Stuck with repair of Spectrum+

[speccyplus]

Thanks again Mark, that is what I thought, and entertained the possibility I am reading it wrong.

I am not a professional, but I know enough if I screwed up a through hole, lifted a trace, etc. I check everything after I pull a chip. But yeah will do more passes again looking around where I suspect work has been done.

I have already checked continuity for all through holes with traces on both sides on the CPU sockets. The Ram before I socket, same deal. Same with the ROM I just pulled.

I scoured the net a little last night to find a pinout for the following chips since they are all the same:

NEC D231128C
GI 9128C
Skk SBB23128

I could only find the GI.




With pins 20, 22 and 27 do you think it is worth tying them all to +5v at once, to see if I can read anything other than FF FF ...? I have only tried one at a time, and no other combinations.

I want to find out if this rom is bad.

[1024MAK]

The control input pins 20, 22 and 27 on the ROMs used in ZX Spectrums are all active low. This means that they all have to be at a logic low level of between 0.8V and 0V in order for the ROM to output data.

This Commodore ROM is similar.
I don’t have links for NEC, Hitachi or GI ROMs.

Mark

[Firefox]

I could only find the GI.

If that pinout is accurate then the ROM links are correct in the "H" configuration, as they are in your latest photo.

[speccyplus]

Was finally able to dump my rom. Mine is the zx16k.rom file dumped as a 256bit chip, I then had to split the file to get the upper and lower halves. I guess a trick to get a burner to toggle pin 27 hi and low as a memory select setting and thinking it is reading 2 16k banks...

me@home:~/tmp$ minipro -p AM27256@DIP28 -r zx.rom -y

me@home:~/tmp$ spilt -b 16k zx.rom

me@home:~/tmp$ mv xa zx16k.rom


me@home:~/tmp$ md5 48.rom
MD5 (48.rom) = 4c42a2f075212361c3117015b107ff68

me@home:~/tmp$ md5 zx16k.rom
MD5 (zx16k.rom) = 4c42a2f075212361c3117015b107ff68
me@home:~/tmp$

I have a TL866+ II EEPROM burner BTW, and the GUI is nice, but the open source CLI too lets you bypass all the silly error checks, enabling me to do the dump above!

[speccyplus]

I went and pulled the last 2 74ls chips, tested OK as suspected. I now have a 16k spectrum in theory

I scoured the board, and found 1 broken trace from the previous work someone did on the board, it was pin 3, a chroma signal pin with a through hole on the video signal chip. I bodged it below to the resistor it should connect too, didn't expect and change any and there was none.

At this point only chips left are the ULA...

I have some EEPROMS coming still, maybe burn a zero memory usage test rom? Any suggestions on the best one?

[speccyplus]

I could only find the GI.

If that pinout is accurate then the ROM links are correct in the "H" configuration, as they are in your latest photo.

Thanks, this is the info that told me this:
The Spectrum computer can use one of four differently manufactured ROM's.

They are types :-

HITACHI HN613128P
NEC D231128C
GI 9128C
Skk SBB23128
The NEC, GI and Skk ROM's are fully interchangeable without any modification to the links, unless you replacing a faulty HITACHI ROM, (links situated top righ of speaker). The 'H' strap would have to be moved to the 'N' link position.

The HITACHI ROM if fitted for replacement for any of the above should have the 'H' links strapped.

http://www.worldofspectrum.org/SpectrumRepairGuide/

So this is wrong, or did I mis-understand that only the Hitachi chip should be H, rest are N?

[1024MAK]

I have some EEPROMS coming still, maybe burn a zero memory usage test rom? Any suggestions on the best one?

https://github.com/brendanalford/zx-dia ... i/Firmware

In terms of ROMs, the ZX Spectrum was originally designed to accept either a Hitachi or NEC ROM. Hence why the selection links are labelled H / N.

See https://spectrumforeveryone.com/wp-cont ... Manual.pdf

Mark

[Firefox]

me@home:~/tmp$ minipro -p AM27256@DIP28 -r zx.rom -y
.
.
.
I have a TL866+ II EEPROM burner BTW, and the GUI is nice, but the open source CLI too lets you bypass all the silly error checks, enabling me to do the dump above!

Ah! I tried that... a year and a bit ago, maybe? Couldn't make it work.

It's perhaps time to give it another blast. Did you install it using a package manager or build it from source?

[speccyplus]

me@home:~/tmp$ minipro -p AM27256@DIP28 -r zx.rom -y
.
.
.
I have a TL866+ II EEPROM burner BTW, and the GUI is nice, but the open source CLI too lets you bypass all the silly error checks, enabling me to do the dump above!

Ah! I tried that... a year and a bit ago, maybe? Couldn't make it work.

It's perhaps time to give it another blast. Did you install it using a package manager or build it from source?

