Internal 32K RAM mod - Help

[TMAOne]

Proteus, yes I do. Not permanently, since I intended to put the AND gate at pin 1, but I have an alligator jumper there. You can see one end in the picture; the other is at a ground.

sirmorris, I don't have a source for 486 boards these days, but do have more of the narrow RAMs seen in the 2nd picture. I was rather trying to avoid that piggyback mess this time around, so the mod would fit back into the original case.

[TMAOne]

I got back around to working on this part of "Dad's Sinclair stuff", and got the mod working on the TS1000.

For the record, I had misinterpreted the instructions and thought that for the HRG8 option to be included, the resistor connection was to be the only thing attached to pin 27. You can see in the photo I had turned the leg up. Oops. Not cool. It pays to walk away sometimes and come back to it later and check things over with a fresh perspective. I hadn't included the HRG8 resistor in my Frankenstein ZX81, but can see in the 2nd photo that the leg is into the socket on that one. Well there's a clue Sherlock. Once I fixed that on the Timex, the mod worked perfectly. I had used one of the D43256C-12L chips recovered from the eBay board.

Now I'm doing the mod on another TS1000 with the faster HY62256ALP-70 chip I'd started with. It's working for the first 16K, so timing doesn't seem to be an issue there. However, I think writing to the 48-64K area clobbered the basic 16K. I may have a problem with the logic gate, which is probably another wiring boo-boo or possibly (less likely) a timing issue. I used one AND gate of a 74LS09PC instead of the two inverting AND gates of an LS00 recommended in the instructions. I'm assuming the author just used what he had on hand at the time, as I have. His way just works out to be an AND after inverting twice.

I really hate to have to open the computer up again since those brittle ribbons can't hack the stress, but I'll have to do that another night to see where I went wrong this time.

Cheers,
Ian

[sirmorris]

There might also have been a method in the 2-gate solution - it would introduce another ~4ns propagation delay which may or may not be significant. I suspect not but it's something to bear in mind..!

[TMAOne]

Okay, this is kind of funky.

The good news is it works now. But I don't understand why,...

I checked my wiring, voltage to the chip, and inspected for shorts. Nothing found. So I started with the substitution of least resistance--swapping the actual gate chip, since the actual wiring was conveniently to a socket. I had a 74LS09PC in there, which is a quad AND gate. I substituted a 74LS08PC, which is also a quad AND gate, with the identical pinout. (1) I don't understand why they're different. (2) Whatever the difference is, the 08 works whereas the 09 didn't. If I believed in lottery tickets I might be tempted to think I picked on the one faulty component of whatever I took the 74LS09PC out of, but I hardly think that's likely.

So, lessons learned about the ZXTeam 32K mod:
- HY62256ALP-70 RAM chip works fine--the fast speed is not a problem.
- A single AND gate can take the place of the tied dual NAND gates.
- Use a 74LS08PC, not a 74LS09PC for the AND.

Miscellaneous lesson learned:
Pick up an extra of RWAP's nifty replacement keyboards to keep uninstalled. It makes a fine way to test a ZX/TS without reassembling it and stressing the original keyboard ribbon. The long and more durable ribbon of the replacement takes the keyboard out past the heat sink when you lay it flat on your bench, and you can run your computer nekkid.

Just have to reassemble now and I think I'm done with this refurb.
Ian

[1024MAK]

A 74LS09 has open-collector outputs. This type of output does not actively drive the output to a logic high. That is to say, it can only pull the output to a logic low. Normally gates with open-collector outputs have a pull-up resistor on the output (connected to the +5V line).

A 74LS08 has a normal output stage that can drive the output both high and low.

Mark

[TMAOne]

Awesome. Thank-you Mark.

So not all gates are created equal,... an aspect to keep track of when I take the course from the back of a book of matches and become a computer designer.

Ian

[sirmorris]

Congratulations on the successful refurb - and that's a cool tip about the external membrane!