SC131 troubleshooting help

[MagnumKale]

So I've put the SC131 together, it's my first kit, but however the yellow status light just stays lit and never blinks off. I've measured all the voltages and they all seem fine except some weird voltages on IC U2, where some pins are higher than they should be and others are lower. I'm a novice to electronics, so any help would be appreciated.

[Mark Pruden]

There is a fairly comprehensive guide here

https://smallcomputercentral.com/sc131-z180-pocket-computer/sc131-v1-0-troubleshooting/

But I would recommend a recheck of your work. e.g.

* check for dry solder joints, if in doubt resolder

* check all IC;s are inserted in correct sockets, correct orientation, and no bent pins.

its a fairly simple board there isn't much that can go wrong.

[Mark Pruden]

If you post a high quality photo of both sides of the board someone might be able to spot an issue

[MagnumKale]

I will post photos when I get home. Thank you

[MagnumKale]

Here are some photos I took

[MagnumKale]

[Greg Holdren]

I defiantly would go over all the solder joints. Looking at the red circled joints that PCB pads are not covered with solder.  Notice the ring of exposed pad.

It looks like there is plenty of heat on some of them but not enough solder fed into the joint. Heat and flow more solder. Make sure the iron is making contact with pin and pad prior to feeding solder.

The joints circled in green have to much solder and or looks like cold joints. Lack of heat.

The pad circled in purple looks good. All the joints should look like this one.

Easy to fix by just re-heating to reflow and/or add more solder. 

Make sure there are no bent pins curled under the DIP parts.

Greg

On Thursday, September 25, 2025 at 1:58:40 PM UTC-7 MagnumKale wrote:

[Greg Holdren]

Just make it clear the whole board need to be touched up not just I what I showed above. These are the things that burn me still to this day and I have been soldering for half a century. :) There is always a joint that needs touching up.

Greg

[MagnumKale]

Thank you Greg. I've touched up most of the board so far but no luck. I will keep at it

[MagnumKale]

I've redone every solder joint, and I've checked everything proscribed in the troubleshooting guide but no luck. Only thing not right are some voltages on IC U2. I don't know where to go from here. I must have really messed something up

[Ed Silky]

Maybe I missed it, but you've said about 'not right voltages', but have you mentioned what they are?

-Ed

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[Richard Deane]

Just to ask the obvious. When testing I assume you are getting power through the usb serial adapter cable, I which case I assume you need the power shunt/jumper in place, which is missing in the photo.

Richard.

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[Steve Cousins]

From my experience, more often than not, problems come down to solder joints or other assembly issues.

I assume the reset LED performs as documented. ie. ON when the reset button held and turns off about half a second after reset button released. If working as described this means you have a good reset and the supply voltage is adequate.

Check the part numbers of U3 and U4 (I can read these on the photos)

I assume the LEDs on the USB adapter do not flash. Flashing indicates serial data activity.

Try slightly lifting the Z180 in its socket just in case there is a poor contact on one of the pins. The last issue reported turned out to be fixed by reseating the Z180 in its socket.

Check for bent pins on the other ICs. Sometimes one bends under and is not easy to spot. You could try reseating them as well.

I can't be sure from the photos but there are a couple of places where there appears to be a bit too much solder, possibly bridging to a place they shouldn't be. Take a careful look at JP1.

Can you see any change in voltage levels on data pins following reset, that then stay roughly the same for a fraction of a second or up to a couple of seconds, then changes again and remains stable. This would indicate the firmware is running but then hangs or is running until initialisation is complete and awaiting a key press on the terminal.

Unfortunately, there is not much to go on from the symptoms described. For peace of mind I have just checked the ROM in one of my stock of SC131 kits. All good. I'm not sure if your kit was from this batch or the previous batch, but in either case it seems likely the ROM is good. I test the Z180 CPUs and all the other ICs are brand new from major distributors, so should also be good. Thus the most likely explanation is an assembly issue. I'm not saying a component issue is impossible, but the odds favour an assembly issue.

Steve

[MagnumKale]

Thanks Steve. So yes the reset LED is performing as documented. The LEDs on the adapter do not flash. I've tried reseating and lifting the Z180 with no luck. No bent IC pins and I've also resoldered every joint that looked even somewhat questionable. The data pins don't really change in voltage after reset. They drop to zero when holding reset, and then when reset is released it just instantly jumps up to and stabilizes at a voltage. At this point I feel like I messed something up catastrophically, and I've spent more time troubleshooting it than I did building it. I am going to throw in the towel and probably order another kit in the near future.

[Bill Shen]

SC131 is a popular product, so what state/country are you in?  There are likely people near you who can help you.  If you are in USA, I’m happy to debug for you free of charge, although I don’t have SC131, but I’ve experiences with SC126 and have designed several my own Z180.
Bill

[MagnumKale]

Thanks Bill. I am in South Carolina

[Richard Deane]

I checked my working sc131 and have JP2 power shunt/jumper installed.

By default the software will boot from port A, which has hardware handshaking. Port B has no hardware RTS/CTS handshake. You can change to boot from B by a small software tweak and rebuild.

Try with JP2 in place. Perhaps voltages are getting to your circuit via other leakage paths.

Richard

On Sat, 27 Sept 2025, 00:28 MagnumKale, <kylecoul...@gmail.com> wrote:

Oh, it was my assumption that the jumper changed which serial port powered the board. I'll give that a try tonight

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[Richard Deane]

False alarm about JP2. Just checked the doc. :

As power can flow either way, the jumper (JP2) also enables serial devices to be powered from SC131. If such a device is used in serial port B, fit a jumper shunt to JP2.

Richard.

On Sat, 27 Sept 2025, 00:28 MagnumKale, <kylecoul...@gmail.com> wrote:

Oh, it was my assumption that the jumper changed which serial port powered the board. I'll give that a try tonight

On Friday, September 26, 2025 at 2:24:12 AM UTC-4 Richard Deane wrote:

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[MagnumKale]

Thanks Richard. Gave it a try and no luck. I really think the CPU is failing to run the firmware, I just need to find out why

[Bill Shen]

I’m in Albuquerque, New Mexico.  Like I’ve said, I’m happy to debug your board for you free of charge.  I have all the parts on hand, I will inspect your board, I have flash programmer to examine the flash contents and logic analyzer to follow program execution.  If needed I can switch out the parts until your board works.  I’ll send you my home address.  

Bill