3K Rampack issues

Discussions about Sinclair ZX80 and ZX81 Hardware
User avatar
mhudson52
Posts: 141
Joined: Tue Sep 27, 2016 2:17 am
Location: Decatur, Alabama (US)

3K Rampack issues

Post by mhudson52 »

I am in the process of building more of my reproduction Ram packs for the ZX80 and recently received 3 more PCB's from OSH Park of the same design as the previous order. I am populating the board with the same components as before (18-pin dual-wipe sockets, 5 VOLT 1KX4 450 NS DIP-18 STATIC RAM, 74LS138 1 of 8 Decoder/Demultiplexer IC in a 16-pin socket, and the two capacitors.

The first board I built worked perfectly, just like the previous batch. The two subsequent boards, however, refuse to work. No magical "K" prompt appears. I have swapped chips from board to board without success. I next printed out the PCB layout files and meticulously did a point-by-point check of all connections, finding no shorts or dead spots.

Right now, I am dazed and confused as to what to try next. I do not have either the equipment or knowledge to do further testing.

Suggestions will be most welcome.

Regards,
Mike
TS 1000 bought in 1982 and brought back to life by Sir Ian
User avatar
siggi
Posts: 988
Joined: Thu May 08, 2008 9:30 am
Location: Wetterau, Germany
Contact:

Re: 3K Rampack issues

Post by siggi »

Hi Mike
mhudson52 wrote: Thu Nov 22, 2018 9:09 pm I next printed out the PCB layout files and meticulously did a point-by-point check of all connections, finding no shorts or dead spots.
Did you check/measure on the PCB or on the contact inside the socket (maybe soldering is the problem)?
Soldering all components again often solves such problems.

Siggi

PS: could you show some pics of the populated board (both sides)?
My ZX81 web-server: online since 2007, running since dec. 2020 using ZeddyNet hardware
http://zx81.ddns.net/ZxTeaM
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: 3K Rampack issues

Post by 1024MAK »

@Mike - try your boards with NO RAM chips installed, just the 74LS138 decoder fitted.

The machine (ZX80 or ZX81) should start up using it’s internal RAM controlled by the 74LS138 decoder on your boards.

I’m assuming that your boards are similar to the boards in this thread and this thread.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
User avatar
mhudson52
Posts: 141
Joined: Tue Sep 27, 2016 2:17 am
Location: Decatur, Alabama (US)

Re: 3K Rampack issues

Post by mhudson52 »

Mark:

I tried your suggestion using only the decoder installed and everything looked rosy (got a nice "K" prompt. The board I am using is based on the Markus Brenner design. The same board as discussed in the first link but corrected by Charlie.

Siggi:

All the connectivity checks I did were with the chip sockets soldered to the board. Also checked to make sure there were no shorts to adjacent connections and found none.

Photos attached.

Thanks, guys!

Mike
Attachments
100_5581.JPG
100_5580.JPG
100_5576.JPG
TS 1000 bought in 1982 and brought back to life by Sir Ian
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: 3K Rampack issues

Post by 1024MAK »

So the next easy step, is to fit just two known good SRAM chips, keeping in mind that as per the discussion in one of those threads, that one has to be suitablely fast. Then try it again.

Which Sinclair machines do you have? And if you have one or more ZX81s, which type of RAM chips do they have, if they use 2114 type, are these socketed?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
User avatar
mhudson52
Posts: 141
Joined: Tue Sep 27, 2016 2:17 am
Location: Decatur, Alabama (US)

Re: 3K Rampack issues

Post by mhudson52 »

The only machine I have to test on is my TS1000, which has shown to be completely compatible with all my previous Ram pack builds.

Due to availability issues at the time I ordered the last batch of RAM chips, what I am using now are 450ns chips which have shown to work in past builds. The 200ns have become available once more and I have another order of them on the way. Should I just wait on them before any further testing?

Which sockets should I use if fitting only two chips?

Mike
TS 1000 bought in 1982 and brought back to life by Sir Ian
User avatar
siggi
Posts: 988
Joined: Thu May 08, 2008 9:30 am
Location: Wetterau, Germany
Contact:

Re: 3K Rampack issues

Post by siggi »

Maybe it does not solve the current problem, but usually each chip should have its own capacitor (around 10-100nF), connected across its power pins to stabilize its power supply. If those C are missing, the board may run (because the logic is OK), but not very stable (because the power supply of a chip breaks down for a short period, when the chip draws a lot of current during an internal state change) ...

