🇬🇧 Sinclair ZX Spectrum: Part 6 (Troubleshooting in circles?) [TCE

Still replacing parts, as indicated, but not making any progress. I would like to replace the Z80 but its soldered in ...
Phil at Retroleum in the UK makes some neat RAM replacement boards, so I got one of each. Have replaced the whole lower 16KByte of RAM (eight 4116) as one was flagged as faulty (despite passing the initial lower RAM test ... ?)
I am also using the Retroleum diagnostic ROM installed on the Speccy board in place of the Sinclair ZX Spectrum ROM. And ... the text is readable now! Well, after a cold boot. Once the machine has warmed up a little it starts misbehaving.
blog.retroleum....
--------------------------------
From Part 3:
Ivan very kindly donated one of his 128KByte RAM expansion boards so I am looking forward to trying that out - my first package from 🇷🇸 Serbia! In order for it to work you need to disable the upper 32KBytes of internal RAM, which I've done by adding a switch to inhibit the !CAS signal for that bank of RAM (make one OR gate input on IC23 always HIGH by connecting to +5V thus forcing the output HIGH, thus disabling !CAS).
I've verified the switch works by executing "PRINT PEEK 23732 + 256 * PEEK 23733" : I get 65535 (full 48KBytes), or 32767 (upper 32KBytes disabled).
However, after this it seems things started to go awry (nothing to do with Ivan's board).
--------------------------------
Check out my Patreon for regular blog-type updates between videos - there is a free membership tier and two paid tiers (US$2 and US$5). There is also a one-week trial for the paid tiers so if you don't think it's worth it, then cancel ... no hard feelings ... I appreciate any support, even if it's just subscribing to my KZbin channel!
/ thecluelessengineer
--------------------------------
Music by Karl Casey @ White Bat Audio
karlcasey.band...
/ @whitebataudio

[@MrWaalkman]

@3:01 C26 could use a touchup on the right hand leg. @3:04 There might be some solder splatter above IC25. @3:09 There's bit of splatter to the right of the top of IC23. Could be affecting which RAM chips the bits are getting read from (hence the RAM errors). Get some cold spray and hit the chips one at a time to see which one is the troublemaker. What does a meter say when you test KB0 - KB4 at the Z80? That should settle it. Don't be a hero, clip that Z80 out. And maybe a keyboard extender for the Speccy would be a handy thing.

[@Brfff]

Man, I need to stop shooting in 4K UHD ... I'm giving you guys too much information about my possible mistakes & errors 😇 I did have a close look at most of the traces with a loupe (because I still haven't gone back to the optometrist to get new glasses), but I'll double check those points you noted. For the keyboard, the output from the ULA is on the data bus ... but yeah, with all the weird shit going on, maybe it could be an issue there! I did check over the board with my thermal imaging camera and didn't see anything obvious (some diodes stood out, but otherwise nothing too extreme). And yeah, ef-it ... I might just clip the Z80 out (like I did ... off camera ... for everything else 😇). Regarding keyboard extender - it has occurred to me, several times ... similarly for the VIC-20 ... to have some easy-to-use keyboard for troubleshooting.

[@MrWaalkman]

@@Brfff Noooo! Hover over both sides of the boards and let us get to inspecting. I'm nearsighted, so this is a piece of cake for me. :) Do the spray trick, and you should be able to see where things are getting too hot. Once you cool down the that is chip causing the problem, the board should start working for a moment or two at least. A cheap digital microscope will go a long ways towards exposing bad solder joints and other issues. Good man on clipping the chip out. I have all of the hardware to do it right, and I still screw it up every fourth or fifth time.

[@charlesdorval394]

Hey mate ! I almost missed it hehehehe Unrelated to the video but I've been meaning to ask ... how did the imperialist adaptors turn out? :/

[@Brfff]

Ahhh ... I need to re-visit them, just like I need to re-visit the Speccy clones ... I did build one ... I think I tried using two 10-way pin headers per side but it was still a bit squishy ... so I was/am planning to try individual pins ... use a stack of the boards as a jig, pull the pins out of the plastic and install them one-by-one. A bit more effort but should fit perfectly. If it works, I think it's worth/maybe interesting to do a video on your converter board and using it.

[@inf0mike]

I would be suspicious of the CPU at this point.

[@Brfff]

Yeah ... that's my suspicion too ... been putting off desoldering it ...

[@thomasloode7711]

When will you work on the mini crt again

[@Brfff]

Ahhh ... which one? One of the Soviet-era ones, the Sony one (I still need to figure out how to add a video in), or converting the Princess one? I did promise the Princess one, didn't I? Okay ... tomorrow (Tuesday 24/9)

[@thomasloode7711]

@@Brfff yeah the Princess one