I have a mac, so brew install minipro did the trick for me. If you are not familiar with brew, it gives the mac a opensource repo, not that apple store junk

[Firefox]

I have a mac, so brew install minipro did the trick for me. If you are not familiar with brew, it gives the mac a opensource repo, not that apple store junk

I have a Linux, but the source will be the same. Time to give it another blast!

[speccyplus]

So been going through all the traces, tested all the address and data lines from ULA to lower ram, started from ULA to ROM, etc.

Man there are quite a few mistakes in the schematic.

One of the other things I did was test voltages around the ULA. They seemed way off based on some other stuff from world of spectrum.

I will do more wire tracing from the ULA to CPU and ROM, but so far not finding anything weird. With the power on, I did notice that the CPU got warm but the ROM was getting super hot. Is that normal?

Not quite as hot as the ULA, but getting there.

So wondering if I do have a short somewhere.....

I am trying to strip the board down to as simple as possible, i.e. no ram, minimal logic gates (IC3 and IC4). etc to get to a good state and work up.

Does anyone have definitive behaviors with ULA no Ram, IC3 and IC4.

[1024MAK]

The ULA normally runs hot (see elsewhere as to the reason why). The Z80 (if an originally fitted NMOS type) will run slightly warm. The ROM should run cool. If it’s getting hot, there’s a problem.

With the schematic, it’s also worthwhile looking at the other issue board schematics. To reduce the complexity of the actual routing of the tracks, any address (in a 7 or 8 bit group) or data line can be exchanged with another. In the same way, logic gates in 74xxyyy series logic chips can also be exchanged with another gate of the same type within the same chip (or another chip of the same type).

And yes, even taking all the above into account, there are still lots of errors in the schematics.

If / when people find errors, can you please post details in this topic.

Mark

[speccyplus]

Thanks again Mark.

Just did 100 reads of the ROM, just to make sure I didn't fry it, all returned matching checksum: 4c42a2f075212361c3117015b107ff68 which is also the same for the 48.rom I downloaded referenced earlier in this thread.

Code: Select all

Found TL866II+ 04.2.112 (0x270)
Warning: Firmware is newer than expected.
  Expected  04.2.105 (0x269)
  Found     04.2.112 (0x270)
WARNING: Chip ID mismatch: expected 0x0110, got 0xCE43 (unknown)
Reading Code...  0.52Sec  OK
MD5 (xaa) = 4c42a2f075212361c3117015b107ff68
me@mac:~/Downloads/split$ for i in {1..100}; do minipro -p AM27C256@DIP28 -r zxrom.rom -y; split -b 16k zxrom.rom; md5 xaa; done

Will continue to check traces and try to source a ULA.

[speccyplus]

The status so far:
I have traced nearly every trace from the ULA, CPU, ROM SOCKETS (Removed the components) either looking for 0 resistance on any trace that has no components (Diodes, Resistors, Caps), or checking for continuity before and after, testing the components.

Cleaned both side of the board, found a few previous owner things that were damaged in regard to traces, removed all the upper ram and logic.

I did a burn in for about 10 minutes the other day with just the CPU, ULA and no ROM or RAM.

My board looks like this now:




After powering it on I now see this, progressing from white, gray to the blue bars:









Any of you with more experience, does this indicate anything new to you?

[1024MAK]

Does it always power up and change the border to white (non-bright white)?

And I do mean power up and NOT reset. Test at least eight times.

Mark

[DouglasReynholm]

Threads like this make me glad JTAG/BSCAN were invented. Then we would only need to worry about the discretest of the discretes.

[speccyplus]

Does it always power up and change the border to white (non-bright white)?

And I do mean power up and NOT reset. Test at least eight times.

Mark

Yep, looks good still.

[speccyplus]

So, I got a 27C128 EPROM, and flashed it today with the test rom recommend by Mark.

Did the wire PIN1 and PIN27 to 5v, then ran the select wire to pin 20.

So I turn it on and get this:

[media]https://streamable.com/vm3qgh[/media]

The first beep is my bench PSU, the rest are what I understand should happen. So it get the same results with no ram, 1 chip and full. I saw that I should be getting instruction text if ANY ram is working, and some sort of ram failure, looks like I just get the halt screen that says all RAM is bad.

I am fubar right? This sounds like it is a lower ram problem. I have traced all the wires back from the lower ram last week, and all back to the correct ICs, everything tested well, I am stumped. So either a wonky ULA or need to look at the small caps, etc.

[1024MAK]

Well, the good news is that it looks like the Z80 is working and can read the ROM/EPROM.

The ULA is at least partly working, as the border works and it is trying to display something.

