Issue 6a ZX Spectrum 48K Repair - From Knackered to New 

wont make much difference..

@@andygozzo72 You have no idea how much energy the 7505 dissipates, it's dropping 9V to 5V, the heatsink gets painful to touch. Without a heatsink the 7505 kills itself in a few minutes. A switchmode regulator(the can be a drop in replacement) can safely run without a heatsink, showing how much cooler it runs. The ULAs are no longer manufactured, and Sinclair over exploited them in the design, so they run hot too. The two together are a recipe for failure. the other thing to do is of course heat-sink the ULA. The heat also contributes to drying out electrolytic capacitors. The 128k Spectrum had an external heatsink and was dubbed a "toast-rack". Sinclair designed their machines to be cheap, and pushed the technical limitations of the components.

@@EmmittBrownBTTF1 yes, without it, it could fry(they're supposed to shut down with excess temperature, though) thats why they have a heatsink ,, loads of other devices use those linear 3 terminal regulators, some running hotter than in a spectrum, with no real issues ,

Good tip. I've got a drawer full of them. I'm often torn though because you lose the authentic heft and warmth that the 7805 and the heatsink generate

Thanks buddy

definitely, a shorted tr4 can destroy the coil, it melts the wires enamel coating causing shorted turns, replacing tr4 without replacing a damaged coil will soon destroy the new tr4! guess how i found out!!!!

My boss still asks me why he was in a pile of leaves!

Good eyesight, that's an interesting observation. If I ever get my hands on another VLA82 I will compare

I've shipped a few over the pond ;)

there was the USAs own variant of the spectrum TS2068, unfortunately not fully compatible unless you fit a spectrum rom in place of the original

The address doesn't matter, as long as it resides in upper RAM, which is all addresses between 32K and 64K. I think I only chose 55000 because a website describing the trick used that address.

Was the board working previously? If so, I would recommend a new 7805, new TR4 and TR5, and then find a video by Joulespercoulomb "ZX Spectrum Initial tests" and follow along. If you get to the end without any funny business then power it up and see what you get on the screen.

@@HappyLittleDiodes Perfect. will do so. actually , since I am at it, think i can replace the 7805 with a newer / more efficient regulator to have less heat dissipation, would you agree? thanks again.

Yes you can use a Traco TSR 1-2450. I think Murata also make one which is slightly cheaper

Hi Mike. You can solder the two wires to either end of C27. See the bottom left of this schematic. RESET is active low, so you need to short it to ground to cause a reset. spectrumforeveryone.com/wp-content/uploads/2017/08/ZXSpectrumIssue6a-Schematics.gif You can see it on a 4S board here ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-NOUfxcpP3Xc.html

@@HappyLittleDiodes Thank you so much for getting back to me and taking the time to show and explain how to do it. Definitely made a subscriber here 👍

I think you are just seeing an interference pattern between the resolution of my camera and the screen

@@HappyLittleDiodes No I am not. Minute 6:00 just a second before also later... about middle... centre of screen... and later appear some more. I have also this board and I changed some toasted memories and the ULA now it works but with those... glitches. Trying to figure this out.

@@ElectronGordo Yes, you can also see that the screen jumps right a couple of columns. Later in the video I swap the ULA and it's OK afterwards.

Do you have another ULA to try?

@@HappyLittleDiodes No I have not, but this one works fine, cause it took it off another Spectrum and has not this problem. But with this Issue 6A appears. I will watch your video again, I thought they were there even after ULA replacement.

In hindsight it's probably more likely that it just needed reseating, as it worked in the other machine I seem to remember

I've yet to come across a speccy with a bad cap that has caused any trouble, you're right though, it can happen. I would certainly do that if any of the caps were clearly failing from a visual inspection.