65XE boot issues

[ziggy]

Hi there,
I've run into some issues restoring some XEs, two 130s and one 65 to be specific. The two 130s have some minor issues but work, the 65 is giving me some serious trouble.

Video output is a solid black, once in a while red and the rare green every 30th time or so
All chips are socketed, but the ones on the 130s are not, so not much chance to change chips. Exceptions are the CPU and ANTIC chips. Both are fine and work in the 130.
I don't have the ominous "Star Raiders" cartridge which apparenty helps diagnosing hardware faults. What I do have is "Star Raiders II", which brings some interesting results. Red & Green appear more often than without it and I just got an interesting screen trying to boot, see attachment.

Has anyone experienced something similar? I did the usual approach of trying different PSUs (no change), resocketing chips (no change) and testing chip temperatures (only CPU gets warm-hot, both the original and the 130 cpu).

photo_2020-06-04_11-34-43.jpg (153.87 KiB) Viewed 5704 times

[rubber_jonnie]

Can you get into the self test (Hold option and power on) at all and run the memory test? If so what is the result, because that looks like a memory problem to me, either RAM chips or the MMU (Freddie I think)

[ziggy]

Can you get into the self test

I've tried and tried before, but no luck so far. At least with the most recent screen with characters I can confirm the video circuit is working and the cpu is doing "something". Can the XE series boot with less RAM than 64kB? If so, I could pull RAMs until it (might) work

Either that or the MMU route, I've read before that might be the problem. Apparently you can program standard PAL16s and replace the MMU, do you have any experience with this?

[ziggy]

Alright, major breakthrough:

I had no luck booting with random RAMs removed, but for fun I decided to put all RAMs back in, just in a different order.
Star Raiders II boots!
But: I can't access diagnostic mode, nor can I boot the system without the Star Raiders II cartridge.

So, what does that mean??
Was the first RAM defective?
Is the ROM broken?
The system runs the game just fine after all. Picture, Colour, Sound, inputs are working.
The only things that don't work are diagnostics (ROM?), OS (ROM?) and the former RAM order (one or more RAMs?)

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[PhilC]

@ziggy the type of ram a 65xe uses is 64k x 8 meaning you need 8 chips as a minimum.

With the case off leave the machine switched on and see which chips get hot.

Also with this computer you can sometimes have success finding out which ram chips have failed by piggyback a known good chip on top of a failed one.

[rubber_jonnie]

Can you get into the self test

I've tried and tried before, but no luck so far. At least with the most recent screen with characters I can confirm the video circuit is working and the cpu is doing "something". Can the XE series boot with less RAM than 64kB? If so, I could pull RAMs until it (might) work

Either that or the MMU route, I've read before that might be the problem. Apparently you can program standard PAL16s and replace the MMU, do you have any experience with this?

I've not tried programming a PAL (I suppose a GAL would do as well) as an MMU replacement, I only have one machine, which I upgraded, details here: https://www.exxosforum.co.uk/forum/viewt ... f=44&t=160

There are details there of a bigger expansion to 320KB in there which does use a 16V8 chip.

It's odd that you can't get into self test, but that you can get into a cartridge. It makes me wonder if you have a ROM problem TBH. I take it on boot you don't get into basic or anything like that? And that holding down start or start+option doesn't try to boot the machine from disk or tape as expected?

Given Star Raiders II worked, that suggests maybe RAM is ok, and that it has mapped in where the normal system ROM would be, again pointing at the ROM. Do you have spare ROMs you can try from any of the 130's? Have you checked all the connections are good on the ROM, it has 5v and GND and there are no bad solder joints?

Also, since the ROM is socketed in the 65, is it in the correct orientation in the socket?

[guus.assmann]

If a cartridge is in the machine, the OS is still essential.
A Rom being partially defective is not very common. There could be an address line stuck, but that's also rare.
And a correct picture does suggest that Ram is ok, at least for some part.
Sitll there could be a part defective, again also an address line stuck.
Mostly, a defective Ram chip does get warm after some time.
So the part that holds the screen memory is ok.
The other symptoms would point towards the PIA. (6520)
That chip does switch the MMU. This MMU could also be defective but can be replaced with a Gal 16V8.
The 6520 may also be replaced by a 6821. Be sure to use the B type for the speed. (So use a 68B21.)

BR/
Guus

[ziggy]

OK, I will try to address everything you said. Thank you for the detailed answers

@PhilC
Does that mean you need 8 working RAMs, or just 8 RAMs occupying the sockets?
No Chips get hot, except CPU (Hot-ish, still touchable)
Don't have excess 4164s right now, as all other XEs are socketed. I might check in an 800XL of mine.

@rubber_jonnie
If I can pinpoint the trouble to the MMU I will look into programming a GAL, thank you
No prompt, no Basic, nothing. No self-test, really. I haven't gotten anything but Black/Green/Red out of this machine without the cartridge.
Start+Option doesn't do anything.
Unfortunately all ICs except CPU and ANTIC are soldered in both of the 130s, so I can't really check
Will check solder joints and continuity on the ROM.
All ICs point to the left, as marked on the board.

@guus.assmann
Does that mean the Cartridge manages to access the OS?
No RAM chips get hot. One has been replaced at some point. Replacement is a Mitsubishi instead of the original MT.
So your guess is PIA or MMU? I will check their solder joints.

[PhilC]

@ziggy you meed 8 ram chips installed.

It is possible on an XL to have a bad basic rom but to be able to boot a cartridge, I think its the same on the XE but I'd need to verify that. That might explain the blznk screen on boot up.

Is the keyboard known to be good? Those XE membranes are really fragile.

[ziggy]

@PhilC
AFAIK the keyboard is fine. I might try one of the others. But I was able to control the ship in Star Raiders II


Here's a photo of the mainboard, in case anyone is wondering. I don't see anything out of the ordinary

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