Broken ZXpander

[questarian]

I recently picked up a ZXpander, and was casing in an old Memotech casing, and everything worked perfectly until I hooked up external busy/error status LED's and removed the surface mount ones.

The screen comes up blank... (occasionally full of junk, scrolling noise, but that may just a bad/loose connector) The busy and error status LED appear to be working normally - the busy blinks on startup and reset, both the busy and error come up when you do a long hold on reset, and I tried putting the most recent firmware update on an SD card and starting it up and the status LED's looked like they were correctly indicating the upgrade. but the screen stays blank.

I've reseated the chips, checked for solder bridges, damaged traces, and cold solder joints... and I can't find a problem. Any ideas/suggestions? Could the SRAM (cy7c199cn-15pxc) have gone bad?

Thanks!

[sirmorris]

Hi,

RAM is certainly a possibility. There are no real delicate areas around the original LEDs or breakout connector and like you say, the interface seems to be booting with the green and red LEDs reporting activity.

Another possible target is the EPROM or memory decoder GAL. If either of those are damaged you'd see the same effect.

Do you have access to an oscilloscope or logic analyser?

C

[RWAP]

John is based in the USA which makes it awkward, however, I ought to point out that this is the ZX80 version - John have you tried it on a ZX81 also (remove the jumper)?

If not, does the ZX80 work on its own still (with the upgraded ROM) ?

[sirmorris]

OK, that narrows it down to the RAM or the GAL. There would be serious issues if the jumper were left off, though - I know it sounds like a ridiculous question - but is the jumper present with its connections intact?

C

[siggi]

I killed one of my Zeddies by eletrostatic discharge, when I tried to reprogram an EEPROM on ZXPAND, when I touched one of the jumper pins on ZXPAND (going to the Z80 /WR line).
Electrostatic discharge can destroy anything

Siggi

[questarian]

Hi,

RAM is certainly a possibility. There are no real delicate areas around the original LEDs or breakout connector and like you say, the interface seems to be booting with the green and red LEDs reporting activity.

Another possible target is the EPROM or memory decoder GAL. If either of those are damaged you'd see the same effect.

Do you have access to an oscilloscope or logic analyser?

C

Hi,

I forgot to mention, I've been using Z81's... It worked (sans the jumper) before I started working on it. I've tried it on several Z81's with the same results... the z81's work without it, and don't with the ZXpander.

No, unfortunately I don't have either a scope or an analyzer... though I do think I see what the culprit was. I'd initially been using the ZXpander with zx81 and TS-1000's in original casings and a Suntronic case. I recently picked up a old metal Canadian made casing, which I was using on my work bench as I'd just upgraded the video. I'd been using the extension cable because the ZXpander in a Memotech casing wouldn't fit on it... once I removed if form the casing, I removed the extension, and it fit, but looking at it now it appears that some pins may have been close enough to short on bare spots in the casing and/or the mounting screws... Stupid me didn't stop to think that a metal casing next to a bare circuit board would be a problem... lol, got excited by the pretty lights...

-John-

[RWAP]

Oops - however, John has now ordered a second ZXpand, so before sending the first one back for repair, at least you will be able to swap over the chips and see if that resolves the issue (or maybe that is too risky??? )

[sirmorris]

Hi John,

Sorry to hear about the board. I think that a short across the back of the board is indeed the most likely culprit. Like Rich says, you could try swapping chips from a known working board which would be low risk - with appropriate static handling - and that could mean a repair is as simple as a new chip, unless it's the RAM which require de-soldering.

The issue three boards which will be shipping from next month do not have the LED signals broken out, as I believed that no-one used them... If desired I can leave the SMD LEDs off your new board (unless Rich has one ready for you??) just let me know how I will be happy to assist you in any way I can with the casing-up.

Regards

Charlie

[questarian]

Hi John,

Sorry to hear about the board. I think that a short across the back of the board is indeed the most likely culprit. Like Rich says, you could try swapping chips from a known working board which would be low risk - with appropriate static handling - and that could mean a repair is as simple as a new chip, unless it's the RAM which require de-soldering.

I'm a little leery about swapping chips between good and bad hardware... I'll definitely be wearing the grounding strap for that one ... I've gone over the board a dozen times and I don't seen any rouge solder blobs/bridges, no metallic contaminants, no broken traces or components, etc, so if you think it's safe enough I'll certainly give it a try. Too bad there's not enough space to socket the RAM. I've been apprehensive about soldering replacement RAM directly, so I typically thrown a socket on as they tend to be quite a bit more forgiving (that and I don't have to desolder it again if it dies) I'm getting older, which is why I can afford the hobby now, but my touch with a iron isn't what it used to be... that and/or Radio Shacks desolder tape also isn't what it used to be.

The issue three boards which will be shipping from next month do not have the LED signals broken out, as I believed that no-one used them... If desired I can leave the SMD LEDs off your new board (unless Rich has one ready for you??) just let me know how I will be happy to assist you in any way I can with the casing-up.

Regards

Charlie

Lol. Well once again I'm the exception to the rule... after I cut out the SD slot the Memotech casing was just BEGGING for external LEDs the front! You can leave the LED's on the second board. I'll shortly be ordering one of the sound daughter boards so I be casing the second one differently... I've been think of possibly building a clear plexiglass extension casing with a bit of extra room for any additional add-ons or mods. Any advice or suggestions on casings would be greatly appreciated!

Thanks Much!
-John-

[questarian]

I killed one of my Zeddies by eletrostatic discharge, when I tried to reprogram an EEPROM on ZXPAND, when I touched one of the jumper pins on ZXPAND (going to the Z80 /WR line).
Electrostatic discharge can destroy anything

Siggi

I've been lucky. With the one possible exception of a TS-2068, I haven't lost any of my retro computer stuff to static... Have lost LOADS of hardware to bad OEM power supplies. The old C-64 bricks are the worst!