General Stuff(tm)

[JezC]

Andgate was very amusing to me

Me too - but in a typical autistic moment I was thinking that since this is a retro electronics/computer forum, maybe NANDGATE would allow any other logic to be synthesised?

I'll just pop outside to sort myself out...

[stephen_usher]

"Nandgate" was the first name I thought of, but thought Andgate sounded better, and more positive?

[rubber_jonnie]

"Nandgate" was the first name I thought of, but thought Andgate sounded better, and more positive?

Andgate was very amusing to me

Me too - but in a typical autistic moment I was thinking that since this is a retro electronics/computer forum, maybe NANDGATE would allow any other logic to be synthesised?

I'll just pop outside to sort myself out...

I also quite like Norgate

[stephen_usher]

Someone elsewhere suggested that... I'd not even thought about it. Sounds a bit like Moorgate.

[rubber_jonnie]

Someone elsewhere suggested that... I'd not even thought about it. Sounds a bit like Moorgate.

[stephen_usher]

Since I got the ZX81 booty I've on-and off been trying to get a Cheetah 16K RAM pack to work.

I desoldered and tested all the 4116 RAM... All fine. Desoldered the three 74LS chips... all fine. Replaced all the caps, which were most definitely not fine! The 10uF 16V caps were testing as ~24uF and with a high leakage percentage. The 220uF one was better, only being about 20% out of spec. After replacing this lot the -5V rail changed from about -3.8V down to the required -5V. Still not working.

IMG_4404.jpeg (329.87 KiB) Viewed 92 times

Reflowed practically every joint. Checked with the oscilloscope and only A6 looked a little strange but not terrible.

Today I decided to test the machine I was using with the other RAM packs I have, first verifying them on my known good original ZX81. All good, so it's not the machine.

Seeing as the Memopak has a pass-through and you can daisy chain RAM packs to give more memory I tried an experiment. I removed all the chips from the Cheetah RAM pack. This should essentially make it inert. Daisy-chain that on the back of the Memopak and switch on... non-working machine!

Seeing as the lines from the edge connector don't actually go anywhere how can this be?

Checked for shorts, and nothing.

As a last ditched effort I reflowed the pins on the edge connector, making sure to bubble any flux out of the solder. That did the trick!

Obviously the flux they'd used had broken the connection of one or more pins but made a connection via capacitance, screwing up the bus(es).

Replacing all the chips and...

IMG_4407.jpeg (212.39 KiB) Viewed 92 times

A working RAM pack!

[rubber_jonnie]

Since I got the ZX81 booty I've on-and off been trying to get a Cheetah 16K RAM pack to work.

I desoldered and tested all the 4116 RAM... All fine. Desoldered the three 74LS chips... all fine. Replaced all the caps, which were most definitely not fine! The 10uF 16V caps were testing as ~24uF and with a high leakage percentage. The 220uF one was better, only being about 20% out of spec. After replacing this lot the -5V rail changed from about -3.8V down to the required -5V. Still not working.


IMG_4404.jpeg


Reflowed practically every joint. Checked with the oscilloscope and only A6 looked a little strange but not terrible.

Today I decided to test the machine I was using with the other RAM packs I have, first verifying them on my known good original ZX81. All good, so it's not the machine.

Seeing as the Memopak has a pass-through and you can daisy chain RAM packs to give more memory I tried an experiment. I removed all the chips from the Cheetah RAM pack. This should essentially make it inert. Daisy-chain that on the back of the Memopak and switch on... non-working machine!

Seeing as the lines from the edge connector don't actually go anywhere how can this be?

Checked for shorts, and nothing.

As a last ditched effort I reflowed the pins on the edge connector, making sure to bubble any flux out of the solder. That did the trick!

Obviously the flux they'd used had broken the connection of one or more pins but made a connection via capacitance, screwing up the bus(es).

Replacing all the chips and...


IMG_4407.jpeg


A working RAM pack!

I had a Cheetah RAM pack many moons ago, was still living at home, build quality was about what you'd expect for the era...

Needless to say it wobbled a lot, and I ended up cutting the edge connector off and soldering it directly onto my ZX81. Much more reliable after that.

[PhilC]

Oh the joys of ram pack wobble, coupled with a poor keyboard. The cause of many early 80s upsets.

[stephen_usher]

After fixing the RAM pack I found that a column of keys had stopped working... SO...

A bit of re-engineering to help out the keyboard membrane:

IMG_4409.jpeg (286.29 KiB) Viewed 77 times

Well, that fixed it.

IMG_4410.jpeg (204.95 KiB) Viewed 77 times

[rubber_jonnie]

Makes me feel nostalgic, should pull my ZX81 out and have a play.