Commodore Plus/4 strange behaviour

[cercamon]

Hi,

I need help with my Commodore Plus/4 I recently unboxed after a while. It apparently boots with no problem, but it behaves strangely with the following:

1. the built-in applications start but only show the first introductory screen and then black screen (ie. pressing F1 at the boot shows the welcome screen and then black screen)
2. when I try to type a simple BASIC program the number of lines don't get stored the way I typed in (see pictures below)
3. simple calculations performed using the PRINT command result in big mistakes (see pictures below)

There may be other errors performing other tasks such as loading and run games, BASIC programs, etc. but I couldn't try so far.

Thanks for reading and for any advice you may have on how to repair my PLUS/4.

 

[jap]

Hi,
You may have a faulty RAM chip. Try for example filling the memory area $1000-$3FFF with $FF using the Monitor and then check if the memory area contains $FF values:

MONITOR
F 1000 3FFF FF
M 1000

If you see other values than $FF, there is something wrong with the RAM memory.

 

[cercamon]

Thanks @jap, I have followed your advice.
All normal, using the monitor filling and reading the $FF value from $1000-$3FFF area.
Same result for $4000-$7FFF area, writing in the $FF value and reading it back.
At $8000 I think it begins the BASIC area, as I see the characters of the boot screen followed by commands and error messages.

What other memory areas should I check?

The fact that the built-in programs don't work still makes me think about ROM chips failure, could also be the BASIC ROM responsible for the curious behaviour when entering a simple BASIC program or mistaken results of the PRINT command calculations?

TY

 

[jap]

Yes, it starts to look like a ROM failure to me too. Try unplugging all the ROM chips, cleaning their legs and sockets, and then reinsert the ROMs.

 

[Deleted member 14264]

Possible ROM failure, possible PLA failure

 

[cercamon]

Thanks to both of you. Now looking for documentation about the Plus/4 motherboard so I can locate the ROM and the PLA chips and try reseating them. Any help appreciated as I'm no electronic expert.