Bright White screen .No cursor

[1024MAK]

Up to you what you do with your early issue 1. I have a LCD TV that is happy with 2C184E ULAs, so if it was mine, as I have other issue 1s that I have already modified, I would leave it as it is.

The issue 3 has developed a further problem, 'if' i get a white screen it goes black after a few minutes then its had to get a white screen back.

That's not good

I'm pretty sure I tested the voltage on the power supply before I used it but cant remember anyway now its outputting 16v....

The so called 9V PSU is an unregulated type. If there is no load (ZX81 not connected), the transformer will produce a higher voltage than the rated value, plus the smoothing capacitor(s) will charge up to the peak of the rectified AC waveform. Typical small transformer off load voltage increase is 15 to 25%. To convert AC RMS to the peak of the sinewave, multiply by 1.414. So 9V AC plus 25% is 11.25V AC, then multiply by 1.414 gives the peak value of 15.9V. This is the voltage that the smoothing capacitor(s) will charge up to. Once the PSU is connected to a ZX81 (loaded), the output voltage will fall to anywhere between 9V DC and 12V DC. ZX81s that have been upgraded to a switching regulator/ DC-DC converter (in place of the 7805 voltage regulator) will draw less current, so the ZX81 mains PSU output voltage will be higher than when supplying a unmodified ZX81.

So I have switched to my bench power supply set to 9v and 500mA and its only drawing about 0.14-0.2 mA.

Are you sure you mean mA and not Amps? 0.2mA will not warm up anything. The CPU on it's own will consume more than this.

I am still getting signals on pin 6 z80 so has the ula been fried ?

A black screen may mean a faulty ULA, or it may mean the machine is somehow entering FAST mode.

Rom still gets warm.

Can you hold your finger in the middle for at least 20 seconds after it has been on for about five minutes? If no, I would say the ROM may be faulty.

Should I risk swapping the 184 ula in the working issue 1 to the temperamental issue 3 which has a 210? will it work?

If the 5V supply is okay (between 4.75V and 5.25V) and you test each ULA socket pin (except the 0V/GND pin ) on resistance between it and a 0V/GND point (modulator case) to check that there are no short circuits, it should not be damaged. Normal long list of exclusions apply...

Mark

[povvercrazy]

Thanks for your replys Mark
Yes i ment Amps not milliamps, my brain is as confuddled as my 81's ula.
The ROM just gets warm nothing too hot i cant hold my finger too even after a few minutes.
I have noticed the ULA in the problematic issue 3 doesn't get warm at all any more, I did put a new z80 in so maybe thats
made the ULA just give up.
When it does display a white screen the out put on pin 16 is good with back porch incidentally.
Think I'll brave a ULA swap after I do the checks you recommended, just need to pry it out the issue 1 seems to be stuck
I now have a Sinclair 16k ram pack but thats non-working plugging that in the working issue 1 causes it not to boot, so thats another project
What do you recommend, putting the 210ula in the working issue 1 or put the working 184 ula in the non-working issue3?

Oh and yeah the 5v supply on both units is good.

[1024MAK]

What do you recommend, putting the 210ula in the working issue 1 or put the working 184 ula in the non-working issue3?

Both! Do a swap. If the problem moves...
Mark

[povvercrazy]

Ok Issue 3 Mobo is good, rom and cpu and ram all work fine with 184 ula from the issue 1 .

Putting the 210 Ula in the issue 1 mobo I get the white screen issue... so thats sorted the 210 Ula is dead

So I need to source a replacement Ula the case and issue 3 mobo are to nice to collect dust.

Thanks for all your advice Mark.

Now... any idea how to fix a 16k ram expansion

[1024MAK]

Well done

Good that you have found the fault. Bad that it is a duff ULA. So someone else waiting for a modern ULA replacement...

There is a thread here somewhere about Sinclair 16k RAM packs and repairing them...

Mark

[Tezz]

It'll certainly be a great day when the ULA replacements are available.

[povvercrazy]

yeah i'll look for the topic on the ram pack, would be nice to get that going too.

I have seen some posts and links to 'remaking' a 81 Ula

http://trastero.speccy.org/cosas/JL/ZX8 ... 1-ULA.html

looks interesting...

Bit beyond me I think...

I'm keeping the issue one as is, given her a good clean lots of alcohol.. and have just ordered a backporch /comp mod from Pokemon

which I will fit intrusively so I can use it, I will just scan ebay for more 81's that need tlc hopefully get lucky.

thanks for your guidance Mark

[mrtinb]

I have seen some posts and links to 'remaking' a 81 Ula

http://trastero.speccy.org/cosas/JL/ZX8 ... 1-ULA.html

looks interesting...

Bit beyond me I think...

I'm trying to create RetroTechie's ULA in this thread. My board is huge single sided, but you can probably make it smaller and simpler.

[povvercrazy]

How far have you got with your project? There are a few spectrum options I'm surprised that this hasn't been done yet as there is definitely a demand.

[mrtinb]

It's been on pause for a while. In my next step I need to transfer the design to copper and etch. (I havn't made homemade pcbs yet.)