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Re: General Stuff(tm)
Posted: 18 May 2025 17:04
by JezC
rubber_jonnie wrote: 18 May 2025 16:33
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...
:smackbum: :slapface: :chairsmack: :chairsmack: :stickwhack: :poke:
Re: General Stuff(tm)
Posted: 18 May 2025 17:47
by stephen_usher
"Nandgate" was the first name I thought of, but thought Andgate sounded better, and more positive? :-)
Re: General Stuff(tm)
Posted: 18 May 2025 18:48
by rubber_jonnie
stephen_usher wrote: 18 May 2025 17:47
"Nandgate" was the first name I thought of, but thought Andgate sounded better, and more positive? :-)
JezC wrote: 18 May 2025 17:04
rubber_jonnie wrote: 18 May 2025 16:33
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...
:smackbum: :slapface: :chairsmack: :chairsmack: :stickwhack: :poke:
I also quite like Norgate :)
Re: General Stuff(tm)
Posted: 18 May 2025 19:13
by stephen_usher
Someone elsewhere suggested that... I'd not even thought about it. :-) Sounds a bit like Moorgate. :-)
Re: General Stuff(tm)
Posted: 18 May 2025 19:34
by rubber_jonnie
stephen_usher wrote: 18 May 2025 19:13
Someone elsewhere suggested that... I'd not even thought about it. :-) Sounds a bit like Moorgate. :-)
:)
Re: General Stuff(tm)
Posted: 19 May 2025 16:13
by 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
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!
Re: General Stuff(tm)
Posted: 19 May 2025 16:38
by rubber_jonnie
stephen_usher wrote: 19 May 2025 16:13
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.
Re: General Stuff(tm)
Posted: 19 May 2025 16:40
by PhilC
Oh the joys of ram pack wobble, coupled with a poor keyboard. The cause of many early 80s upsets.
Re: General Stuff(tm)
Posted: 19 May 2025 19:12
by 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
Well, that fixed it. :-)
IMG_4410.jpeg
Re: General Stuff(tm)
Posted: 19 May 2025 21:17
by rubber_jonnie
Makes me feel nostalgic, should pull my ZX81 out and have a play.