RetroPCCupboard wrote on 2026-01-28, 08:50:

CharlieFoxtrot wrote on 2026-01-28, 08:10:

Looks about correct and you can definitely see SMD legs and I believe the chips on your pic is also HM628512ALFP-7, which means they are identical. That is for 10 chips, so if you don't need more than two, it may be worth looking at few other sales for individual or 5 x chips to save some money,. Of course spares and parts for future projects do you no harm, but just a thought.

That's the cheapest I can see with that exact part number. There are cheaper ones that begin HM628512 but the letters afterwards are different. I am not sure if the final letters matter. Anyhow, this 10 pack is barely more expensive than the prices of those other chips in lower quantities. It gives me some spares if I screw up the soldering, or if one dies again.

I will see if I see anything different between the chips with the oscilloscope.

All right. You'd need to look at the data sheets if there are differences and how interchangeable they are. They might be the same chip, but with different packaging or there might be something else.

About your second post, bodge wires aren't uncommon and especially in products like this. They are fixes to design flaws in the PCB, but they aren't a sign that the board works somehow unreliably. Especially when they are indeed made as fixes. The most likely thing that would cause chip failures is overvolt, so it doesn't harm to measeure PSUs and regulator outputs. I bet these chips run at 5V, so it may be worth to measure 5V line that it is approximately in the spec. Again, check the data sheet and which one is the voltage pin and measure the 5V input voltage with multimeter.

I personally bet that it is just a chip failure without any particular reason or fault behind it. AFAIK these are vintage chips and have not been in prodction for ages, so the age alone will affect the reliability overall.

CharlieFoxtrot wrote on 2026-01-28, 09:47:

All right. You'd need to look at the data sheets if there are differences and how interchangeable they are. They might be the same chip, but with different packaging or there might be something else.

About your second post, bodge wires aren't uncommon and especially in products like this. They are fixes to design flaws in the PCB, but they aren't a sign that the board works somehow unreliably. Especially when they are indeed made as fixes. The most likely thing that would cause chip failures is overvolt, so it doesn't harm to measeure PSUs and regulator outputs. I bet these chips run at 5V, so it may be worth to measure 5V line that it is approximately in the spec. Again, check the data sheet and which one is the voltage pin and measure the 5V input voltage with multimeter.

I personally bet that it is just a chip failure without any particular reason or fault behind it. AFAIK these are vintage chips and have not been in prodction for ages, so the age alone will affect the reliability overall.

Ok. Well, I guess it will be good to have some spares if that is the case then. I do quite like this little machine. Its a good price compared to getting a genuine XT. Even the NuXT motherboard is double the price of this machine.

I am kinda tempted to get a NuXT though. Mostly because I would be able to use it with a floppy drive. Something this laptop sadly can't do.

RetroPCCupboard wrote on 2026-01-28, 09:54:

CharlieFoxtrot wrote on 2026-01-28, 09:47:

All right. You'd need to look at the data sheets if there are differences and how interchangeable they are. They might be the same chip, but with different packaging or there might be something else.

About your second post, bodge wires aren't uncommon and especially in products like this. They are fixes to design flaws in the PCB, but they aren't a sign that the board works somehow unreliably. Especially when they are indeed made as fixes. The most likely thing that would cause chip failures is overvolt, so it doesn't harm to measeure PSUs and regulator outputs. I bet these chips run at 5V, so it may be worth to measure 5V line that it is approximately in the spec. Again, check the data sheet and which one is the voltage pin and measure the 5V input voltage with multimeter.

I personally bet that it is just a chip failure without any particular reason or fault behind it. AFAIK these are vintage chips and have not been in prodction for ages, so the age alone will affect the reliability overall.

Ok. Well, I guess it will be good to have some spares if that is the case then. I do quite like this little machine. Its a good price compared to getting a genuine XT. Even the NuXT motherboard is double the price of this machine.

I am kinda tempted to get a NuXT though. Mostly because I would be able to use it with a floppy drive. Something this laptop sadly can't do.

If you actually like electronic projects, build a Micro8088 with backplane and IO/controller cards if you need those.

