder sucker (with bulb) to remove the fresh solder as you use a small pair of needle nose to
remove the IC pin from the top side. Doing this will leave the board clean and ready for the IC Socket. My
Weller iron is a 25 watt, and the Desoldering Iron with suction bulb is 35 Watt. Large wattage irons will lift
the PCB traces.

Make sure your IC socket is a good Brand. I like the Double wiping IC sockets.

I'm curious if you Model 1 has Lower Case? While you are in there you can check the Character Generator's
number and you might be able to easily add Lower Case if you don't already have it.

Also I upgraded my ROM Version 1.1 to the latest version 1.3, years ago. That got rid of a few problems I couldn't
live with.

Larry.
 
Hi Larry,

Thanks for the insight! I'll follow your advice!
As for the lower case mod -- I looked at what's involved in that. I'd need at least another 74 logic IC that I don't have (I have a few 21L02's on order). Not sure if I want to do that now or maybe later.

Thanks!
Robbert
 
Ian has a method that might work in you case.
I simplified it this way which has served me well since the 80s.

If your character generator (Z29) last 4 numbers are 3001 or 6670, then you do the mod Lowercase with Upper from Custom TRS-80

If your character generator last for numbers are 6673 or 6674, then you do the Radio Shack lowercase mod, from the same book.

All other LC mods are complicated or unnecessary in this day and age, and in fact when I buy systems with other mods I undo
them and set them up as listed, above.

For the record:
3001 : early systems, lowercase but no descenders, and a flying 'a' due to Motorola manufacturing fault (so they sold these
chips cheap to RS who figured they didn't need LC anyway)
6670 : mid-production systems, same as above but with no flying 'a'
6673 : late systems, custom part made especially for the M1 for Radio Shack and included with most late 79 and 1980 machines.

By this time RS was selling the LC modification "kit" for $99 but if the computer already had the 6673 then the 1-min job
was to cut the trace, install the stacked 2102s into the already fitted socket, and solder two wires. It probably took
longer for the soldering iron to warm up than to make the mod.

Ian.
Click to expand...

http://www.vcfed.org/forum/showthre...l-1-in-2016/page2&highlight=Model+1+Lowercase

Larry
 
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