Jump to content
Sign in to follow this  
xrbrevin

Advice needed for RAM board repair

Recommended Posts

Can anyone give me any pointers as to how to diagnose a 400/800 RAM board? I have a 48k one and a 32k one and i feel ive done all that i can with them but I just keep getting the red/brown screen.

ive checked traces for continuity where there should be and shorts where there shouldnt be. Ive checked the 4116 DRAMs work in a standard board and ive checked that the 4164 DRAMs work in an XL.

The 48k one receives it's SEL4, SEL5, A14 and A15 connections from the Z103 socket (via a small piggyback PCB) and it also gets the RD4 and RD5 from the cartridge socket (via 2x wires).

Does anyone know of a procedure for troubleshooting? Im wondering if the 74LS glue logic is at fault.

cheers!

Share this post


Link to post
Share on other sites

I'm no expert but there isn't much else on those boards to go wrong 

Share this post


Link to post
Share on other sites
Posted (edited)

Yes, the 74LS chips, while generally durable, can go bad. If they're socketed, swap them out.

 

Also check for shorted caps on the power busses. Use an ohmmeter between each bus and ground.

Edited by ClausB

Share this post


Link to post
Share on other sites

If you have a logic probe, you can probe the inputs and outputs of the 74LS chips to try to get an idea.... generally the data should be changing..... if you see something stuck high or low then you would focus in on those chips first when diagnosing.  You may also run into neither high nor low.... (open).  Unless the chip is a tri-state device, this would indicate something is wrong (if it is an output) (if it is an input, it simply may not be connected to anything).

 

Share this post


Link to post
Share on other sites

Thanks for the suggestions. I have a logic probe and the back of the RAM board is accessible so i'll have a mooch about.

I thought maybe i would take a macro photo of the PCB and then zoom in on the computer to check if any traces are in amiss.

On the 32k board I did some continuity testing across the DRAMS and all lines appear to be behaving as they should. All address lines are linked, -5v 12v 5v and GND are linked with no crossovers. RAS and CAS lines are linked but only on every other chip, i assume this is due to it being 2x16k on a single board.

If the worst comes to the worst, i will salvage the 4116s and make my standard Atari RAM board into a 32k one. These '80s aftermarket PCBs dont have a silkscreen and the traces are less than robust!!

Share this post


Link to post
Share on other sites

A TL866 programmer can test function of LS logic chips with a few clicks if you're in the market for one. Otherwise the aforementioned methods are of course cheaper. :)

Share this post


Link to post
Share on other sites

I have a TL866 and ive occasionally used that feature. altho, it has never found a bad 74LS thus far so im unsure if its accurate...

Share this post


Link to post
Share on other sites
4 hours ago, xrbrevin said:

I have a TL866 and ive occasionally used that feature. altho, it has never found a bad 74LS thus far so im unsure if its accurate...

It probably is - when I got my TL866, I tested it with a 4050 hex buffer that I know was bad (pulled from an 800XL video circuit that had lost color output). I also tested it with a couple 74-series logic chips I pulled from a damaged C64 a few years ago and it successfully ID’d the damaged gate. 

  • Thanks 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...