Zx spectrum 128 +2 iss3 Z70500, problem with tr9

[Muttley Black]

Hello all!

I have here a zx 128 +2 Issue3 Z70500 motherboard that didn’t have 12v. I measure TR9 ztx650 and it was faulty. I replace it with ztx651 but keeps burn it. I see in my desk power supply that pulls almost 2A when i connect the motherboard. I replace also tr8 with ztx751.

Any help please?

[Muttley Black]

The problem was in 12v rail, that was short to ground and blows TR9. The solution made by replacing capacitors C6 and C41. Measurements on this two capacitors shows the one as resistor and the other one as diode. After that replacements voltages are ok. The only problem now is that TEA2000 color encoder damaged from that short mess cause I have nice boot screen but no color.

[Muttley Black]

Hello all.

Anyone have measurements of TEA2000 ic with oscilloscope?

With new ic still have no colors.

[cj7hawk]

Can you let me know what pins you want oscilloscope pics from on that chip?

Thanks
David.

[Muttley Black]

Hello and thank you for your answer. If it is no problem i want for all (not the voltage pins ofc) cause I want to compare with my results.

Thank you very much.

[cj7hawk]

No worries. Just recorded it with my phone. Seems to work OK.

https://youtu.be/3ehCIXH_d1M

Let me know if you have any issues. I just track from Pin 1 to Pin 18 and leave it running for a few seconds on each.

Hope this helps

David

[Muttley Black]

Thank you so much!!!

I will report back!

[Muttley Black]

I have the same scope with you! Also all my measurements are about the same with you. I don’t understand why I don’t have a color. I have made an av mod by get the signal from rgb connector. Everything is good but black and white picture only… I have test also 2 more tea2000. Same results.

[cj7hawk]

Silly question... Have you tried another composite monitor?

Some monitors had problems with the Spectrum color burst timing IIRC.

David.

[Muttley Black]

Yes. Two monitors. Monitors that working great with other spectrum computers.

[cj7hawk]

OK, time to go deep.

Read this: https://html.alldatasheet.com/html-pdf/ ... A2000.html

It's the chip datasheet.

Prior to checking, MEASURE the voltage on pin 11. It MUST be above 10.8v. From around 5 to 10.8v, you get image, but no colour. From 10.8 to 12v, you get some colour but your clock can't be more than 50hz high. From 12 to 13.8v, you should get colour but your clock can't be more than 50hz low. I noticed this when I bypassed the oscillator and drove the 12v rail directly from a lab power supply to check if it was my oscillator that had failed. If your oscillator isn't pushing enough voltage ( and STABLE voltage ) to this pin, that will cause the problem you have. Ground is on Pin 9.

If that's all OK, then check the video really is broken. Try taking your video signal directly from Pin 6 on the chip and see if that works.

If still nothing good, then check the oscillator ( and the oscillator frequency ) on pins 12/13 just about the 12v supply. Should be 8.867238 MHz. If it's lower, you might have a NTSC computer.

Then check pin 14. It should be TTL logic HIGH. If it's low, it will encode NTSC and you won't get colour.

This is probably all you can do at this point. Start debugging the circuit like you're making it for the first time. You're outside of the repair envelope now and working in the engineering envelope.

Regards
David

[cj7hawk]

There is also this document that will help -

https://retrocomputacion.com/yabbfiles/ ... 28_RGB.pdf

Regards
David

[Muttley Black]

Thank you so much for your help! I have this pdf already printed. Just no much free time. I will post when I have news!

[Muttley Black]

Finally I got some time

I made some tests.

1) volts on pin 11 of TEA2000 is 11,4V. I also gave my self 12+ and nothing change.

2) Pins 12 / 13 have correct MHz.

3) Taking directly video signal from pin 6 didn’t change anything. No color.

4) Pin 14 is High.

I am lost……..

[zx64]

If you probe composite output from the chip (using scope), can you see color bursts? They must look like few cycles of sine wave at the start of each line, before the active video. Last page of datasheet (link above) shows how they look. Post us pictures or video.

Make sure you border is not black color, so you can see where the active video starts