Learning Things About The 1200XL...

[DavidMil]

I just read an old article written by, Tom Halfhill, about Atari 1200XL's that says Atari put a current limiting resistor (R53) in the SIO port to discourage

anyone from designing devices that drew 5 Volt power from the computer itself. Has anyone ever had a problem with this.? He specifically points to

to the XM301 modem as not working till this modification is made. The article goes on to explain how to get around this by replacing the resistor with a

wire jumper. He also said that Atari recommends that you take computer to a distributer to have this done. Anyone have any more info about this?

 

Thanks,

DavidMil

[Rybags]

The solution is to replace or bridge R63 with a piece of wire which will let the SIO device draw more power.

 

http://atariage.com/forums/topic/221719-sio2sd-and-atari-1200xl-having-problems/?do=findComment&comment=3136311

[DavidMil]

The solution is to replace or bridge R63 with a piece of wire which will let the SIO device draw more power.

 

http://atariage.com/forums/topic/221719-sio2sd-and-atari-1200xl-having-problems/?do=findComment&comment=3136311

 

So the article I read is incorrect! And the correct resistor to bypass is R63 (right beside R53) and not R53 as the article states.

 

Thanks for the correct info,

DavidMil

[+DrVenkman]

I replaced the existing resistor with a 1 ohm jumper rather than just a wire. Looks neater.

[Rybags]

The operative thing would be to test before doing anything - simply check continuity between the resistor and the +5V / Ready SIO pin.

[Paul Westphal]

This is correct. You can do the same to improve the video, but I forget which ones to do..

[+DrVenkman]

This is correct. You can do the same to improve the video, but I forget which ones to do..

 

Here's the ClearPic mod. I've done it on one of my 1200XL's, but one of Bryan's little UAV boards is an easier install and works even better.

 

http://retrobits.net/atari/clearpic.shtml

[Paul Westphal]

Ahhh... thanks Dr.Venkman. I restored 10 1200xl's two years ago. I just did the first three to improve the video. NEW 1200XL KEYBOARD MYLARS ARE NOW AT BEST ELECTRONICS !

[ACML]

 

Here's the ClearPic mod. I've done it on one of my 1200XL's, but one of Bryan's little UAV boards is an easier install and works even better.

I have modified more than a few 1200XLs with the ClearPic 2002 mod and have found it to be very good. I have never tried Bryan's mod. Any chance you could post photos of the two with same image for comparison. I have another 1200XL mobo to mod and I'd like to see if this is something I should try.

 

Thanks

Edited June 15, 2017 by ACML

[+DrVenkman]

I have modified more than a few 1200XLs with the ClearPic 2002 mod and have found it to be very good. I have never tried Bryan's mod. Any chance you could post photos of the two with same image for comparison. I have another 1200XL mobo to mod and I'd like to see if this is something I should try.

 

Thanks

 

Sadly, no. I have only two 1200XLs - one that had the ClearPic mod that now has one the UAV board in it, and a second totally-stock board that doesn't work properly; I'm still trying to figure that one out. Photos of my results with the UAV, however, are posted here.

[1050]

I had a 1200XL that would not boot up a disk drive until I

soldered a wire around that resistor. After that it worked

fine. The problem is real.

 

This is from Atari_1200_XL_Home_Computer_Field_Service_

Manual_Rev01.pdf, page 3.

http://www.atarimania.com/documents/Atari_1200_XL_Home_Computer_Field_Service_Manual_Rev01.pdf

 

 

R63 at 100 ohms will be brown, black, brown. The non-

players will be violet, yellow, red, 4k7 as they say

it in Europe. At any rate the PIA is U23 and note the

horrible handwriting for U23 - that three matches

exactly the three in R63. It looked like 65 to me

first time thru there but that one is actually unreadable.

Have no clue why they said 53.

 

This exact area is in error across ALL other Atari XL/XE

schematics, L10 which takes the place of this resistor is

not in fact taking the place of this resistor. L10 instead

feeds +5 to MMU, cartridge and the center portion of the

mother board where the 800XL and 600XL have a lot of TTL

logic chips.

 

On other XL/XE, +5 buss is actually fed directly to READY

line despite how it is depicted on schematics. L10 is not

used for READY line at all.