Commodore 1700 RAM and C128 issues

[rwicksall]

So I got my first Commodore 1700 RAM expansion and I'm having some issues.  It will not work 100% on my C128 but appears to work fine on my C64.

 

If I fill the screen with text and then stash it to the RAM:

stash 1000,1024,0,0

And then clear the screen and fetch the contents back to the screen:

fetch 1000,1024,0,0

Every other 64 bytes is missing starting with the first 64.  The odd thing is on the 64 byte blocks that are not returned, text on the screen is not overwritten and no garbage is returned.  Like the transfer just doesn't happen.  Image attached.

 

The 128 seems to work perfectly otherwise.  I have a couple versions of the Final Cartridge and a few early Commodore game carts.  The Cart slot seems to work fine.

 

Any ideas where I should start looking?  The system was recapped earlier in the year.

 

Edit: the image attached was run on a cleared screen so you can see the gaps but not the original text that is not overwritten.

Edited December 15, 2020 by rwicksall

[rwicksall]

I'm starting to think there is a stuck address line or maybe bad power supply issue.  My 64 has a brand new power supply and I know these 17XX's require a bigger supply.  I recapped the motherboard but the power supply hasn't been refurbished.  I don't have the proper tools to test but is seems like the REU is having trouble controlling the 128's address lines.

[rwicksall]

Well it is not the power supply.  Recapped and I get the same result.

[+OLD CS1]

Shot in the dark: I was thinking DMA controller, but since the problem is specific to your 128 -- maybe the MMU?  I am not sure of the interaction between the two, but IIRC the MMU is involved in memory access from the REU as well as the CPU.  I suppose you do not have another 128 to test on.

[rwicksall]

I just have the one 128.  I used this as an excuse to pick up a dead test cart.   Maybe that will find something.

 

Based on the pattern on the screen I think address line 6 may be the issue.  If it is stuck high I think it would cause this problem.  I also found a blog post with similar issues and looking at the schematic there a handful of chips that I could shotgun onto the board.

 

http://ode2commies.blogspot.com/2015/08/the-commodore-128d-journey-ends.html

 

This is my personal 128 from 1985ish and it saw a few years of heavy use.  I wouldn't be surprised if something got damaged swapping the final cartridge a million times. 

[rwicksall]

Figured it out.  Looks like the 74F245N chip in U55 was bad.  I swapped it and the SN74ALS373N in U17 with new chips and now the memory expansion works as expected.