I took a punt on a cheap ram pack (£3.75). the only bummer was the postage, which was nearly the same price again.
It's in pretty good condition, but I gave the edge connector a clean with IPA, as well as the edge connector on the zeddy.
I tried it on my issue 1 ZX81, but it won't come on with it plugged in. The machine works fine without.
I'm looking for some suggestions on fault diagnosis of the ram pack; I've drawn a blank online. A quick visual inspection of the internals yielded nothing obvious.
Which version is it (yes, there is more than one revision). Best post some photos of the internals.
Did the ZX81 show a white screen, or did you get something else?
Is the DC/DC converter running and producing the -5V supply (pin 1 on the 4116 or equivalent DRAM chips)? What voltage is it?
Is the +12V supply present on pin 8? What voltage is it?
Mark
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
I don’t see a location key / peg in the edge-connector. So it is extremely hard to line up the contacts when fitting it. So I suspect when you tried it, the contacts were misaligned and were shorting out adjacent edge-connector contacts ZX81 board.
So please don’t try to use it until you have a solution to provide a location key / peg in the edge-connector.
Some options are:
Cut up a old cassette tape case. Then superglue a suitable sized piece into the correct slot of the female edge-connector.
Use an old piece of circuit board, the proceed as above.
Use a suitable string washer
Drill holes in the top and bottom of the plastic of the female edge-connector in line with where the key should be fitted, then use a large hot tip of a soldering iron to push a suitable size nail through the holes. Then use a Dremel or similar tool to cut the top and bottom off the nail leaving a nice flat surface. The nail then forms the key.
Mark
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
I've just received a new credit card so I cut out a piece off the old one and slotted it in, and checked everything lined up when I connected it. Still no joy.
I took a reading off one of the 4116's and I get zero volts at pin 1 and at pin 8. I metered the edge connector on the ZX81 and I'm getting just below 5v and 10v. Unfortunately, I took the readings before I noticed your reply
Removing the ram pack, the ZX81 still works.
***Edit ***I've just checked the auction pics and its missing from them too, so its not come out in transit. I suspect then, its been shorted out some time in the past
Have a good look at the coil. Can you see any discolouration? Does it smell of overheated electronics?
If no, the next step is to replace the nearby transistor.
Then test for the -5V and +12V voltages again.
Mark
Last edited by 1024MAK on Fri Oct 25, 2019 11:04 pm, edited 1 time in total.
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
SpaceFlightOrange wrote: ↑Fri Oct 25, 2019 10:27 pm
The ULA? On the ZX81? It seems to be fine without the ram pack installed? Is there anything in particular I should look out for?
Nah, ignore the comment about the ULA. I had a brain fart and was thinking about a later issue of the RAM pack where Sinclair replaced some of the logic chips with a small ULA.
Mark
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
1024MAK wrote: ↑Fri Oct 25, 2019 11:03 pm Nah, ignore the comment about the ULA. I had a brain fart and was thinking about a later issue of the RAM pack where Sinclair replaced some of the logic chips with a small ULA.
Mark
Ah.I was scanning the ram pack board, thinking, "There's no ULA on here"
I'll swap that transistor out. I don't have any of those right now, so I'll get some ordered
So a quick update. I haven't really done much on this, due to other things, but I got around to whipping the Transistor out and testing it (I haven't ordered any replacements) My tester gave it a clean bill of health, so I stuck it back in. It did originally look as if there was a dodgy joint on both the collector and emitter, so I made sure it was properly soldered in.
Something different happened. Previously nothing happened on the TV, but this time, I get black static for a second before the picture goes off.
Not sure what that means to be honest so I'll go over the board looking for other dry joints.
I've got the schematic now, so I'll have probe round with the scope at some point.
Standby alert 