Siggi
My ZX81 web-server: online since 2007, running since dec. 2020 using ZeddyNet hardware
http://zx81.ddns.net/ZxTeaM
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: 3K Rampack issues

Post by 1024MAK »

mhudson52 wrote: Sat Nov 24, 2018 2:21 pm Due to availability issues at the time I ordered the last batch of RAM chips, what I am using now are 450ns chips which have shown to work in past builds. The 200ns have become available once more and I have another order of them on the way. Should I just wait on them before any further testing?

Which sockets should I use if fitting only two chips?
@Mike

See the extract of the Sinclair instructions shown below:
Sinclair 1K-3K RAM Pack instructions #3
Sinclair 1K-3K RAM Pack instructions #3
Note also the two different speed rated memory chips, 2114L in the 'A' positions and the 2114L-1 in the 'B' positions.

Here is a photo I pulled from the internet archive of a Sinclair 3K RAM Pack:
Sinclair 1K-3K RAM Pack photo of the board
Sinclair 1K-3K RAM Pack photo of the board
zx80memboard.jpg (22.64 KiB) Viewed 4133 times
2114 SRAM timing / speed information

2114, 2114L ............. 450ns max. access time (tA or tACC)
2114AP-12 ............... 120ns max. access time (tA or tACC)
2114AP-15 ............... 150ns max. access time (tA or tACC)
2114-1, 2114L-1 ........ 150ns max. access time (tA or tACC)
2114-2, 2114L-2 ........ 200ns max. access time (tA or tACC)
2114-20, 21L14-20 .... 200ns max. access time (tA or tACC)
2114-25, 21L14-25 .... 250ns max. access time (tA or tACC)
2114-3, 2114L-3 ........ 300ns max. access time (tA or tACC)
2114-30, 21L14-30 .... 300ns max. access time (tA or tACC)
2114-4, 2114L-4 ........ 450ns max. access time (tA or tACC)
2114-45, 21L14-45 .... 450ns max. access time (tA or tACC)

The type marked "2114L" or "21L14" are lower power versions (typically 70mA, 370mW rather than 100mA, 525mW). The "2114AP" are also a low power type.

I hope this helps ;-)

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: 3K Rampack issues

Post by 1024MAK »

siggi wrote: Mon Nov 26, 2018 10:20 am Maybe it does not solve the current problem, but usually each chip should have its own capacitor (around 10-100nF), connected across its power pins to stabilize its power supply. If those C are missing, the board may run (because the logic is OK), but not very stable (because the power supply of a chip breaks down for a short period, when the chip draws a lot of current during an internal state change) ...
@Siggi, yes, one decoupling capacitor per IC is good practice. This RAM board is however a recreation of an existing Sinclair design. So it "should" work if suitable 2114 SRAM chips are used.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
User avatar
siggi
Posts: 988
Joined: Thu May 08, 2008 9:30 am
Location: Wetterau, Germany
Contact:

Re: 3K Rampack issues

Post by siggi »

1024MAK wrote: Mon Nov 26, 2018 5:00 pm
siggi wrote: Mon Nov 26, 2018 10:20 am Maybe it does not solve the current problem, but usually each chip should have its own capacitor (around 10-100nF), connected across its power pins to stabilize its power supply. If those C are missing, the board may run (because the logic is OK), but not very stable (because the power supply of a chip breaks down for a short period, when the chip draws a lot of current during an internal state change) ...
@Siggi, yes, one decoupling capacitor per IC is good practice. This RAM board is however a recreation of an existing Sinclair design. So it "should" work if suitable 2114 SRAM chips are used.

Mark
The first new board worked, so number 2 and 3 also "should" work ;)
The issue 1 board above has a big ground plane next to the connector and maybe below the ram chips. Does the issue 3 board also have such a ground plane?

Siggi
My ZX81 web-server: online since 2007, running since dec. 2020 using ZeddyNet hardware
http://zx81.ddns.net/ZxTeaM
Post Reply