I made mine couple of years ago and it was a fun project. It is nothing really complex, but still something that is fun and it is interesting to bring a whole system alive from a bunch of components and parts. Sure, it is not that cheap, depending on the other cards you want to build the total can be easily over 200€ with all the required chips and passive components (CPU board, backplane, Floppy/serial controller, 8-bit VGA card, XTIDE), but it is definitely worth it. Most of the parts you can buy new from Digikey/Mouser/equivalent, but you also need some vintage chips which you need to source from somewhere else, but I managed to find everything without that much of a hassle or searching. In the end you get a reliable small form factor XT machine.

This is the link for the CPU board, you can find rest of the related stuff including printable case stl files from Sergeys repositories. There are posts/threads about Micro8088 here in Vogons too. I think it was here that I also got the idea to make one myself.
https://github.com/skiselev/micro_8088

CharlieFoxtrot wrote on 2026-01-28, 10:12:

If you actually like electronic projects, build a Micro8088 with backplane and IO/controller cards if you need those. […]
Show full quote

If you actually like electronic projects, build a Micro8088 with backplane and IO/controller cards if you need those.

I made mine couple of years ago and it was a fun project. It is nothing really complex, but still something that is fun and it is interesting to bring a whole system alive from a bunch of components and parts. Sure, it is not that cheap, depending on the other cards you want to build the total can be easily over 200€ with all the required chips and passive components (CPU board, backplane, Floppy/serial controller, 8-bit VGA card, XTIDE), but it is definitely worth it. Most of the parts you can buy new from Digikey/Mouser/equivalent, but you also need some vintage chips which you need to source from somewhere else, but I managed to find everything without that much of a hassle or searching. In the end you get a reliable small form factor XT machine.

This is the link for the CPU board, you can find rest of the related stuff including printable case stl files from Sergeys repositories. There are posts/threads about Micro8088 here in Vogons too. I think it was here that I also got the idea to make one myself.
https://github.com/skiselev/micro_8088

How cool is that project! That sounds very much like something I would like to do at some point. I know that I don't have the time to build such a thing for a while though.

I think for now I just want hardware that works, so I can play with the software side of things.

From looking at it, I think you can make a smaller form factor machine with the NuXT, as long as you don't want to put any add-on cards in. The only card I might want to add would be adlib card. I am not sure if you can get low profile versions of that card? If so, then putting a NuXT in a SFF PC case might be kinda cool.

RetroPCCupboard wrote on 2026-01-28, 10:38:

How cool is that project! That sounds very much like something I would like to do at some point. I know that I don't have the t […]
Show full quote

CharlieFoxtrot wrote on 2026-01-28, 10:12:

If you actually like electronic projects, build a Micro8088 with backplane and IO/controller cards if you need those. […]
Show full quote

If you actually like electronic projects, build a Micro8088 with backplane and IO/controller cards if you need those.

I made mine couple of years ago and it was a fun project. It is nothing really complex, but still something that is fun and it is interesting to bring a whole system alive from a bunch of components and parts. Sure, it is not that cheap, depending on the other cards you want to build the total can be easily over 200€ with all the required chips and passive components (CPU board, backplane, Floppy/serial controller, 8-bit VGA card, XTIDE), but it is definitely worth it. Most of the parts you can buy new from Digikey/Mouser/equivalent, but you also need some vintage chips which you need to source from somewhere else, but I managed to find everything without that much of a hassle or searching. In the end you get a reliable small form factor XT machine.

This is the link for the CPU board, you can find rest of the related stuff including printable case stl files from Sergeys repositories. There are posts/threads about Micro8088 here in Vogons too. I think it was here that I also got the idea to make one myself.
https://github.com/skiselev/micro_8088

How cool is that project! That sounds very much like something I would like to do at some point. I know that I don't have the time to build such a thing for a while though.

I think for now I just want hardware that works, so I can play with the software side of things.

From looking at it, I think you can make a smaller form factor machine with the NuXT, as long as you don't want to put any add-on cards in. The only card I might want to add would be adlib card. I am not sure if you can get low profile versions of that card? If so, then putting a NuXT in a SFF PC case might be kinda cool.

It is cool indeed!

