Another Elk to play with - It's a doer upper.

[rubber_jonnie]

This fellow arrived in the post for just £15 from a nice chap over at Stardot.

IMG20240729125742.jpg

As you can see, it's not in the best of shape, the list of issues as follows:

1. No ULA or ULA socket
2. No ROM.
3. CPU soldering looks a bit off.
4. No case or motherboard screws.
5. Dead PSU.

I've fixed the PSU already, the starter cap was a tiny fraction of what it should have been, so I've done a full recap bar the bulk cap as they never seem to have issues, and the PSU is working great on another machine.

Need to order some machined pin socket strips now and some Hydrogen Peroxide to see if the sunny weather and a warm bath will improve its' looks :)

[rubber_jonnie]

OK, some success with this machine.

Scoping out the clock circuit, there was nothing at the CPU, and there was no 16Mhz clock at the ULA, but the crystal had 16Mhz. So I checked IC8, which should have had 16Mhz on pin 8, but nothing good was on that pin.

IC8 removed and tested, and it failed, so I dropped another 74S04 in and lo and behold I have good clocks now:

IMG20240730112259.jpg

Plus I am getting an image on screen:

IMG20240730112303.jpg

Still needs work, but this is good progress IMHO :)

Since it has a known good ROM and CPU, my money is on some bad RAM at this point...

[HigashiJun]

A nice summer project in fact.

:D

[rubber_jonnie]

A nice summer project in fact.

:D

Indeed.

I plugged it in and gave it a quick scan to find IC11 rather hot:

20240730-130208.jpg

Yep, that is 104C about 60 seconds after powering on. IC11 is a 74S74 and shouldn't be that hot. Needless to say it is bad after testing and I've not got any replacements, so have put a 74LS74 in for now.

I get a solid image now:

IMG20240730133754.jpg

And a good video signal with a colourburst:

IMG20240730133203.jpg

So clock and video out look fixed, I just need to find the culprit for the non booting white screen now.

[rubber_jonnie]

Quick check of the /CS line on the ROM and nothing is doing, nor on the R/W line of the CPU, so I need to find pin 46 of the ULA and see what that's doing.

[rubber_jonnie]

Quick check of the /CS line on the ROM and nothing is doing, nor on the R/W line of the CPU, so I need to find pin 46 of the ULA and see what that's doing.

OK, so CPU 'might' be bad, just testing with an alternate and R/W has a signal now. /CS on the ROM is actually tied to GND though, so that is a red herring.

I've also replaced the RAM and one IC was dead, another marginal. I do now get more than a white screen now, I get corrupt graphics.

I think next step will be to buzz out the address and data lines, see if any are missing in action. There was some dodgy soldering on the CPU and some evidence of other rework, so lots of potential for badness.

[stephen_usher]

I'm not sure that it's going to take very long for you to get this one running actually.

[rubber_jonnie]

I'm not sure that it's going to take very long for you to get this one running actually.

Having a known good ULA certainly helps 😁

If I'm honest, I only wanted it for the case, but the PSU and mainboard called to me...

[HigashiJun]

Slowly but surely, you'll find out what's wrong with your Elky.

[rubber_jonnie]

Slowly but surely, you'll find out what's wrong with your Elky.

:D