Voltage Regulator, TR4 and TR5 replaced - this did not correct the missing -5v issue.
C65 part of the -5v circuit and all capacitors replaced as matter of course anyway. The -5v now working and therefore C65 was obviously at fault.
Boots and diagnostic ROM tests passed.
...But already done…
Check the Z80 clock signal...
Xavier ...on the Facebook groupe : "Zx81 France"(fr)
Yep test pin 6 (clock) on the Z80 using a ‘scope or logic probe.
If that is okay, test the following Z80 pins:
Pin Function
16 /INT (should be pulsing at about 50Hz)
17 /NMI (should be high)
18 /HALT (should be high)
19 /MREQ (should be pulsing)
21 /RD (should be pulsing)
22 /WR (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Without answers to help, I still try to work on this case. I checked everything I could physically on the motherboard, and saw that the reset button cable was in short circuit ... That's why the CPU had strange readings on its pins. Now the CPU seems to work fine.
The display is still the same when powered on (see two posts earlier). But now the Spectrum boots on my Retroleum diagnostic ROM.
The ROM says there is a problem with ram addressing, and asks to check IC1 / IC3 / IC4.
I tried to replace IC1 (ULA) but no change. My motherboard is an issue 6A, so IC3/IC4 are now included in the IC27 "PCF1306P". I replaced it, but still no go. I also tried again to replace the following components :
- the Z80
- the 74LS04N (IC28)
- all the low RAM
- IC15 on upper RAM
But still the same.
However, I noticed something a bit strange : the ROM (IC5) is quite hot, I measured it to be at 58°C. I compared to my other working Spectrum, and on them the temperature of the ROM was a lot lower (at least twice lower). So I wonder if the ROM is dead or not. But unless I'm wrong, my Retroleum diagnostic ROM should override the Sinclair ROM, right ? So if the ROM is defective, at least my diagnostic ROM should not talk about RAM address problems.
Since my Spectrum is now able to boot on the Retroleum diagnostic ROM, I could see a new problem on this Spectrum : no sound ! I will take care of this later.
Today I will try to investigate about the ROM, and maybe replace all the upper RAM.
I just found a neat resource here: http://hardware.speccy.org/reparar-i.html. It would suggest C44 from comparison with your picture. You did a complete recap, but maybe there's still a fault there, or you got a bad part? I don't know.
Strange ! I was just checking the same web page !
But as you said, I have the exact same symptoms but I already replaced all caps, including C44. I will replace it one more time but I'm 99.99% sure it will be useless.
But this shows that the problem may come from any electronic component other than an IC. As I am a total noob in electronic I need help to find out which component may be the culprit.
EDIT : you will see here https://www.markmorton.co.uk/zxMaintProjects.html that the same display may also be a Z80 problem (but not in my case as I also replaced it). So there are close to no chance that C44 would be the culprit
Here is an update of the list of replaced component :
- the Z80
- the 74LS04N (IC28)
- the ULA
- all the lower RAM
- all upper RAM
- all capacitors
- TR4/TR5
- IC27 (replaces IC3 IC4 etc ... on issue 6A).
Still no go. My diag ROM still says that there is a RAM addressing problem which may be related to IC1, IC3 or IC4
My next step will be to check the Z80 with an oscilloscope, along with ULA and IC27 (but I have no references for IC27 so I do not know what I should see with my oscilloscope).
Standby alert 
