exxos's Atari, Amiga & retro forum

Well I returned from the Northwest today following a 2 day karate seminar, and there were some parts waiting for me, namely the power transistor, 74LS156 and some sockets.

I fitted a socket for the LS156 and dropped one in, then added in the power transistor, popping on a bit of thermal paste to help it along. It should be a higher rated component than the original and be more capable.

With the new 723 VRMs in place, this nicely fixed the 12v and 5v circuits, so the machine could be powered up using its original PSU once more!!! :)

However, I was still getting garbage at boot up, but everything suggested that I had all the signals across the address bus and I was seeing data. I decided to pull my nasty 41256 RAM hack and see what happened, and got a screen full of @ characters instead!!

I checked all the 41256 RAM chips and found a bent pin on one of them (ARRRGHHH!!), so straightened it all and put them all back in taking extra care not to bend any more, and got this:


I tried entering the MEM size of 16384, but it didn't like that, but I found after hitting enter a couple of times, it dropped me to the basic prompt, notably R/S (Radio Shack) Level 2 basic, so I wrote a little program:


And it ran just fine :)


I don't honestly think the RAM is quite right, as I don't think I should see the MEM SIZE prompt, but it is working, and the RAM has scope for improvement :)

Fingers crossed the last piece of the PSU puzzle, the Zener diode for the crowbar circuit, will arrive tomorrow, and though it runs fine without, the lack of protection is a worry.

What was also fantastic, is that my IDE based keyboard cable repair works perfectly, as does the whole keyboard, so I'm pretty happy at this point.

I think a 'Woo Hoo' is in order!!!

WooHoo indeed @rubber_jonnie. What ram do you need? I’ve probably got some you can have?

:yay: :yay2: :dualthumbup: :cheer: :girldance:

PhilC wrote: 06 Sep 2022 22:42 WooHoo indeed @rubber_jonnie. What ram do you need? I’ve probably got some you can have?

The original was 4116's, I have modded 41256's in there at the moment. 4164's will work too, but first I need to figure out if it's just the way it's supposed to be or if I am really missing some RAM or have another issue.

Let me do some more digging. I do have some RAM on the way, but if it'll work with the 41256's, that's fine as I have a truck load of those.

When my RAM tester, as suggested by @stephen_usher arrives, I'll be interested to see how much RAM is dead.

So far, the tally is:

8x 4116 RAM chips
1x 74LS04 for the clock circuit
1x 74LS156 for RAM/ROM multiplexing
2x 723 VRMs
1x Zener diode
1x Power indicator LED

Not the worst dead part count I've had, but pretty well up there.

I'll let you know regarding the RAM, thanks so much, I just need to verify the situation first.

@rubber_jonnie no worries, I’m sure I’ve got some NOS 4116 somewhere

PhilC wrote: 06 Sep 2022 23:00 @rubber_jonnie no worries, I’m sure I’ve got some NOS 4116 somewhere

So I think it's working...

It seems 'MEM SIZE' is the prompt you get at boot time on these machines so you can reserve RAM space. I just need to verify if the ? MEM command should come back with a result different to 15570. it's only 814 bytes short of the total it's supposed to have of 16384.

I'll let you know, thanks again :)

You lose some ram to basic and the z80 needing a stack area

@rubber_jonnie

Tom

I must admit that I was surprised how you did those RAM chips with bus wires. I always do my hacked chips individually so I can replace them independently. It must have been "fun" getting them all back into the sockets!

rubber_jonnie wrote: 06 Sep 2022 23:14 It seems 'MEM SIZE' is the prompt you get at boot time on these machines so you can reserve RAM space. I just need to verify if the ? MEM command should come back with a result different to 15570. it's only 814 bytes short of the total it's supposed to have of 16384.

Does the Commodore 64 report the full 64K in the BASIC boot banner?... :-)

Tomswork wrote: 07 Sep 2022 00:09 You lose some ram to basic and the z80 needing a stack area

@rubber_jonnie

Tom

Yeah, that's what I was thinking, but wanted to verify.