The bad news is that the Z80 does not appear to be able to access the lower RAM. If you have checked and tested all the address lines to the multiplexer chips (IC3 & 4) and the address lines from these to each DRAM chip, and the data lines, that leaves the control lines to/from the Z80, the ULA and the DRAM chips.

So start at the ULA.
Test pins 1, 2, 3, 4, 27, 35, 36, 38
IC24 pin 1, 2, 3, 11, 12, 13
IC3 & 4 pins 5 (also check pins 8 & 15 are connected to 0V/GND)

May also be worthwhile removing the Z80 and the ULA so that you can do a static logic test of IC4. Use a thin wire to feed in a valid logic signal to ULA pin 35 and check to see if the rest of the circuit through IC24 to IC3 & 4 works correctly.

If you still don’t find any problems, then unless the ULA is faulty, I’m stumped...

Mark

[speccyplus]

Well, the good news is that it looks like the Z80 is working and can read the ROM/EPROM.

The ULA is at least partly working, as the border works and it is trying to display something.

The bad news is that the Z80 does not appear to be able to access the lower RAM. If you have checked and tested all the address lines to the multiplexer chips (IC3 & 4) and the address lines from these to each DRAM chip, and the data lines, that leaves the control lines to/from the Z80, the ULA and the DRAM chips.

So start at the ULA.
Test pins 1, 2, 3, 4, 27, 35, 36, 38
IC24 pin 1, 2, 3, 11, 12, 13
IC3 & 4 pins 5 (also check pins 8 & 15 are connected to 0V/GND)

May also be worthwhile removing the Z80 and the ULA so that you can do a static logic test of IC4. Use a thin wire to feed in a valid logic signal to ULA pin 35 and check to see if the rest of the circuit through IC24 to IC3 & 4 works correctly.

If you still don’t find any problems, then unless the ULA is faulty, I’m stumped...

Mark

I have a nebula on order, the ULA all traces back OK with those pins.

In order to simplify things, I removed all the upper ram and what I thought were the Logic ICs needed for it, do I still need IC24 in place?

Your IC3 and 4 pins check out ok.

[HappyLittleDiodes]

Your clip of the machine running with no RAM looks a lot like what I had here
https://youtu.be/OYsv7Jaseu8?t=326

That came down to a broken trace under the RAM sockets.

Have you checked continuity on all the lower RAM pins (Except Din and Dout) between the farthest left (IC13) and the farthest right (IC6)?

[1024MAK]

In order to simplify things, I removed all the upper ram and what I thought were the Logic ICs needed for it, do I still need IC24 in place?

Yes, IC24 is definitely needed for issue 4A, 4B and 4S boards, as two logic gates are used to improve the timing for multiplexer chips IC3 and IC4.

Issue 1 to issue 3B boards don’t use IC24 for this purpose, so IC24 can be removed. Issue 5 and issue 6A boards use a custom chip to replace most of the 74LSxxx series chips, so there is no IC24.

Mark

[speccyplus]

In order to simplify things, I removed all the upper ram and what I thought were the Logic ICs needed for it, do I still need IC24 in place?

Yes, IC24 is definitely needed for issue 4A, 4B and 4S boards, as two logic gates are used to improve the timing for multiplexer chips IC3 and IC4.

Issue 1 to issue 3B boards don’t use IC24 for this purpose, so IC24 can be removed. Issue 5 and issue 6A boards use a custom chip to replace most of the 74LSxxx series chips, so there is no IC24.

Mark

Doh! I was looking at the wiring diagram and saw the NAND gates and was like wait, wah? Thats what I get for thinking a issues 3 and 4 are almost the same.

[MaxS]

Thank you Mark!
I was dealing with an issue 4A, the first 4 I ever had, and it was the worst Speccy motherboard I've ever seen.
Build quality is very poor, traces in the brown upper side without protecting coating and very easy to lift.
And it was very abused by previous owner.. too much love may be..
Anyway after fixing every single trace and checked every IC it won't boot up!!
I was completely puzzled..

when I've read your post, late in the night... very late actually , I jumped in the basement to check schematics:
I've removed the upper RAM as it was an issue 2/3..
put IC24 back in his socket...
and TA-DA!!!
another Speccy of my growing collection is now fully working again!
Many Many thanks
max

[MaxS]

Thank you Mark!
I was dealing with an issue 4A, the first 4 I ever had, and it was the worst Speccy motherboard I've ever seen.
Build quality is very poor, traces in the brown upper side without protecting coating and very easy to lift.
And it was very abused by previous owner.. too much love may be..
Anyway after fixing every single trace and checked every IC it won't boot up!!
I was completely puzzled..

when I've read your post, late in the night... very late actually , I jumped in the basement to check schematics:
I had removed the upper RAM as it was an issue 2/3..
put IC24 back in his socket...
and TA-DA!!!
another Speccy of my growing collection is now fully working again!
Many Many thanks
max