5150 mainboard diagnostics (partially dead)

[rvdbijl]

Hi All,

I picked up a 5150 (16-64kb mainboard) from eBay in a non-working state (as in, dead) and kind of hobbying around trying to get it to come back to life. Turns out the power supply was bad (really bad, as in a blown fuse.. Replacing the fuse got a puff of smoke and a fried NTC.. Not going to debug that one any further), and it looks like the motherboard is bad too (As a new power supply didn't get any POST or beeps from it).

So I downloaded and programmed the Landmark / SuperSoft test ROM and popped it in. That appeared to work, so the board isn't completely dead.. But that said, it fails on the RAM Refresh test. Doing some reading, it looks like the RAM Refresh test is done on the first 16k only (which passes in the critical memory test, see picture attached). The Landmark / Supersoft manual is saying that you should replace the 8253 timer and try again. Before I de-solder that device and scour eBay for a replacement -- does anyone know how you can diagnose if this thing is working properly? I have some test tools here (scope, multimeter, and I can get my hands on a logic analyzer if need be), but I'm not sure what I should see where...
I also understand that the error can be with one or more of the RAM chips in bank 0. But since those are soldered down, not sure if it would help to try to peel them off one by one and try replacing them..

Any suggestions would be appreciated!

Thanks!

 

[Chuck(G)]

Does the speaker make a beep? If so, the 8253 is probably good.

 

[rvdbijl]

Does the speaker make a beep? If so, the 8253 is probably good.

Yup.. The speaker makes noise when the Landmark ROM finds an error.. Guessing it's probably RAM.

I did put a scope on 2 of the 3 timer outputs of the 8253 (OUT0 and OUT1), and I see square waves at 5V on the OUT pin for both of them. Just don't know what frequency I should see there and if that's maybe off or something.

Ordered some 4116 IC's and I'll see what I can figure out once I get them in. Too bad that first bank isn't socketed....

 

[David_M]

Its not a faulty RAM, it passes that test. Trace the logic back from the RAS pin of one of the DRAMS to the 8253, I expect you will find your fault along that path.

 

[rvdbijl]

Looks like it was RAM. On a hunch I de-soldered the Bit 7 IC in bank 0, and replaced it with one from bank 3. The bit 7 indicator was lit in the Landmark diags, so I figured that was a good place to start.
After that, no more RAM Refresh errors, and the diagnostic continued to the other tests (obviously failing the memory check of bank 3). I replaced the standard BIOS and it booted fine (after configuring the motherboard for 48K). Managed to boot into BASIC and into Dos 1.0.
Until I get the 4116's in that I ordered, I won't be able to put the 576K memory board back in there, but it looks like everything is ok!

Thanks for the suggestions!