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AwkwardPotato

32K Memory Card PHP1260 Repair Tip

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Hi all,

 

Thought I'd share this in case anybody with a PEB 32K card (PHP1260) runs into a similar issue, since this was a bit confusing at first. It would appear that this also applies to the original PHP2200 32K sidecar, but I haven't verified that past a quick glance at the schematics.

 

The symptoms of my card's failure:

  • XB or EA5 programs loaded from disk either error out or crash the system, respectively
  • Doing a rudimentary RAM test with the Minimem cart (as described by Willsy here) reports seemingly random failures
  • Listing of an XB program looks like below, where e.g. "DSK1.SPEAK" becomes "DSK1, @EAK", and where incorrect characters change with each successive program listing

 

2127908454_screenshot(2).thumb.jpg.be6614bdda3c3cd46d80a8ecfc38c5f2.jpg

 

In my case, the culprit was one of the two 74LS257 (U15, U16), although I accidentally removed the wrong one at first and ended up replacing both at the same time.

 

While the 32K card is now functional, moving forward I'll look into possibly picking up a SAMS card. Been a while since I got new 4A goodies...

 

 

 

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Posted (edited)

I've heard from others that the buffers go bad bad once in awhile.

Great fix!!

Edited by GDMike
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Thanks for the tip. I have card that when I power up with my Card I just get a blank screen. When I replace my 32k PEB card (PHP1260)  with another, it works. So I need to figure out which chip(s) are causing the issue. 

 

I will start with the 2, 74LS257 (U15, U16). I will test those with my retro chip tester. Hopefully that will fix my 32k board!

 

Thanks!

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Posted (edited)
8 hours ago, videofx said:

Thanks for the tip. I have card that when I power up with my PCard I just get a blank screen. When I replace my 32k PEB card (PHP1260)  with another, it works. So I need to figure out which chip(s) are causing the issue. 

 

I will start with the 2, 74LS257 (U15, U16). I will test those with my retro chip tester. Hopefully that will fix my 32k board!

 

Thanks!

just an edit. my 32k PEB card fails with my Card Enabled. I will check the 74LS257s tomorrow

Edited by videofx
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On 5/19/2022 at 9:28 AM, videofx said:

Thanks for the tip. I have card that when I power up with my Card I just get a blank screen. When I replace my 32k PEB card (PHP1260)  with another, it works. So I need to figure out which chip(s) are causing the issue. 

 

I will start with the 2, 74LS257 (U15, U16). I will test those with my retro chip tester. Hopefully that will fix my 32k board!

 

Thanks!

The LS257s on the 32K card are used to multiplex the row and column addresses onto the 4116 DRAMs. If any of the LS257s' outputs fails (hi-Z/floating in my case), reads and writes to certain parts of the 32K expansion will also fail. For a card like yours that locks up the TI at startup, I'd suspect one of the parts closer to the edge connector. The 74LS245 buffer comes to mind, and the voltage regulators are also worth checking just to rule out the obvious. The schematics for the 32K card can be found on Mainbyte.

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2 minutes ago, AwkwardPotato said:

The LS257s on the 32K card are used to multiplex the row and column addresses onto the 4116 DRAMs. If any of the LS257s' outputs fails (hi-Z/floating in my case), reads and writes to certain parts of the 32K expansion will also fail. For a card like yours that locks up the TI at startup, I'd suspect one of the parts closer to the edge connector. The 74LS245 buffer comes to mind, and the voltage regulators are also worth checking just to rule out the obvious. The schematics for the 32K card can be found on Mainbyte.

It only happens at power up when my P-Code card is turned on in the PEB. If I switch the P-Code card off then everything works fine until something accesses the 32K card.

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Do other programs accessing the 32K cause the system to lock up too? If not, it might be worthwhile to try one of the memory test programs out there.

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