Spectrum Computing

With multimeter in continuity mode i got:

ZTX 651 values:
670, 671, 1, 1

ZTX 751 values:
670, 668, 1497, 1

I power on without ram for some seconds and the screen is bit better then before. THERE IS STILL LIFE!

Now with new ZTX 651 and 7651 (without new RAMS)
After some seconds I pressed the reset button and this is result:

https://imgur.com/a/Se0bVKu


this was before:

https://imgur.com/raae8HW

Looks like the CPU can fetch and execute the first few instructions, including a jump, from the ROM, which is great news.

Ast A. Moore wrote: ↑Tue Jul 09, 2019 11:38 am Looks like the CPU can fetch and execute the first few instructions, including a jump, from the ROM, which is great news.

Wow! I'm really, really, really happy today! Next step will be RAM!

Hyuma wrote: ↑Tue Jul 09, 2019 10:43 am Can i do some tests now? (I still don't have ram)

Sure!

Test the voltages on pins 1,8 and 9 in the position where IC6 was (or any other of the 4116 or equivalent DRAM chip positions). With the negative probe to 0V/GND (e.g. the case of the modulator or the heatsink).

Expected voltages:

PIN 1: between -4V and -5.5V
PIN 8: between 10.8V and 13.2V
PIN 9: between 4.75V and 5.25V

If all these are good, then the next stage is to sort out the ‘lower’ DRAM chips.

Mark

1024MAK wrote: ↑Tue Jul 09, 2019 3:57 pm

Hyuma wrote: ↑Tue Jul 09, 2019 10:43 am Can i do some tests now? (I still don't have ram)

Sure!

Test the voltages on pins 1,8 and 9 in the position where IC6 was (or any other of the 4116 or equivalent DRAM chip positions). With the negative probe to 0V/GND (e.g. the case of the modulator or the heatsink).

Expected voltages:

PIN 1: between -4V and -5.5V
PIN 8: between 10.8V and 13.2V
PIN 9: between 4.75V and 5.25V

If all these are good, then the next stage is to sort out the ‘lower’ DRAM chips.

Mark

pin 1: -5,21v
pni 8: 12,38v
pin 9: 4,98v

yesssssssss!

Keep on walking!

-If it’s not problem for you, please use Postimage to upload your photos. It is easier as Postimage button is pinned on “reply” mode, but also is way better and safer for you photos to stay online for long time.-

I just noticed that retroleum website has lower ram module out of stock... damn!

See here for another source of Lower RAM modules
I have bought from here before and got good service

Mark

I sent an email to the guy of retroleum and he re-stock the item! I just bought 1 lower ram! Let's wait for it!

Ram arrived! Check pictures. There is still issue.



I uploaded also a video, I pressed reset button after a bit.

http://imgur.com/a/0UGyyEQ

As a test, i checked all voltage on upper Ram module on pin 8 and i have all 4.98V.

I double checked the wires I soldered for Lower ram traces and I discovered a wire not connected!


I download OTLA software and try to load some .tap:

Hyuma wrote: ↑Wed Aug 07, 2019 7:26 pm I double checked the wires I soldered for Lower ram traces and I discovered a wire not connected!

Hurrah! Really happy to see this mate, I applaud your perseverance.

Also big thanks to everyone who helped Hyuma, I knew I could rely on you when they first enquired about their problems on another (inferior) website, and I suggested a better place to ask!

*sheds small tear and salutes

I've noticed that with OTLA loading games some are working, some others not, dunno why... and not all keys are responding when i press... maybe the keyboard need some clearing or some other to check?

DouglasReynholm wrote: ↑Wed Aug 07, 2019 7:35 pm

Hyuma wrote: ↑Wed Aug 07, 2019 7:26 pm I double checked the wires I soldered for Lower ram traces and I discovered a wire not connected!

Hurrah! Really happy to see this mate, I applaud your perseverance.

Also big thanks to everyone who helped Hyuma, I knew I could rely on you when they first enquired about their problems on another (inferior) website, and I suggested a better place to ask!

*sheds small tear and salutes

Thank you so much, friend!

Hyuma wrote: ↑Wed Aug 07, 2019 7:39 pm I've noticed that with OTLA loading games some are working, some others not, dunno why

OTLA can only modify the standard loader. It can’t magically scan and figure out every non-standard loading scheme. Also, keep in mind, that due to the way the OTLA loader reads from the port, you can’t press any keys while a game is loading; it’ll introduce errors.

Hyuma wrote: ↑Wed Aug 07, 2019 7:39 pmand not all keys are responding when i press... maybe the keyboard need some clearing or some other to check?

You can manually short each of the pins of KB2 sequentially with each of the pins of KB1. If some are not responding, than the problem is either with the ULA, diodes D1–D8, or the PCB traces. If they check out, then the problem is with the keyboard membrane, which, in all likelihood, will have to be replaced.

I'm now cleaning the keyboard sheet, I found these black dirty lines on the tracks, with IPA very easy!

Usually the ribbon part develops microscopic fractures on the bend as the plastic degrades which eventually breaks continuity in those metallic paint tracks. Since you've got it open you should replace the membrane. The new ones are made of a better material.

I also applaud your persistence, it's been an interesting thread to check back on. It must be satisfying to have resurrected this machine.

I know nothing about electronics but like others enjoy following the progress of threads like these..

If it's at the point where it just needs a new membrane it's a great success story (even I can change one of those!).

It shows all your work and persistence have finally paid off.

Congratulations! Don’t bother keep or fix that membrane.
Change it with a new quality one. You can buy it from Retroleum.

Or if you can fix it, do that and keep it for testing

I want show you something curious...


In this picture, you can see on my mat a white/beige shadow line pointed by my red probe. That is the result after cleaning the black dirt from the membrane. Seems like that IPA removed with the dirt also the conductive material of the tracks on the membrane . So after the cleaning I've 0 keys working and I will buy one new the next weeks. I don't know if that is the result of corrosion or not, but on my mat now I have some some part of the membrane's tracks .

Yeah, I strongly advise against using IPA or any other solvent cleaner on any keyboard membrane.

For cleaning the plastic part of the membrane, only use a cloth moistened with soapy water, then another with clean water, then dry.

For the conductive copper "strips", you have to be rather careful as they are only a thin film. Apart from the areas where they connect to the PCB connector, and the area where two membrane layers connect together (plus keyboard only), don't worry about them going black in colour. The black colour may well be tarnish due to the copper actually being an alloy rather than pure copper (which goes green rather than black when exposed to moisture and oxygen).

Having said all that, it was probably unrepairable anyway.

Mark

Now that you job is done and you have a working spectrum, don’t forget to give a good cleaning to your motherboard with isopropyl alcohol and toothbrush.

It is very important. I saw lot of flux on your soldering work.

Hey long time i didn't post nothing, this week i'll go to an important flea market for electronic in Modena. I hope to find some spare parts for my ZX (rubber keyboard, capacitors, etc..). Lately i was busy to repair some C64.