Hello from Nashville, Tennessee USA!

[DrVenkman]

No it doesn't. But 16k RAM chips are very uncommon. Most SRAM chips with 28 Pins are at least 32K. And vLA81 can modify the ROMCS and RAMCS behaviour so that 32k can be accessed.

Per the vLA81 documentation, I need to wire pin 26 of the SRAM to pin 35 of the ULA socket, which I haven’t yet done. The current way I’ve got things wired, pin 26 is wired to one of the address line diodes. I’m unclear if I need to remove that wire, or if I can run a wire under the board from that diode over to the ULA socket. And since 35 of the ULA socket is the oscillator input, I’m unclear if I need to cut a trace or if the vLA81 or remove the stock oscillator entirely (the vLA81 contains its own).

Also, the “easy” mod above has pin 2 of the SRAM tied to the existing Vcc at the socket, so being logic-high all the time, half the address space is unusable. I need to take pin 2 out of the socket and wire it to the address bus. There are user-configurable options to set the memory mapping in the vLA81, but I lack enough knowledge or appreciation of ZX81 software yet to determine which way to configure the chip.

So, tl;dr: I’m gong to continue with the rest of my repairs (new phono jacks and replacement keyboard, case lettering paint) before worrying about going to 32K.

[Paul]

There is no need to desolder the oscillator. Pin35 of VLA81 isn't connected to the Mainboard.
Only connect vLA81 pin 35 to SRAM, disconnect this pin from the diodes. That's all. There is no need for soft configuration, just use the setup ram from 8-40 K in the beginning. This will make most programs work

[DrVenkman]

There is no need to desolder the oscillator. Pin35 of VLA81 isn't connected to the Mainboard.
Only connect vLA81 pin 35 to SRAM, disconnect this pin from the diodes. That's all. There is no need for soft configuration, just use the setup ram from 8-40 K in the beginning. This will make most programs work

Oh, excellent! Thanks for the clarification! Sounds like next weekend's project.

[Paul]

I just reread the manual. The SRAM Chip must be connected to both. A13 and vLA81 pin 35.

[DrVenkman]

I just reread the manual. The SRAM Chip must be connected to both. A13 and vLA81 pin 35.

Thanks for the further clarification.

I used the Wayback Machine capture of the blog post here for my 16K mod:
https://web.archive.org/web/20201103191 ... n.html?m=1

I did make one change: I used LK2 to tie pin 21 into A10 rather than lifting the pin and running a wire to a diode.

So I also need to lift pin 2 of the SRAM and connect it to the address bus rather than leaving it tied to Vcc as well.

[Paul]

Yes if you followed the blog you need to connect pin2 to A14

[DrVenkman]

I had a few minutes this morning so I completed the rest of the 32K mod. Lifted pin 2 from the SRAM socket and connected it to A14 at diode D7, then ran a wire under the board from A13 at diode D5 to ULA pin 35. After about a 4 second white screen, I get the expected K prompt. So far so good.

Are there any easy BASIC programs I can run to verify the available system RAM?

[BarryN]

I had a few minutes this morning so I completed the rest of the 32K mod. Lifted pin 2 from the SRAM socket and connected it to A14 at diode D7, then ran a wire under the board from A13 at diode D5 to ULA pin 35. After about a 4 second white screen, I get the expected K prompt. So far so good.

Are there any easy BASIC programs I can run to verify the available system RAM?

Code: Select all

10 CLS
15 FAST
20 DIM P(8)
30 FOR M=1 TO 8
50 LET A=8192*(M-1)
60 LET T=PEEK A
62 LET A2=A-32768
63 IF A2<0 THEN LET A2=A
64 LET T2=PEEK A2
65 LET A3=A-8192
67 IF A3<0 THEN LET A3=A
69 LET T3=PEEK A3
70 POKE A,M+1
75 IF A<>A2 THEN POKE A2,M+3
77 IF A<>A3 THEN POKE A3,M+5
80 LET M1=PEEK A
90 POKE A,M2
95 IF A<>A2 THEN POKE A2,M+4
97 IF A<>A3 THEN POKE A3,M+6
100 LET M2=PEEK A
110 POKE A,T
110 POKE A2,T2
115 POKE A3,T3
120 IF M1=M+1 AND M2=M+2 THEN LET P(M)=1
125 IF M1=M+3 AND M2=M+4 THEN LET P(M)=2
127 IF M1=M+5 AND M2=M+6 THEN LET P(M)=3
130 NEXT M
135 SLOW
140 FOR M=1 TO 8
145 IF M<=2 THEN PRINT " ";
150 PRINT (M-1)*8;"K TO ";
155 IF M=1 THEN PRINT " ";
157 PRINT M*8;"K RAM:";
160 IF P(M)=1 THEN PRINT "YES"
170 IF P(M)=0 THEN PRINT "NO"
175 IF P(M)>1 THEN PRINT "MIRROR/";
180 IF P(M)=2 THEN PRINT (M-5)*8;"K"
182 IF P(M)=3 THEN PRINT (M-2)*8;"K"
185 NEXT M
190 PRINT
200 PRINT "MEMORY AVAILABLE TO BASIC=";INT (((PEEK 16389)*256+(PEEK 16388)-16384)/1024);"K"
210 STOP
220 SAVE "CHECKMEM"
230 RUN

[DrVenkman]

I appreciate that BASIC program. Since my new keyboard is still not permanently attached to the case, I tried rather awkwardly to type it in this afternoon - twice! - but the computer crashed to a black screen before I finished. After the second crash, I powered up the computer again and started pressing chips. First the new SRAM in its new socket - no problem. Second, the vLA81 and its new socket - no problem. Then the ROM - at soon as I pressed a tiny bit I got a bunch of garbage on the screen and then a black screen. So I removed the chip, cleaned the legs and the crappy, crappy single-wipe socket with Deoxit, then reinserted the chip and powered up. This time the "chip press test" had no effect so it clearly helped.

I expect I still should replace the socket entirely, but I'm not up for that until next weekend. I'm also not up for trying to type in a lengthy program with the keyboard lying loose so I'll just refrain for now until I finish the hardware mods.

[DrVenkman]

Update time!

I’ve now replaced the damaged original phono plugs with new ones, removed the US-model channel output select switch, replaced the stock 40 year old 7805 and its old, giant heat sink with a new Traco TSR 1-2450 and done some basic power tests. All seems to be working in a 32K configuration. I need to get to work on my case top by removing the old keyboard, washing the case thoroughly, then installing the new keyboard and repainting the logo.

But in the meantime, having replaced the old Z80 with a new CMOS version, the old SRAMs with a new CMOS SRAM, and the vintage ULA with a vLA81, plus the new regulator, current draw is about 71% less at idle than it was in stock configuration.