exxos's Atari, Amiga & retro forum

Hmmm, the ZX81 memory plot thickens.

Checking the 4116 RAM chips on the RAM pack, there should be 3 voltages:

Pin 1: -5v
Pin 8: 12v
Pin 9: 5v

Pins 1 and 9 read fine, but pin 8, I'm seeing just under 8V.

Checking the edge connector, there are two voltages consumed by the RAM pack, 5v taken from the 7805 regulator, and 9v consumed directly from the unregulated power input. The 5v line is fine, but the 9v line is reading about 2-3 volts with no RAM pack connected, so clearly something is wrong there.

Interestingly, I'm using my bench PSU and the ZX81 is drawing just over 300Ma when on, and about 350Ma with the RAM pack plugged in, and I'd expect to see it pull more. I guess I need to see what's going on with that 9V unregulated line where it's picked off an taken over to the edge connector. Fingers crossed once that's restored, the RAM pack will work once all the correct voltages are in play.

Well, it turns out that the RAM pack power problems were all down to really crappy edge connector on the ZX81.

A good cleanup later, IPA followed by Electrolube, plus a light skimming of solder over the bottom edge connector pads to bring them back to life, and the machine now runs a treat with the 16K Memotech RAM pack.


What's on screen is the program that queries RAMTOP and gives you a value:

17407 for 1K internal RAM
18431 for 2K internal RAM
32767 for 16K RAM Pack
49511 for 32K RAM Pack
65535 for 64K RAM Pack

I don't think these values are 100% correct as my little machine comes back with 32768, but that's close enough, and I get a different value when I remove the RAM pack, consistent with 1K. Both my results were one higher than the table below.

What was a useful exercise was to run it off my bench PSU, set for 9v & 1.2A. I was able to see the current draw at 450ma with the RAM pack working, 400ma without, so I know the original PSU I have, a 600ma one, will and does work.

Personally, I don't really like the old PSU, so it will get replaced with something more modern, but it'll do for now.

Happy days, my order of loop test hardware for the ST is finally on the way :)

So in conjunction with the diag cartridge I will have loop back testing facilities for midi, serial, parallel, mouse/joystick and DMA/ACSI external.

I'll do an overview once it arrives, but it'll be nice to have another tool for testing all the above in one compact tool.

Well, it was Pokey MAX time earlier today!!




This upgrade gives a dual Pokey where the other once was, and works very well with the VBXL video upgrade, as it uses the same 13 pin socket for output to SCART as the ST, but in addition, when wired up for the 1088XEL SCART cable, allows you to wire up a L & R audio channel for games that support it.




As you can see, it's a relatively simple 5 wire install, and with the Ultimate 1MB plug in also supporting it, it shows up as stereo right away. If you don't want to bother with stereo, it is also a plug in replacement for the Pokey.

If you don't have an Ultimate 1MB it can still be controlled easily, and the install guide gives some very good instructions.

Sounds great too!!

I've been looking at some of the Atari 8bit Stereo demos now I have the right hardware, and oh my god, my mind is well and truly blown!!

If Atari had put a pair of Pokeys in the ST, IMHO the sound would have been way better than what the AY could offer, I'm quite astounded just how good it is.

Just for giggles, I dug this out and made it work on one of my STFMs:


Works with the very latest in DOS too :)

The CPU is a Sony V30/8088 equivalent. The things we do for fun.....

:girldance: :girldance: :girldance:

Time for doom ;)

exxos wrote: 01 Jan 2021 18:57Time for doom ;)

Oh, if only....

rubber_jonnie wrote: 01 Jan 2021 21:07 Oh, if only....

Worm it is then ;)

Leisure suit Larry in CGA?