600XL almost working. Advice needed.

[nama]

I recently got a 600XL and on power I am greeted by a BLACK screen.
I was able to install my SALT cart and it booted, and told me ROM error (but not sure if I can believe that), and RAM is all ok.
Coincidentally most other cars don't work except Defender that seem to run perfectly!!!! 

Symptoms and observation:

- Black screen (with brief red flash)

- Keyboard dead. Have been using the one from my 800xl for testing

- Holding option does nothing at power-up. Does no go to Self test.

- Have tried some other carts. The Defender Cart seems to run ok. Not tested much beyond that.

Things I have done. Tried to make a 2764 to 2364 adaptor and replace the BASIC ROM. Not sure if the adaptor is right, and it didn't change anything.

Burnt a 27128 to replace the OS ROM...that didn't work either, and in fact seemed to make things worse. Maybe I burnt the wrong .bin file.
Thats about it. 

Any help or recommendations are VERY welcome.

 

Thanks

[Beeblebrox]

@nama Can you swap it out for a known working original OS ROM chip, (C061598B)?  Ebay sells them at dirt cheap prices.

 

If it's anything like my experiences with one of my 600XLs, it may well be several issues. 

 

Keyboard issue sounds like it needs a new membrane if the 800XL keyboard you are using temporarily is working.

 

I would still definitely try replacing both the RAM chips first. (Chose at least 120ns RAM chips - I got HM50464P 12 120ns). I had the original RAM chips pass tests on a 600XL and it still turned out they were bad. Again it's dirt cheap to get the chips and the black screen/flash of red smacks to me, (in my limited experience), of RAM issues.

 

Worth also looking at the MMU chip (CO61618) and swapping this out for a known working one.

 

Also can you swap out the PIA chip with a know working one? 

 

In my recent experience with my aforementioned 600XL detailed here I had bad RAM, BAD OS rom and a bad PIA causing the issues.

 

Have you also tried lifting each IC in turn and reseating it (whilst system off of course) and powering it up after reseating each time?  (Just in case there are some oxidised socket/leg contact points).

 

 

 

 

 

[+kheller2]

If you have an 800XL --- then start swapping chips INTO the 800XL from the 600XL.

The OS chip is the same.

 

Unless your 800XL isn't socketed.

 

[nama]

Thanks guys,

I should have mentioned that the machine looks to have the 64k mod done to it.

 

I have done as you suggested and swapped out as many of the 'big' ICs from my 800xl into the 600xl and I could not find any bad ICs. I could not however swap the OS Rom, BASIC ROM and the 6520 as those are soldered in on the 800xl.

 

Anyway, I keep coming back to the BASIC ROM...
Removing the BASIC rom altogether doesn't change the behaviour of the machine (boot to black screen, but can run SALT and Defender from cart). 
I also noticed a cut trace. In the picture attached the bottom trace is intact, the top trace is broken. The top trace used to go to pin 18 on the BASIC ROM. I have tried jumping it, but no change. I have searched hard for anything about cutting this trace or images of 600xl motherboards and can't find anything about this. To me it looks intentional.

Cheers again

 

[bfollowell]

Well, to me, that looks more like a crater, like something hit the board, rather than an intentional cut. I realize that the image is really magnified, and that the damage is a really small spot, but still, if I cut a trace, I use a razor knife and cut the trace. This looks more like it was dug out with a screwdriver tip or something, and it was too close to the other trace to look intentional, in my opinion.

 

If you hadn't said that you'd already tried to jumper the spot, and that it made no difference, I'd swear you'd found at least part of the problem.

 

Edited November 3, 2021 by bfollowell

[mimo]

the machine will work with the basic rom removed, so it's not that causing the issue.
why are those traces cut/hacked/screwed? no cuts required for the 64k mod.
 

[Beeblebrox]

 

@nama The severed trace dink defo doesn't look right - looks like someone too a soldering iron point to it.

 

Also as mentioned definitely try installing new RAM chips - don't assume the ones in place are both working properly. I had both chips test ok using Sys check II but they were still not 100% right.

 

My money is on RAM AND poss also the OS rom chip. Do you feel comfortable unsoldering the OS chip for example and popping a socket in? Or know someone who is?

 

As @mimo mentioned above the Basic Rom being absent doesn't fully stop the machine booting. 

 

Also how does the underside of the board look? If it has taken a knock maybe some solder connection as also cracked around that area of the board, either side.

 

When you swapped out the ICs one at a time, checking each time, did you try putting the 600XL's ICs into the 800XL as well as the other way around, so see if the 600XL's chips worked in the 800XL. Good practice to do so.

 

 

 

 

 

 

[+DrVenkman]

1 hour ago, mimo said:

no cuts required for the 64k mod.

No, but if someone has replaced the BASIC rom with an EPROM, there are some jumpers that have to be cut and some others jumpered. You can see the vias/pads involved off to the left of the pic along those traces. It’s possible someone didn’t quite know what they were doing and attempted that mod but then gave up without restoring the machine. 

[flashjazzcat]

If the machine happens to assert chip select on the back socket, for example, when the OS is being addressed (or fail to assert chip select at all), you'll have big problems. The traces to those jumpers need to work.

 

[manterola]

In my experience (not that much) the problems were located in: Delay Line (the weird chip with some feet missing), RAM and finally Sockets (not making contact with one Antic Chip pin).

You should be able to swap the delay line as those are mostly socketed. If it work maybe you can get the 64KB expansion from lotharek that replaces the delay line among other chips, since it is difficult to get the original replacement.

Regarding RAM, it is easy to get and cheap. I actually have probably 6 chips available (TMS4416? or something like that?) from 64KB upgrades I have done to 600XL. I can put a couple of those in an envelope if you live in USA. 

Sockets, it just getting the multitester in continuity mode, the schematics and a LOT of patience and time to go thru the each chip "shoulder" and some convenient point in the motherboard.

 

 

 

[hueyjones70]

I have a similar problem with a 600XL.  It boots to a black screen but I often get the audio click that usually comes before a boot.  The really really strange thing is that Pole Position will boot.  I haven't tried to see if the program will run but it always boots.  No other cart boots.

[Beeblebrox]

Bad ram? 

[hueyjones70]

Syscheck says bad ram but I tried several sets of drams that work in another computer.

[hueyjones70]

I also replaced the MMU, ROM, BASIC, and all of the 40 pin ICs. I need to dig out a working 600xl because there are several ICs that aren't common with the 800xl.

[Beeblebrox]

Then poss bad ram sockets, or bad mmu sockets. Yup I'd swop the ICs from this 600XL into the known working ones one at a time and make sure they work in the working 600XL. 

 

What is the Ram set up anyway - an original 16K set or an upgraded 64k set? 

[+DrVenkman]

11 minutes ago, hueyjones70 said:

Syscheck says bad ram but I tried several sets of drams that work in another computer.

 

8 minutes ago, hueyjones70 said:

I also replaced the MMU, ROM, BASIC, and all of the 40 pin ICs. I need to dig out a working 600xl because there are several ICs that aren't common with the 800xl.

Have you replaced the 74-series logic in the memory circuits? If not, that should be your next step. 

[hueyjones70]

That's why I need to bring out the other working 600xl.

[hueyjones70]

I thought I had tested the MMU but it was the culprit.