Sinclair ZX80 / ZX81 / Z88 Forums

Have you carefully inspected the board for PCB track damage or for a solder splash (which can be as fine as a human hair)? By this, I mean using a good light and a magnifying glass.

If you don’t find any problems with the visual inspection, next up is to electrically test using your multimeter. This is with the power off, using the continuity range to test that each leg/lead/terminal of any component that you have worked on, is correctly connected to every other component for that PCB track network.

Also using the 20kΩ resistance range, test for short circuits between adjacent pins, or where PCB tracks run close to others where you have been working.

Mark

Hi Mark,

I've been over my soldering both by eye and with a meter also testing the sockets are good. As my eyesight isn't the best even with my glasses and a magnifying glass I will redo this and try and test the connections between components from the 40058 as well to ensure my soldering is ok and I didn't break a track during the desoldering process.

Progress. It seems I did indeed lift or break a track or two. Even with the magnifying glass I couldn't make out the break but it's there somewhere.
So I've patched what I have found so far either by testing the connections from the chips I removed and now it seems to want to boot into 48k basic or crash.
Diag cartridge doesn't want to work with it yet though.

More investigation tomorrow hopefully.

So I thought I had it there for a while. Found a faulty IC socket for one of the Ram chips.
Desoldered and managed to break another track Either its due to the age of the PCB or it's my eyes and hands.
Soldered in another IC along with the other 7 I had to remove to track it down (Murphy's Law, eight sockets and it's the one on the end you didn't start at). Bridged the broken track. Tested continuity of the pins (All good) and... White screen with a few lines and squares.
I'm starting to think this board just does not want to be fixed.
Oh well it'll have to wait a couple of days now until I have another chance to look at it.

Keep going, you are making progress, even if it doesn’t always appear so.

Some of the 16K/48K/+ boards are a bit delicate. I’ve worked on a 128K toastrack without too much difficulty, but I was being extra careful.

Although I have two +2 greys, I’ve not done any desoldering on either of them.

Mark

Progress after almost putting it aside for a rainy day.

After checking everything again I decided to try replacing TR7 as it's connected to A0 and I now get the correct boot screen. I cannot however get my diag cart to work so will now bell out the edge connector.

Yet to also test keyboard input and tape.

So one more bridge for the keyboard and one for the edge connector and the machine appears to be up and running again.

Bad news is that the tape loading still fails... Good news is that it no longer seems to be doing the same with the Divide+

Changes made so far Z80 replaced, ULA replaced with ULA128, 40058 replaced with ZXMUX, TR7 replaced with a BC547 (was a 2n3904 fitted the wrong way around). And just because this is/was going to be my last Spectrum I also replaced the AY with a AY-3-8192A TS and a Unrainer mod.

Update

I tried replacing the transistor Q301 on the tape PCB which was a C1815 with a 2N2222a (its what I had that was equivalent) but that didn't change anything and loading still fails more often than not.

I then build an amplifier circuit as described here (https://www.youtube.com/watch?v=qUIv-A_DOc0) and added an external ear socket. It loads every time from a TZXDuino but the internal tape is still the same. I'm guessing the issue is either with the tape head or something on the PCB.

I have also had a go at trying to get the AY louder and close to even with the beeper and the AY is louder but the beeper is non existent. Probably should bell it out as I may have broken yet another track, although I think it'll be the resister values need changing.