Help with new build please :)

[JonPowis]

Hi All,
Have just built a new issue 3B using the new replica board. Have used all new components apart from the ULA and a couple of logic chips. Have done the DCDC mod from new. Have tested the ULA/Z80/Rom in a working board and all three are fine. I did put sockets on the board from the lower/upper ram boards and now have tried putting them direct on the board. All voltages are measuring fine to ram but all I get is a scrambled flashing image on the screen as attached. I have a diagnostic rom fitted at the moment but will not boot to that and have also fitted my dandanator but no difference. Any ideas please, thanks in advance.

[azesmbog]

First you need to install a test ROM instead of the main one in the panel. There are quite a lot of test firmware )
http://zxbyte.ru/test_rom_for_zx-spectrum.htm

Show a screenshot of the screen without the ROM. Should have beautiful vertical stripes)

[JonPowis]

Thanks for the reply, the rom that is plugged in at the moment is a test rom, has two on the switchable rom but still boots to this.

[dfzx]

Is that a static image you see (other than the FLASHing) or is the pixel data changing?

What does it do if you take the lower RAM board out?

[JonPowis]

The image is flashing all different colours, with the ram out I get lots of thin vertical lines which is what I guess you would expect without any ram.

[1024MAK]

What test gear do you have (other than a multimeter)?

Mark

[Tiger]

Have you tried to make the bridges as for an Hitachi-ROM? Possible, that you have to connect pin 1 of your ROM with pin 28 of it, like an EPROM. Maybe pin 24 must be modified - look for instructions to replace the original ROM with an EPROM ...
Using a Lower RAM Module it isn't necessary to make the DCDC mod, because -5V (and 12V) isn't used by it. 12V is only used by the LM1889 at your board ...

[spider]

Slightly off topic (sorry please ignore) but I'm curious because you have the modern upper and lower RAM boards installed, negating the need for the 'extra voltages' aka a coil being fitted. I could be wrong, sorry.

[1024MAK]

As Tiger says, a +12V supply is still needed for the colour encoder (IC14).

Mark

[spider]

As Tiger says, a +12V supply is still needed for the colour encoder (IC14).

Mark

Oops sorry. Somehow I missed that!

[JonPowis]

What test gear do you have (other than a multimeter)?

Mark

I have a logic probe and an oscilloscope which I am still learning to use properly

Cheers
Jon

[JonPowis]

Have you tried to make the bridges as for an Hitachi-ROM? Possible, that you have to connect pin 1 of your ROM with pin 28 of it, like an EPROM. Maybe pin 24 must be modified - look for instructions to replace the original ROM with an EPROM ...
Using a Lower RAM Module it isn't necessary to make the DCDC mod, because -5V (and 12V) isn't used by it. 12V is only used by the LM1889 at your board ...

I will try that thank you regarding changing the jumpers to Hitachi from NEC. I have tried working ROM's from my other boards and get the same result. The diag rom I have fitted is one from retroleum which doesnt need any mods like an eprom as it works fine in other boards. Never even crossed my mind regarding the DCDC that's a very good point, oh well I'm still learning Do you think the DCDC mod could have any adverse effect or should it not matter?

Cheers

Have just tried it with jumpers set for Hitachi but still the same result I am afraid, have also tried with a stock ROM and my JHG rom but still the same.

[1024MAK]

So, using your ‘scope preferably, test pin 6 of the Z80 to see if it’s getting a 3.5MHz (approximately) clock signal.

If this is okay, test pin 19 (MREQ) to see pulsing, to see if it’s trying to access memory
If it is, test pins 20, 22 and 27 on the ROM socket if you are using a ROM.

If you are using an EPROM, how is it configured/connected? Only the mask ROMs that Sinclair used have two chip select inputs plus an output enable input. EPROMs only have one of each.

Mark

[JonPowis]

So, using your ‘scope preferably, test pin 6 of the Z80 to see if it’s getting a 3.5MHz (approximately) clock signal.

If this is okay, test pin 19 (MREQ) to see pulsing, to see if it’s trying to access memory
If it is, test pins 20, 22 and 27 on the ROM socket if you are using a ROM.

If you are using an EPROM, how is it configured/connected? Only the mask ROMs that Sinclair used have two chip select inputs plus an output enable input. EPROMs only have one of each.

Mark

Hi Mark,

Thanks for the response, please see images of readings below. This is using a stock ROM.

Z80 Pin 6 Clock



Z80 Pin 19



Rom Pin 20



Rom Pin 22



Rom Pin 27





Thank you for the support.

[1024MAK]

Okay, for some reason your ’scope appears to think it’s measuring current. We are interested only in voltage (U or V symbols).

The waveform for the Z80 clock on pin 6 looks okay, but I can’t tell if the waveform has the correct voltage range.

The other waveforms just show garbage, which could be noise if it’s being used on automatic mode.

Can you check the voltage (using a multimeter) between Z80 pins 11 and 29 please.

You could also recheck your other results with your logic probe.

Mark

[Tiger]

Imho it seems that the "RAS" isn't recognized. I would check R32. Is it the right value (100 Ohms)? Are the connections (IC3/4 to R32 and R32 to ULA and pin 4 of the RAM's) ok?

[JonPowis]

Okay, for some reason your ’scope appears to think it’s measuring current. We are interested only in voltage (U or V symbols).

The waveform for the Z80 clock on pin 6 looks okay, but I can’t tell if the waveform has the correct voltage range.

The other waveforms just show garbage, which could be noise if it’s being used on automatic mode.