I can't emphasis enough how small footprint this has with sergey's case (not to say NuXT can't be even smaller).

https://github.com/skiselev/micro_8088_case

The 3.5" floppy drive in the pic kind of gives the scale, but 2x3 soda/beer cans is probably quite close. Case is designed for PicoATX PSU so it uses 12V DC and there is naturally external lap top style DC PSU.

CharlieFoxtrot wrote on 2026-01-28, 14:50:

It is cool indeed! […]
Show full quote

It is cool indeed!

I can't emphasis enough how small footprint this has with sergey's case (not to say NuXT can't be even smaller).

https://github.com/skiselev/micro_8088_case

The 3.5" floppy drive in the pic kind of gives the scale, but 2x3 soda/beer cans is probably quite close. Case is designed for PicoATX PSU so it uses 12V DC and there is naturally external lap top style DC PSU.

I saw a video with someone using that case. I researched a bit after you made me aware of this project. I get a sense of scale from about 2mins 40 secs into this video:

https://www.youtube.com/live/u9o0PMgx6vc?si=Pnd7A4Xj9RR9CaaE

It does look cool. Though I was thinking of using a case with 5.25" drive. Since I have a few games in that format to try. Starglider (Rainbird Software), Metropolis (Melbourne House), Sid Meier Railroad Tycoon, Sid Meier Civilization, Battlehawks 1942, F-19 Stealth Fighter....

I have tried running it for a few hours. Nothing getting even slightly warm. Though the VGA card is unplugged in order to have access to the RAM. So not sure if that is stopping the boot process in its tracks. I would have thought the CPU would be doing something though wouldn't it? Even if it's just a loop that does nothing?

Ok. Well I have used my multimeter to confirm that both chips are getting voltage (seems to be 4.8v on both).

I then used my pocket MiniDSO oscilloscope to look at the I/O pins. It doesn't have the resolution to actually see any square waves, but I am seeing that one chip seems to be showing activity on the I/O, and the other is showing nothing. So, I am going to remove the one that shows nothing and then try to start the laptop again. My assumption is that it will boot with same error as before after getting to 512kb in the test.

Assuming that's the case I will then try to put a new memory chip. My first time doing and surface mount stuff!

Chip removed.

Now to test to see if it still sees 512Kb.

When I put new memory chips on, should I remove all solder from the pads and use new solder? Or do I just re-melt what's there?

Ok. Well good news I think. Now I have removed that chip there's no BIOS error. After reaching 512kb it starts to boot DOS. But then it freezes on the useUMB program in the boot process. Not entirely surprising, as there is no longer any memory there for it to use.

RetroPCCupboard wrote on 2026-02-06, 07:36:

When I put new memory chips on, should I remove all solder from the pads and use new solder? Or do I just re-melt what's there?

Standard procedure is to remove old solder and re-apply, but you can get away with reusing the solder if it still looks clean and shiny, and if it has been submerged in clean flux while desoldering. Once molten without flux it becomes oxidized and needs to be replaced. Be very careful when removing the old solder, it's easy to damage the board with a solder sucker or wick.

asdf53 wrote on 2026-02-06, 09:54:

Standard procedure is to remove old solder and re-apply, but you can get away with reusing the solder if it still looks clean and shiny, and if it has been submerged in clean flux while desoldering. Once molten without flux it becomes oxidized and needs to be replaced. Be very careful when removing the old solder, it's easy to damage the board with a solder sucker or wick.

Ok. Thanks for the tip. Wish me luck!

asdf53 wrote on 2026-02-06, 09:54:

Standard procedure is to remove old solder and re-apply, but you can get away with reusing the solder if it still looks clean and shiny, and if it has been submerged in clean flux while desoldering. Once molten without flux it becomes oxidized and needs to be replaced. Be very careful when removing the old solder, it's easy to damage the board with a solder sucker or wick.

I tried to use a solder wick, but it kept sticking to the board. So I abandoned that, and just added a bit of additional solder to the pads, then put the chip on and used my hot-air soldering gun.

