Building a ZX Spectrum and I have some voltage questions

[DistantStar001]

I've posted about this before looking for parts, but now I've found all the hard-to-get stuff (thank you Retrolium!). So I figured new questions, new thread. Also, cross-posted in Spectrum Computing forums and World of Spectrum.

In any case, as seen in the pictures below, all the soldering is done, and I've begun testing the voltages. Figured I'd make sure that was right before I started putting in the ICs.

I've tried two different power supplies but haven't noticed any significant difference in voltage on the board. The first is an original European Sinclair supply that came with my Spectrum+ (yes I have a step-up transformer giving me 220v). It's rated at 9v, 1.4 amps, center negative.

The other is an American (110v) generic brick, rated at a similar 9v DC, 1 amp center negative.

Both supplies work on my Spectrum +, and the latter was what I was using on my original 3B before it went bad (re-capping mishap but I'm going to bring it back). Also, both bricks are unregulated and seem to be putting out about 12 to 12.5v (a bit high, but within expected ranges for these kinds of bricks).

Side question: Would either of these work for a +2? I have one coming in a few days.

Anyway, here's what I got:

+5V to "RF": +4.82V (probably not important since I'm not installing an RF Modulator)
+5V to Lower RAM: +4.98V
+12V to Lower RAM: +12.56V
-5V to Lower RAM: -4.31V
+5V to Upper RAM: +4.98V

So far I think the voltages look good, but I want to know if I should be concerned that the 5-volt lines (both positive and negative) are a little low since the board isn't populated yet (hence a minimum load)? I should also note that I was offered (and installed) a more robust version of the 7805 than was used in the original. So I don't know if that has something to do with the lower voltages I'm seeing?

[Simon_Carr]

Hi Distant Star,

Just a quick response from my experience, and I'm sure there are many other more experienced people who will also respond. Your voltages all look okay to me, although the -5V is a bit low. I'm looking to do something similar to rebuild a spectrum from components taken from a board where there are just too many broken PCB traces to resurrect, so your project looks pretty cool to me...

Simon

[hatman72]

That looks great! If you don't mind me asking, where did you get that metal bracket to replace the modulator from? The connector on my modulator is so worn I struggle to get a reliable connection, so something like this would be great but I haven't been able to find anything similar.

[DistantStar001]

That looks great! If you don't mind me asking, where did you get that metal bracket to replace the modulator from? The connector on my modulator is so worn I struggle to get a reliable connection, so something like this would be great but I haven't been able to find anything similar.

I had to cut it out of a sheet of 18 gauge aluminum. Honestly, I probably should have used copper or some brass for this, as solder doesn't stick to aluminum that well.

[DistantStar001]

IT BOOTS!!!!

There still are a few issues. I tried loading a game from a .wav file, but it couldn't hear it. Not entirely unexpected, as all three of my 48k Spectrums need a volume boost to load programs. I'll have to dig out my cassette player. The computer did react to the input though, so at least I know the port works. Also, the video port is a bit loose on the bracket, so it's not well-grounded, causing the video to cut out if moved. But that can be fixed.

Other than that, it seems to be a fully functional Spectrum!