32K only memory problem on 1200XL

[Gunstar]

I've been working on one of my 1200XL's repairing and upgrading it off and on in my spare time. I thought I had everything working and then apparently something has gone bad.

 

Everything works except that only 32K is being recognized out of 64K (I did a memory upgrade, and when the problem started I reversed the upgrade to 64K to discern if it was the upgrade or base machine with the problem).

 

The memory test will test 32K (two rows) and then start over with the OS chips and not read any bad memory with red squares. Basic works, programs 32K and under work.

 

I've swapped out memory chips with no success, so I figure it must be something else (unless it has to do with the sockets). I think I swapped out the MMU (CO1618 from 800XL-the 1200 has an OS&basic upgrade) and I think I tried swapping out the LS158 chips.

 

It's been a few weeks since I last worked on it so I'm not postive. I'm just looking for some possible insight from possibly more experienced hardware-hackers.

 

Besides the 800XL OS and basic installed, other current upgrades include a stereo pokey board (MetalGuy66's) and SV 2.1 video, everthing else was returned to stock until I figure out this problem.

Edited January 18, 2012 by Gunstar

[sloopy]

The machine possibly thinks it has a 16k cart installed, or more specifically RD4 is probably being held high (Line A on the cart port), this would make the machine think it only has 32k of ram with no errors...

 

sloopy.

[+tf_hh]

Do you have tried to build-in a DRAM based memory enhancement? Most of them have instructions to cut off some wirings on the PCB...

 

If the hint one post before doesn´t success, I would try...

 

Check if addressline A15 (Pin 25 on CPU) is well connected to the Pin 10 of 74LS158 (U10)

Check if addressline A14 (Pin 24 on CPU) is well connected to the Pin 13 of 74LS158 (U10)

 

If not, connect them each.

[Gunstar]

Do you have tried to build-in a DRAM based memory enhancement? Most of them have instructions to cut off some wirings on the PCB...

 

If the hint one post before doesn´t success, I would try...

 

Check if addressline A15 (Pin 25 on CPU) is well connected to the Pin 10 of 74LS158 (U10)

Check if addressline A14 (Pin 24 on CPU) is well connected to the Pin 13 of 74LS158 (U10)

 

If not, connect them each.

 

I'll look into that, the DRAM upgrade that was in it was the Rambo XL, with that one no traces are cut, I'll double check those connections.

 

@ Sloopy, if RD4 is being held high, would that be a short or open circuit?

[sloopy]

Do you have tried to build-in a DRAM based memory enhancement? Most of them have instructions to cut off some wirings on the PCB...

 

If the hint one post before doesn´t success, I would try...

 

Check if addressline A15 (Pin 25 on CPU) is well connected to the Pin 10 of 74LS158 (U10)

Check if addressline A14 (Pin 24 on CPU) is well connected to the Pin 13 of 74LS158 (U10)

 

If not, connect them each.

 

I'll look into that, the DRAM upgrade that was in it was the Rambo XL, with that one no traces are cut, I'll double check those connections.

 

@ Sloopy, if RD4 is being held high, would that be a short or open circuit?

 

short... open it should be GND...

 

sloopy.

[Gunstar]

Thanks for the help and insight guys, I found the problem, a broken trace where it connects to a jumper. Self-test shows all 48K now, and programs over 32K now work again!