48K issue 3 - colours all wrong!

hellraiseruk
Posts: 18
Joined: Tue Oct 12, 2021 5:58 pm

Re: 48K issue 3 - colours all wrong!

Post by hellraiseruk »

Anyone got any ideas on this? I've sourced another Speccy, but before I do a ULA swap any assurances the higher voltage on pin 16 is not liable to nuke another ULA?

Cheers

JD
hellraiseruk
Posts: 18
Joined: Tue Oct 12, 2021 5:58 pm

Re: 48K issue 3 - colours all wrong!

Post by hellraiseruk »

Well, I've found the problem....

Swapped out the ULA from the machine with the fault into the working Spectrum I picked up, lo and behold the fault followed complete with 3.94v on pin 16. Re-fit the original ULA into the new Speccy, and it works fine. So, it looks like the ULA is goosed.

Next question, is it safe to put the working ULA into the duff Speccy as a test, or am I better of just getting a vla82/nebula and sticking that in? Am I liable to nuke the ULA? A part of me thinks I'd be better off getting a ULA replacement and just putting that in.

Anyway, off to order the bits to refurb this spectrum+, I'll be back :)

Cheers

JD
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1024MAK
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Re: 48K issue 3 - colours all wrong!

Post by 1024MAK »

If you measure the voltage on the socket contact with no ULA fitted, that will tell you if there is a problem with the board…

If you don’t get any significant voltage, power down. Wait for a couple of minutes. Then test between the socket contact (with no ULA fitted) to the +5V rail, the +12V rail and to the 0V/GND rail using the resistance range on your multimeter. Remember to each test twice, swapping the leads around for the second test.

If you don’t get any low results, it’s safe to try a known good ULA in the board.

As the video output pins from the ULA also go to the edge-connector, the original ULA may have been damaged by edge-connector misuse.

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hellraiseruk
Posts: 18
Joined: Tue Oct 12, 2021 5:58 pm

Re: 48K issue 3 - colours all wrong!

Post by hellraiseruk »

Thanks for that, I did check pin 16 on the socket with no ula fitted and it was showing 0v, I'll do the remainder of the checks tomorrow. I just need to find a Nebula or vla82, noone seems to have stock which is annoying!

I've actually quite enjoyed figuring out the fault on this, might keep an eye out for a few "untested" ones on eBay and have a play :)

I never considered the edge connector, not knowing the history of this particular spectrum it is a possibility I can't rule out.

JD
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