ZX Spectrum + : no boot

[dizzy33]

After having checked the Z80, I saw a lot of strange things.
I compared this Z80 to a fine working one. Here are the results :
On pin 1, I should have 5V but I only have 2V for a few seconds when the Speccy has been turned on, then it goes down to 0V ...
On pin 2 and 4, I should have 4.8V but I only have 3.2V.
On pin 6 to 10, and pin 12 to 15, I should have around 3.8V, but I only have 1.6V (and sometimes this value vary from 1.2V to 2.2V).

In fact, the only Z80 pins which show good readings are pin 6 (CLK), PIN 16. I did not check yet the other side of the CPU (21 to 40).
So it seems that this Z80 does not get enough power, but this problem does not come from the Z80 itself as I already replaced it.
TR4 and TR5 have also been replaced and all RAM show good voltages (+5V, -5V, and 12V are OK).

So I need help to find out which component may be failing and responsible for all these bad voltages reading on the Z80 ...

[1024MAK]

Z80

Pins 1 to 5, and 30 to 40 are the address bus. The signals here depend on what part of memory is (attempting to) being accessed. Depending on what code the Z80 is trying to run, the signals here will vary.

Pin 6 is the clock input, should be a pulse at 3.5MHz

Pins 7 to 10, and 12 to 15 are the data bus. Signals here very much depend on what data is being read from, or written to memory.

Pin 11 is the +5V supply
Pin 29 is the GND pin.

The rest are the control and status pins.

Keep in mind that for 5V TTL based systems, anything at 2V to 5V is considered to be a logic high (1).
Anything at 0V to 0.8V is considered to be a logic low (0).

You said earlier that the ROM chip is hot. Is it too hot to hold a finger in the middle? If yes, then that is very unusual. The ROM normally runs cool. Either one of it’s output pins is shorted to something else, or it’s faulty.

Occasionally a second hand ROM chip may be found for sale. Alternatively this page shows the alterations needed to fit an EPROM chip. You will have to ask someone to programme (burn / blow) the EPROM for you unless you have a suitable programmer.
The type of EPROM can be either a 27128, 27C128, 27256 or 27C256.

Mark

[dizzy33]

Thanks a lot Mark ! Glad to see you back !
I should receive soon a 28 pins socket, so I can swap a working spectrum ROM with the "bad" one. But something let me think the ROM is not (the only) the culprit : when my Diag ROM (from Retroleum) runs, it does override the existing ROM (even without ROM on the motherboard, my diag ROM works).
And my Diag ROM says there is something wrong with IC1 or IC3 or IC4. I guess the diagnostic would be the same without the "bad" ROM on the motherboard.

According to what you said about the Z80 (thanks a lot !), I guess my Z80 signals are correct. But I will double check it.

[1024MAK]

According to what you said about the Z80 (thanks a lot !), I guess my Z80 signals are correct. But I will double check it.

I would not put it like that, I would say that so far, as the Z80 can run the Retroleum “Diag ROM” (do you mean the SMART Card?) code, it appears that it’s okay.

If the ROM chip is damaged internally, it could be causing problems on either the address bus or the data bus. Or there could indeed be other problems. But we try to sort out the most obvious faults first.

Mark

[dizzy33]

Yes, what I call the "Diag ROM" is the Smartcard from Retroleum.
Do you mean that a failing ROM on the motherboard could cause the diagnotic ROM to fail its RAM tests ?
If yes, maybe I should just remove the ROM from the motherboard and see how the diagnotic ROM behaves.

[1024MAK]

Do you mean that a failing ROM on the motherboard could cause the diagnotic ROM to fail its RAM tests ?

Yes, that’s exactly what I mean.

Mark

[dizzy33]

YYYYYYYYYEEEEEEEEEEESSSSSSSSSSSS !!!!!!!!
Thanks a million again Mark !!!!! That was it !
I removed the ROM from the motherboard, and now all the diag ROM pass !

So now, I need to find a second hand ROM, or to use an EPROM instead. Retroleum sells some programmed EPROM for a very decent price.
If I don't find an old ROM easily, I will buy this EPROM (but I know the EPROM needs some modifications on the motherboard, I should be able to do it).

Mark, do you have an idea of where I should be able to find an original ROM ?
Does the ROM release depends on the motherboard issue ?
my motherboard is an issue 6A, this is a "Spectrum +", I think the "Spectrum +" ROM are a bit different, isn't it ?

[1024MAK]

No, there was only ever one production ROM version (in terms of the contents) for the whole ZX Spectrum line until the 128K machines came out. Although there were different manufacturers of the physical chip itself.

So the content in a ROM on a issue 1 board is exactly the same as that in a ROM on a issue 6A board.

The only source for ROM chips, is from people who repair ZX Spectrums or who cannibalise boards / computers.
If Phil at Retroleum does not have one, try Ben at bytedelight or Ian at mutant-caterpillar.

Or on trading sites like eBay...

Mark

[dizzy33]

Thanks a lot Mark !
But now I remembered I have another problem on this Spectrum to solve :
no sound ...
I replaced TR7, same issue : no sound.
I tried to connect the existing speaker to one on a working Spectrum : still no sound.
I know that D9 could be the culprit, but I need to find one to buy, it may take time before I receive it.

I have read on the service manual here https://spectrumforeveryone.com/wp-cont ... Manual.pdf that I need to check load input waveforms. But I do not know how to do it as I am not able to understand the diagram on this documentation (I'm a noob).
On which pin of which component should I put the probe of my oscilloscope ?

[dizzy33]

The only source for ROM chips, is from people who repair ZX Spectrums or who cannibalise boards / computers.
If Phil at Retroleum does not have one, try Ben at bytedelight or Ian at mutant-caterpillar.

Or on trading sites like eBay...

Mark

Mark, I could find some ROMs on the Retroleum website, but there is something I don't understand : there are ROMs "H" or "N" ...
It seems that some ROMs are specific to some motherboards. Phil says : "Please ensure the links on your Spectrum PCB are set correctly for this version of the ROM chip, IE: The two links should be between the ROM select pads marked "H")"
I don't understand what is he talking about. Which "link" is he talking about ? Where to see that on the motherboard ?
I checked around the ROM position but could not see anything marked "H" or "N" ...
I sent an email to him but for now 2 weeks he does not answer anymore ...