Topside image wanted of XEGS bare PCB C100417 rev A

[Beeblebrox]

I am currently diagnosing an issue with an XEGS. Try as I have on Google, AA, etc I cannot source a photo or silkscreen of the topside of the PCB. Does one exist?

 

My board is the C100417 rev A

 

 

There is lots of info available for XE and XL boards but finding anything for the XEGS is pretty tricky it seems.

 

I have schematics but ideally need an actual visual of the board itself without any chips (or sockets) installed so I can see the traces underneath. In particular the 2 x ram chips location.

 

Basically I found bad ram which I replaced and had got to the stage of a yellow screen, so aside the usual I am revisiting the RAM sockets I installed to double check I didn't screw up. 

 

thanks in advance 

[Rybags]

Row/Col select for each Ram come from Freddie, also the Ras and Cas.

Data should be shared with everything else that uses it - one chip gets D0-D3, the other D4-D7.

 

You could do continuity test for those fairly easily.  Note both Ram chips are involved in all Ram accesses.

I've got a PAL XEGS packed away here, not sure of the revision but there's probably someone who could more conveniently get pics to you at the moment.

 

Simple diag that works with practically all - if you can get clean A/V then at startup you should hear a click, and again when you press Reset.

Sometimes a reddish brown background will be seen on powerup, if it changes to black it means the early OS initialisation is good.

If you have a diag mode game cart like Star Raiders or Asteroids it's worth a shot as it starts up bypassing the OS testing and might give better life signs.

Edited September 10, 2022 by Rybags

[Beeblebrox]

@Rybags  Thanks for the reply.

 

I should point out on starting this thread this morning I've already been working on this XEGS for a good few days and have done quite a bit of work.

 

So just to confirm at this moment in time all I get is a red screen from the outset that doesn't change. I did have a yellow screen for a while also which I'll explain below. There is no audio click at any stage and the console's reset doesn't respond when pressed. With a Star Raider cart in place I still gets red screen. 

 

I will explain the time line ...........

 

So on receiving this XEGS last week it had a red screen on power up from the very start.

 

Opening up it was obvious at least one problem was ram - both ram chips which were red hot very quickly.

 

I replaced them with the same brand of chip, installing double wipes sockets as I did - no change on power up. Red screen.

 

I felt IC's just to see if any other chips were getting hot over 20mins on. Nothing stood out.

 

Next I checked Freddie as it already came socketed so easy -  I just swapped it out for another known working Freddie and tested the chip in my 130XE, (socketed test A8). Red screen.

 

Next I socketed and replaced the following ICs one at a time in the following order, testing each after installing:

 

CPU

ANTIC

PIA

 

(So each time I socketed a chip with a precision socket I popped it in another XE, and the chips checked out ok. I popped working chips in each newly installed XEGS socket too. No change. Red screen)

 

After each - no change - still a red screen

 

I socketed the OS rom and MMU and popped em back in (since I don't have spares given they are bespoke to the XEGS). Red screen.

 

Next I decideded to install a U1MB module which I took from my 800XE since U1MB replaces the OS and MMU ICs. I connected the ribbon cables and hooked up the 4 x CPU hook up wires. Still a red screen.

 

I socketed the GTIA - this time I got a yellow screen on boot up 95% of the time and sometimes on occasions a boot would result in a red screen again.

 

BTW I also socketed the 74LS08 and replaced with a 74F08 whilst I was there. (I do this with all machines when installing U1MB).

 

I checked out SAMS computer facts symptoms checker - the 130XE being the closest as there isn't one for the XEGS specifically:  https://www.manualslib.com/manual/1248642/Atari-130xe.html?page=47

 

So having had a yellow screen I saw that the SAMS symptom suggestions for green and yellow screens were as follows:

 

Replace U13 (74LS138n); U12 (MMU); U27 (??? maybe not on XEGS?); U1-9

 

U27 doesn't exist on the XEGS.

 

So I'd already at this stage socketed and replaced U2 and U3, (bad RAM chips); U5 (CPU); U6; (Freddie - just replaced - socket already there); U7, (OS Rom); U8, (ANTIC); U9, (74LS08); plus of course U12, (MMU)

 

This left three ICS to socket and replace/test: U1, (555), U4, (POKEY), and U13, (74LS138N)

 

I socketed and replaced U1 (555) and U13, (74LS138N), testing them one at a time and putting in working replacements from my 130XE. Same predominantly yellow screen and occasionally red.

 

Before I tackled the POKEY, (I am always reluctant to touch PCB soldered POKEY's as they are hard to come by), I decided to revist my RAM sockets installation. I removed the ram sockets again and replace them once more.

 

Worth noting here that I I took pics of the bare PCB as I did this re-socketing only to later find my phone hadn't stored the pics having run out of space!!!

 

Turned out removing the RAM sockets was a pain. In the process of removing them I'd damaged a few vias to heat which was the first time this has happened on this board whislt working on it. I've been very careful.

 

I think I've accounted for the lost Ram socket pin connections by addiing wiring to ground points I think there were connected to but can't be 100% sure since they are obscured by the latest sockets, hence why I want to see if anyone has pics of the top side of the board without sockets or ICs in this area.

