Bodged 16K RAM upgrade in issue one

[Lardo Boffin]

Sooooo. I managed to bodge an issue one 16k RAM upgrade. I somehow forgot to do link 2 before I tested it connected to the telly... The moral of the story is to not do these things when very tired I guess!

When connected via a ZX8-CCB I get nothing. When connected via RF I just get a white screen no K.

Am I likely to have killed something switching it on with the wrong link set? It was working fine before I started.

I have tested that I have continuity in the right places and not where it shouldn't be so don't think I have a short anywhere.

I will try a bit of chip swapping...

Lardo

[Lardo Boffin]

I have just put the ULA into a working zeddy and it boots up to the K. So I guess that still works.

[Lardo Boffin]

ROM works ok in my test zeddy.

[Lardo Boffin]

So does the Z80. RAM to go and then I guess that proves I bodged something track / soldering wise I guess.

[Lardo Boffin]

The RAM is fine.

[Lardo Boffin]

Below are photos of the finished board.

The obvious candidate for the problem is my fix after managing to lift one of the pads on the right hand RAM chip (there were two 1/2 K chips) - it came off with the chip.

I pulled the track back a bit and snipped it off. I soldered a wire between the leg of the socket and the nearest end destination via and have continuity between the legs of the the RAM and the Z80. I have the same continuity on my working issue one RAM upgrade board.

Any suggestions welcome!


(There is a break between pin 6 and the track to its left - I removed a bit of green solder mask to double check)

IMG_6878.JPG

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My bodge fix.

IMG_6877.JPG

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Lardo

[Lardo Boffin]

I have just tried it with a working 16k RAM pack and still nothing.

[1024MAK]

(There is a break between pin 6 and the track to its left - I removed a bit of green solder mask to double check)

Can you confirm with a meter, because it does not look cut in your photo. Oh and it's not pin 6...

Here is the bottom side track cuts from my guide:

ZX81 issue 1 - 04 - bottom of PCB after tracks cut 560.jpg

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Mark

[1024MAK]

Other possible problems:
Is it possible that a plated-through hole linking the tracks from one side of the board to the other came out when you removed the two SRAM chips?

Is it possible that there is a solder splash under the new socket?

You can test for breaks by testing that each "node" (connection point) connects to the other "nodes" for each track (meter on continuity range) - cross reference to the schematic. If you use a copy, you can mark off each in turn.

Next test all signal lines for a short to +5V or 0V. Use the 2000 / 2k resistance range.

You can test for short circuits by expanding the table below:

Code: Select all

First  2nd    3rd
pass,  pass   pass
Test   Test   Test
A0 to  A1 to  A2 to
-----------------------
A1     A2     A3
A2     A3     A4
A3     A4     A5
A4     A5     A6
etc

until all signals going to/from or through the area of the RAM has been tested.

Mark

[sirmorris]

There's this:

Capture.PNG (734.61 KiB) Viewed 4004 times