I've borked it

[msknight]

Hi Dessony,

It has 210E. It is going to an RGBtoHDMI.

Currently outputting a white screen. Has been replaced with a vLA81 replacement, but the result is the same white screen. Presumption from this is that the video output is fine, but that the boot process isn't happening for some reason.

Michelle.

[1024MAK]

I've found a schematic and pin 26 reset on the CPU starts at zero and rises to 5v over a half a second and stays there.

So that's working, R15 and C5 are also going to be okay.

As I've got a good clock, I assume that TR2 is working.

If the waveform at pin 6 of the processor is going rail to rail (very near to +5V and very near to 0V), yes, TR2 and associated components would be okay.

That leaves two capacitors, two resistors and the X1.

If you mean C3 and C5, nope, it's not those. C3 helps decouple the +9V rail, as long as the +5v from the regulator is within spec (4.75V to 5.25V) and has less than 100mV of ripple or noise, it's not C3.

C5 is okay, as you have already watched the reset signal work.

X1 is the 6.5MHz resonator, this must be okay if the clock to the processor is okay.

Using your 'scope, can you test these signals on the following pins of the Z80 CPU?
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Mark

[msknight]

The differences are...

Pin 17 is high - NMI
Pin 18 is high - HALT
Pin 22 is high - WR
Pin 24 is high - WAIT

Screen shot of one of the data lines attached. They're all more or less like this.

[msknight]

Address lines...
A0, A1, A2, A4, A5, A6, A8 and A10 are all active.

The following lines are 0v flat...

A3, A7, A11-A15

[msknight]

That clued me in that I had solder re-work problems.

Now address lines 0 to 7 are active, A8 and upwards are silent low.

But the system still doesn't want to boot.

[msknight]

EDIT - Not Solved.

So it seems that the Ferannti had died.

The white screen wrong footed me to the ROM, because I had been changing ROMs and experimenting with them.

The existing dry solder joints caused me to address that, and I badly soldered a few joints myself, resulting in another error.

That then wrong footed me when I was changing out chips, so it continued to fail ever after I swapped out the ULA.

So, tracking down the bad soldering and solving that, then enabled me to swap chips again... and then I found the ULA swap fixed the system.

Thanks, everyone, for your time. At least we have another success story after all this!

[msknight]

Not solved. Same problem still exists. I spoke too soon.

As an aside... I'd like a mat that I can rest circuit boards on and power them up safely, because popping chips in and out of the board when its in the case is just risking bending the thing.

Grateful for suggestions please.

[msknight]

Symptoms are the same as before, but the address lines 3 and 7 are obviously fixed. Two new ULA's have been tried, a new processor, I've swapped the ROM and the RAM.

However, the previously reported problems with pin 17, etc. on the processor, remain.

I've obviously got a short or break somewhere... but where...

[msknight]

This is where I am at the moment...

NOTE - The ROM is an EPROM at 32k. Pins 1, 27 and 28 are high, pin 2 is pulsing, as expected

[msknight]

OK - I've returned the original chips, ULA, processor and ROM... with the exception of the RAM. And this is how it's staying for people to catch up with this mess, otherwise people will be making suggestions and I might have changed things... which won't help. (and I don't have another socketed zeddy)

This is what I've got, noting that some of the address lines are coming in with low voltages.

Also, nothing is coming back on A8, although they do test for continuity and resistance is the same as A7, so I assume it's working as connected, and no shorts. This is baffling me to heck and back.