 

Anyways with the new sockets in and wiring in place I now just get a red all the time on power up. So back to the same Red screen all the time.

 

Again pressing reset doesn't do anything still - no response and no audio click. Just a red screen.

 

So - last thing I've tried is socketing the POKEY and popping a new working POKEY in the XEGS. Still a red screen.

 

Therefore at present I am focusing on the RAM sockets PCB area and also the socket for Freddie, (the latter being the only one I've not installed as it was there already). 

 

Again I reslise socketing XE boards especially always increases the risk of adding more failure points. I've checked sockets out each time visually and with my multimeter for continuity and they seem ok.

 

There is no obvious trace or passive component damage anywhere on the board. 

 

All ICs are socketed and replaced with the exception of the 4050, 358p, and 74LS74B

 

So a topside bare board photo or at least bare board under the Ram sockets shot would really help. Plus any other suggestions now I've outlined all the investigative work and socketing/testing done so far. Hopefully someone on AA has such a pic or a link to it. 

 

Thanks

 

 

 

Edited September 10, 2022 by Beeblebrox

[Beeblebrox]

Managed to find an image...  We'll a still taken from one of FJC's vids:

 

This still shot below I've taken from the video will really help me check that my ram socket wiring bodge is correct at the very least, and correct it if I missed anything. 

Edited September 10, 2022 by Beeblebrox

[Beeblebrox]

Well using the aforementioned image to double check the ram sockets it is all checking out in that respect. No change still though. 

 

I've re socketed Freddie with a precision socket.  No difference.  I've reflowed all sockets.  Still a red screen. 

 

I've swapped out all the 40pin chips once more with known working ones. Still a red screen. 

 

No obvious breaks in traces.  

 

So a bit stumped. I don't own a scope either unfortunately. 

 

BTW I've also tried the setup again without u1mb and the original is rom and mmu.  So at this stage aside from the 358 and 4050 ic's all there is left to check are passive components and re check again for traces. 

Edited September 11, 2022 by Beeblebrox

[Beeblebrox]

On 9/10/2022 at 1:50 PM, Rybags said:

Row/Col select for each Ram come from Freddie, also the Ras and Cas.

Data should be shared with everything else that uses it - one chip gets D0-D3, the other D4-D7.

 

You could do continuity test for those fairly easily.  Note both Ram chips are involved in all Ram accesses.

I've got a PAL XEGS packed away here, not sure of the revision but there's probably someone who could more conveniently get pics to you at the moment.

 

Simple diag that works with practically all - if you can get clean A/V then at startup you should hear a click, and again when you press Reset.

Sometimes a reddish brown background will be seen on powerup, if it changes to black it means the early OS initialisation is good.

If you have a diag mode game cart like Star Raiders or Asteroids it's worth a shot as it starts up bypassing the OS testing and might give better life signs.

@Rybags  So I am revisiting the continuiting of the RAS and CAS connections between Freddie and the Ram chips.

 

I am not great interpretting schematics at the best of times I have to say.

 

However I think I have possibily found a likely mislabelling on the schematics, which are the reworked ones from Graham Rolle in 2020 which was based on Jerzy Sobola's initial work on the XEGS schematics.  (I believe Jerzy's work has since been cited for errors and some accidental mislabeling).

 

Either that or it is part of the problem with my board. I don't think it can be though.

 

So according to Graham's schematic RAS is on pin 33 on Freddie which is providing the signal to pin 5 of the ram chips. I can confirm I get continuity here between these.

 

However according to the same schematic the CAS connection on pin 35 of Freddie should connect to pin 4 of the Ram chips. I don't get a connection here at all, until I switch to pin 34 on Freddie which then shows a connection with pin 4 on the ram chips.

 

Furthermore where the schematic says pin 34 of Freddie provides the WRT signal to pin 16 (R/W) of the ram chips, instead I find continuity between pin 35 of Freddie to pin 16

 

Is this just an error on the schematic which in fact should have WRT and CAS swapped around for the Freddie chip?

 

  

 

 

 

 

 

 

 

EDIT: It's also worth noting that with my attempt at bypassing the XEGS's O/S Rom and MMU by installing U1MB earlier, I assume that in the event the OS rom and or MMU were faulty and causing the issue  - installing U1MB would work as it is a straightfoward install. (Where it is the two ribbon cables to the OS rom and MMU sockets, the 4 x CPU lines as well as the U1MB PCB's XEGS jumper).

 

However I noticed on Graham's scehmatic he states a resistor needs installing to the EXTSEL line for U1MB to work:

 

 

I did not see this detailed or required on FJC's XEGS install video so I assume this won't have a bearing? 

 

 

Edited September 12, 2022 by Beeblebrox

[Rybags]

The Sobola schematics also have Ras/Cas on 33/35 but aspects of his overall docs are known to have some errors.

 

You can easily enough test continuity there from the Freddie pin to where it goes on the Ram IC.  Without working Ras/Cas nothing will be running right for Ram accesses.