Half dead 8300

Post Reply
frax
Posts: 2
Joined: Mon Nov 02, 2015 7:16 pm

Half dead 8300

Post by frax » Tue Jul 10, 2018 6:55 pm

Hi,

trying to fix my semi-dead Lambda 8300. It started out almost stone dead, no picture but the CPU was running and the speaker was making some spooookey noises. BUSACK and a few others didn't quite make sense so I replaced the CPU and got the following.

Image
hi-res here!

Grey-ish, instead of black background, a few pixels missing and no Y in READY, blinking cursor but no response from the keyboard.

I have swapped both CPU and ULA from a working machine but the it still won't startup correctly. Running out of ideas. RAM could be bad but I don't see how that could cause the above problem.

Anyone? Anything?

--frax

User avatar
mrtinb
Posts: 786
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Half dead 8300

Post by mrtinb » Tue Jul 10, 2018 10:31 pm

Is it a ULA from a Lambda or a ZX81 you have swapped in?
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

User avatar
mrtinb
Posts: 786
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Half dead 8300

Post by mrtinb » Tue Jul 10, 2018 10:37 pm

Sorry! Stupid question. Since you have characters on the screen it is a ULA from a Lambda. (Since the characters on the Lambda is inside the ULA and not the ROM)

But when you have a working Lambda you can insert the CPU and ULA in there and measure all IC pins on all ICs with eighter voltmeter, digital probe or oscilloscope. Then repeat the process on the faulty one.

The pins that differ might give you a clue.
Last edited by mrtinb on Tue Jul 10, 2018 11:15 pm, edited 1 time in total.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

User avatar
1024MAK
Posts: 2151
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Half dead 8300

Post by 1024MAK » Tue Jul 10, 2018 10:50 pm

Have you tested the supply voltage to the ULA? And is the 5V supply okay as well?

I'm mobile at the moment, but will have a look at the picture later. Meanwhile, incorrect characters on screen can be due to address or dataline faults (including memory chips).

Mark

frax
Posts: 2
Joined: Mon Nov 02, 2015 7:16 pm

Re: Half dead 8300

Post by frax » Sun Jul 22, 2018 10:28 pm

Baaah, to much vacation, to little hacking. I hope I have some more time this week. Will check your tips and report back.

Btw, anyone have any ideas what the 74LS05 is for? Seems like D0-5 + 7, but not D6, is "controlled" or just overlayed on the databus is a way I can't understand. And it's all controlled by pin 39 from the ULA?

Post Reply