Compaq Portable I Plus - Only Detects 256k RAM - 640k Installed

[kb9moe]

I have recently picked up a Compaq Portable I Plus and other than a few scuffs, the unit is near perfect. I made a disk with Checkit and another with Sysinfo8 and both programs would not load (Not enough free memory). I made the Compaq Diagnostic floppy, and ran the inspect program. I see that the memory is detected as 256k. Thinking that there may be a bad memory chip, I decided to try and remove a dram chip from Bank 3 and the computer showed a memory error on power on. I repeated the steps on Bank 2 and Bank 1 (Bank 0 is soldered). All will give an error. I put everything back the way it was, the system shows no error, but still only detects 256k. I removed all of the chips in Bank 3 and the system reports 192k present and no errors. I am at a little bit of a loss on what direction to go from here.

I have the system board version 2, with ROM version G. Bank 0 and 1 contain 4464 chips and Bank 2 and 3 contain 41256 chips.

Any ideas?

Is there a good service manual for this version?

 

[Juror22]

Not sure about a manual, but there are switches for setting the memory amount (see the table available on the link, about 1/2 way down)
https://oldcrap.org/2020/10/08/compaq-portable/

 

[JohnElliott]

There is a service manual at Bitsavers: Compaq_Maintenance_and_Service_Guide_198411.pdf at https://bitsavers.org/pdf/compaq/ . Don't know if it's considered to be a good one.

 

[jafir]

Can you see what part number is in U35?

https://minuszerodegrees.net/rom/bin/Compaq/Compaq Portable - U35.htm

 

[kb9moe]

Can you see what part number is in U35?

It is 100340-001. So that answers it. Someone must have replaced this chip at some point because this machine has 1986 dates all over the inside.

So now I need to get a hold of the correct U35 for this model.