Can you check the voltage (using a multimeter) between Z80 pins 11 and 29 please.

You could also recheck your other results with your logic probe.

Mark

Hi Mark,

Z80 pin 11 gives 4.94v, pin 29 -20mv as i see its ground, pin 19 is low

Rom pins 20,22 & 27 all show as low

[JonPowis]

Imho it seems that the "RAS" isn't recognized. I would check R32. Is it the right value (100 Ohms)? Are the connections (IC3/4 to R32 and R32 to ULA and pin 4 of the RAM's) ok?

Thanks, checked the 100R resistor and that's fine will check other connections

[zx64]

Check all your soldering using magnifying glass. Sometimes pin may appear to be soldered but it's not

[JonPowis]

So, using your ‘scope preferably, test pin 6 of the Z80 to see if it’s getting a 3.5MHz (approximately) clock signal.

If this is okay, test pin 19 (MREQ) to see pulsing, to see if it’s trying to access memory
If it is, test pins 20, 22 and 27 on the ROM socket if you are using a ROM.

If you are using an EPROM, how is it configured/connected? Only the mask ROMs that Sinclair used have two chip select inputs plus an output enable input. EPROMs only have one of each.

Mark

Hi Mark,

Have gone through this again, the clock signal at pin 6 of the Z80 is fine at 3.5mhz.

There is no pulse at pin 19 and no pulsing at pins 20,22 and 27 on the rom

So its as if nothing is coming out of the Z80. I then have swapped the Z80 into a fully working board which is fine so I know the Z80 is fine, what would stop the Z80 pulsing on pin 19 or am I going in the wrong direction.

Cheers
Jon

[cj7hawk]

It looks like the ULA is working OK, so I'd suspect the RAM/Z80 or something around there unless the test rom does something at least.

Doesn't seem to be a colour issue from the posted images.

If you have a logic probe or an oscilloscope, have a look through the ROM signal lines, make sure the ROM is being selected and that there's activity on each of the lines.... Is there any change in screen activity at all after the image stabilises on powerup?

Key things to check.

1. z80 is operating ( getting clock at right frequency, is setting MREQ and RD and is getting data from the ROM. That the ROM lines are all working correctly, none are shorted with solder, etc. Shorted lines generally look different to others on an oscilloscope.

2. If the ROM is working, that the RAM is working and getting write signals from the CPU, since the Spectrum should be trying to write the ram on power up.

3. It can be worth removing the upper memory and upper memory select ICs as a 16K speccy is easier to debug.

4. Begin checking that each important line (eg, from the ROM ) gets to where it's going.

and so on...

Regards
David

[JonPowis]

It looks like the ULA is working OK, so I'd suspect the RAM/Z80 or something around there unless the test rom does something at least.

Doesn't seem to be a colour issue from the posted images.

If you have a logic probe or an oscilloscope, have a look through the ROM signal lines, make sure the ROM is being selected and that there's activity on each of the lines.... Is there any change in screen activity at all after the image stabilises on powerup?

Key things to check.

1. z80 is operating ( getting clock at right frequency, is setting MREQ and RD and is getting data from the ROM. That the ROM lines are all working correctly, none are shorted with solder, etc. Shorted lines generally look different to others on an oscilloscope.

2. If the ROM is working, that the RAM is working and getting write signals from the CPU, since the Spectrum should be trying to write the ram on power up.

3. It can be worth removing the upper memory and upper memory select ICs as a 16K speccy is easier to debug.

4. Begin checking that each important line (eg, from the ROM ) gets to where it's going.

and so on...

Regards
David

Thank you for the info David, I went through testing all the lines from the Z80 to Rom and the ULA last night and cannot find any issues but I do not seem to be getting any life at the ROM just high or low signals. I took another rom out of a working machine but still no joy. The coloured screen you see is all that happens and most of them are flashing.
I think I am going to take off the upper RAM board and its associated logic as you say to start from there. The only thing that is an unknown at the moment is the logic ships at IC3/4 which I have new ones on order from Retroleum which should hopefully arrive today.

Will post when I have more info.

[cj7hawk]

All a z80 needs to work is
* A stable suitable clock.
* Reset low, then rising high.
* A Rom with suitable code.
* Decoding so that the rom is read.

Given the ULA is separated bus-wise by the resistors, if you get this much going, then the z80 should start, so it's just a matter of making sure all the address and data lines go where they should, and aren't shorted to anything they shouldn't be.

From there, it's just a logical troubleshooting exercise.

Regards
David

[1024MAK]

There is no pulse at pin 19 and no pulsing at pins 20,22 and 27 on the rom

So its as if nothing is coming out of the Z80. I then have swapped the Z80 into a fully working board which is fine so I know the Z80 is fine, what would stop the Z80 pulsing on pin 19 or am I going in the wrong direction.

This is a strange one.

If the Z80 has power, a good (as in within specification) clock signal and has received a suitable reset pulse, it should be trying to access memory. The only things that will stop this, is if pin 35, BUSRQ is at logic low. Or something else is shorting the Z80 control lines to 0V/GND.

Mark

[cj7hawk]

This is a strange one.

If the Z80 has power, a good (as in within specification) clock signal and has received a suitable reset pulse, it should be trying to access memory. The only things that will stop this, is if pin 35, BUSRQ is at logic low. Or something else is shorting the Z80 control lines to 0V/GND.

Mark

!WAIT would also do it, as would enough noise on the clock line so that it didn't recognize the clock. But I agree - as long as the main signals are OK, and z80 will at least try to do something. I never did see an update on this so I don't know what happened next?