## Search found 398 matches

Sun May 21, 2023 12:56 am
Forum: ZX BASIC
Topic: I can´t explain this (math precision)
Replies: 3
Views: 177

### Re: I can´t explain this (math precision)

Which ROM is your machine using? Using the last and latest Sinclair official ZX81 ROM I get: IMG_6117.jpeg The above was done on an emulator (the excellent one for iOS) using 16K RAM and the 649 ROM. I also tried on a emulator for a ZX Spectrum 128, but without the timing parts of the program (so o...
Sat May 20, 2023 10:37 pm
Forum: ZX BASIC
Topic: I can´t explain this (math precision)
Replies: 3
Views: 177

### I can´t explain this (math precision)

I know that roughly the mathematical precision of the ZX81 starts to lose after 9 digits, one would expect rounding and exponential expressions. What I can't explain is why the same statement "LET a=-0.999999999999999999999999" gives two different results in this case. I would expect -1 as rounding,...
Mon May 15, 2023 2:15 am
Forum: ZX BASIC
Topic: What exactly do Plot and Unplot do?
Replies: 15
Views: 1205

### Re: What exactly do Plot and Unplot do?

A character can also be 4 pixels. a bit like character 129 to 143 on the ZX Spectrum. PLOT will set the large pixels in a character on the screen. Since the screen is character based you can't plot a pixel at the position of a character unless the character is the graphical character 0 to 7 or 128 ...
Mon May 15, 2023 1:40 am
Forum: ZX BASIC
Topic: FOR SETEP is faster calculating parameters than using variables
Replies: 3
Views: 202

### FOR SETEP is faster calculating parameters than using variables

:shock: I was playng arroud drawing circles an elipses and found that using FOR A=0 TO 2*PI STEP 2*PI/L Is faster than using variables replacing both the counter end and the STEP paremeter with precalculated variables. it was almost a seccond from 77s but it means several bytes less too thet wasn´t ...
Sun Mar 12, 2023 8:07 pm
Forum: Hardware
Topic: ZX +128 12v rail issues
Replies: 13
Views: 1556

### Re: ZX +128 12v rail issues

1024MAK wrote: Fri Mar 10, 2023 10:23 pm Right, not going to get back to this tonight, sorry.
Don't worry I had a busy weekend too..
Fri Mar 10, 2023 2:04 pm
Forum: Hardware
Topic: ZX +128 12v rail issues
Replies: 13
Views: 1556

### Re: ZX +128 12v rail issues

I can’t remember how power hungry the TEA2000 is. But it’s not going to be too much for a boost converter run off the +5V rail. The alternative arrangement would be to link the +9V and +12V rails again (and remove any of the existing components that would not like this) and use a 12V regulated PSU....
Fri Mar 10, 2023 5:51 am
Forum: Hardware
Topic: ZX +128 12v rail issues
Replies: 13
Views: 1556

### Re: ZX +128 12v rail issues

Well I replaced both TR4 and TR5 with BC635 and BC557 (I double checked pinouts and had to cross Tr4 leads) I've used successfully this combination on 48K issue 6A boards.. lifted the bypass wire. And lifted r62 just in case... Didn't work at all, 12v is about 0.6v... I am starting to think about a ...
Thu Mar 09, 2023 11:27 pm
Forum: Hardware
Topic: ZX +128 12v rail issues
Replies: 13
Views: 1556

### Re: ZX +128 12v rail issues

Can you compare the current drawn on the faulty unit compared to the working okay unit? both are getting 0.90A (tested both at 11v ) If the faulty unit is drawing a lot more current than normal, this would increase the 100Hz ripple. The +12V voltage being the same as the +9V input indicates a BIG p...
Thu Mar 09, 2023 1:11 pm
Forum: Hardware
Topic: ZX +128 12v rail issues
Replies: 13
Views: 1556

### Re: ZX +128 12v rail issues

If your PSU ripple remains after updating the electrolyte Capacitors then change the diodes. There might be one at the edge. Or just replace them all as I did. Not seen often but our Sinclair's are worth the effort. Ripple is not there without a charge, nor with another similar computer. And it's a...
Thu Mar 09, 2023 4:41 am
Forum: Hardware
Topic: ZX +128 12v rail issues
Replies: 13
Views: 1556

### Re: ZX +128 12v rail issues

2V ripple on the 9V nominal output of the original PSU is not good. Best to renew it’s internal electrolytic capacitors. I did it.. ripple is still there, it has more a serrated shape than a Sinusoidal one, I tested with another toastie and ripple is not present, same PSU. So I guess it comes from ...