It seems to be stuck, but I am not sure if it's on properly, as I am not 100% convinced that the solder melted, and it doesn't look like the legs have sunk into the solder. Maybe that's because I didnt have enough solder on the pads. What do you think? (The one I replaced is on the right)

That looks quite good, except for the one leg at the top right. The solder pad for it already looked a bit strange on your previous photo, is it just discolored or has it been ripped off? Follow the trace of the pad to see where it goes and use your multimeter to see if the memory chip has a connection. For the other legs, just test the connection between the chip leg and the pad to see if it makes contact.

If the solder wick is sticking to the board, it can rip off pads and vias when you pull on it. Practice your technique on a scrap board before. Temperature 360-400C, large iron tip, always use flux, use a tinned tip for better heat transfer, use small strips of wick.

Good solder wick tutorial here, from 11:30 minutes onwards: https://www.youtube.com/watch?v=IjOh5ShVX_w

The fact that BIOS shows "512 KB OK" suggests that it has completed the RAMtest without errors.

I don't have this machine ether, but the next line looks to me like a BIOS error (ie: before successful book)

Since initally you got "cannot load COMMAND.COM" (a DOS error), I would suspect the hard drive (or whatever this machine uses for "permanent storage")
- is it perhaps an SD card or something similar? Something you can try reading elsewhere?

asdf53 wrote on 2026-02-06, 20:06:

That looks quite good, except for the one leg at the top right. The solder pad for it already looked a bit strange on your previous photo, is it just discolored or has it been ripped off? Follow the trace of the pad to see where it goes and use your multimeter to see if the memory chip has a connection. For the other legs, just test the connection between the chip leg and the pad to see if it makes contact.

If the solder wick is sticking to the board, it can rip off pads and vias when you pull on it. Practice your technique on a scrap board before. Temperature 360-400C, large iron tip, always use flux, use a tinned tip for better heat transfer, use small strips of wick.

Good solder wick tutorial here, from 11:30 minutes onwards: https://www.youtube.com/watch?v=IjOh5ShVX_w

Thanks for the reply. The top right leg is VCC. So I used my multimeter to touch that leg with the same leg on the other memory chip, and they seem to be connected.

Yeah, when it started sticking I got worried. So I just turned the temp of the iron up tinned it, and then put it on the wick until it let go of the board. I didn’t pull it at any point. I think all pads are there. At least all of the spots grabbed solder when I slowly dragged my soldering iron across them to add solder.

I definitely need practice!

Well, since you said it looks ok, and I am getting continuity on the VCC and VSS pins between the two chips, I will see if it works!

Just tried it. Nothing shows on the screen now. So I have made it worse. It is late here now, but I guess next thing to try is to confirm all legs are connected to the pads. If they are, I assume I need to remove the chip again, and see if it works again.

If all connections are good, I would remove both chips and then solder one of the new chips in place of the old working chip to see if they work at all.

DaveDDS wrote on 2026-02-06, 20:15:

The fact that BIOS shows "512 KB OK" suggests that it has completed the RAMtest without errors. […]
Show full quote

The fact that BIOS shows "512 KB OK" suggests that it has completed the RAMtest without errors.

I don't have this machine ether, but the next line looks to me like a BIOS error (ie: before successful book)

Since initally you got "cannot load COMMAND.COM" (a DOS error), I would suspect the hard drive (or whatever this machine uses for "permanent storage")
- is it perhaps an SD card or something similar? Something you can try reading elsewhere?

Its a CF card. When I removed the faulty RAM chip it seemed to be booting ok. Though it did freeze when loading the UMB driver. So, I guess it could be storage.

asdf53 wrote on 2026-02-06, 20:51:

If all connections are good, I would remove both chips and then solder one of the new chips in place of the old working chip to see if they work at all.

All pin connections seemed fine. So I thought maybe I killed it by taking too long with the air gun. So I took off the new RAM chip and put a new one in it's place. This time just using the air gun to pin two corners and then used soldering iron to finish.

So the machine is now booting again but it still only reports 512kb RAM. I am wondering if the CHIP is not working or if the BIOS needs some kind of configuration to detect it now, since it last booted with only 512kb. I can't find any documentation regarding this BIOS.

I mean, I could still use this machine with 512kb. At least its booting now. If I remove the known working chip to put one of these new ones, then there's a chance that I end up with it completely dead?