exxos's Atari, Amiga & retro forum

What type 74 serries chips are you going to use?

I know that the Lotherek version uses HC varients, which may be the issue when the rest of the system is LS or S logic. I know they're supposed to be signal compatible, for certain values of compatible. The HC varients are also slightly slower than LS in most cases.

stephen_usher wrote: 19 Jun 2020 16:54 What type 74 serries chips are you going to use?

I know that the Lotherek version uses HC varients, which may be the issue when the rest of the system is LS or S logic. I know they're supposed to be signal compatible, for certain values of compatible. The HC varients are also slightly slower than LS in most cases.

I'll be honest, I haven't started looking yet, so any suggestions are welcome.

As some of you may know, I recently became the proud owner of a lovely MSTE. Apart from some slight case discolouration, it's in truly great condition and came along with a factory fit 1.44MB FDD too. I've also added an FPU, though I've yet to find anything that uses it!

One minor problem reared it's head in the week though, which I discovered trying to play Wolfenstein3d of all things. The fire button is the CTRL key, and I couldn't fire. Hmmm, so I popped in the diag cart, and sure enough there was no CTRL key response.

Stripping down the MSTE keyboard is super simple, 3 case screws and 1 earth screw inside, then lift the kbd out.

There are 2 ribbons, and you have to lift the locking blocks on the PCB connectors to release these.

Then you can lift the mechanism out, and gently pull back a ton of plastic clips that hold the metal base plate to the kbd. The mechanism is a sandwich with the keys on top, a metal base plate, and a mylar film flexible keyboard matrix in the middle.

The keys have a carbon rubber 'bridge' that contacts carbon pads on the mylar film to make contact and register a key.

Looking at the mylar film, I tracked the traces from the CTRL key and found the following where there seems to have been a scuff on the top of the mylar. How it got scuffed is anybody's guess!!


So there's the problem, and it kind of looks a little burned.

After first covering the other track with kapton tape, I had to clean up the green plastic film so I could see the ends of the tracks, but scraping was too harsh, so I used this instead:


And was able to gently abrade the plastic until the trace was visible either side. I then used a carbon paint pen like this:


To join the 2 ends of the track.

Unfortunately the connection still wasn't working, but by using my microscope again, I found a kinked spot that also looked a little burned about 10cm further down the track, and sure enough that was a break too.

I followed the same process to gently abrade the plastic film away and reveal the traces,then used the pen again to join the broken ends.

To my delight, there was continuity again! I waited for the paint to dry and then covered the repairs with more kapton tape to protect it, reassembled and voila, ctrl key working again!

The microscope proved invaluable for this, and I'm very happy I have a means to repair small failures in these membranes, and thankfully these repairs were in areas that don't flex and are well supported, I'm not sure it would be suitable for flexible areas though.

I'll get one of those carbon pens today! I have some traces to fix up like this on an STE board. Nice job, sir!

Thanks, @rubber_jonnie

tony.reynolds wrote: 21 Jun 2020 17:11 I'll get one of those carbon pens today! I have some traces to fix up like this on an STE board. Nice job, sir!

Thanks, @rubber_jonnie

I got mine from Amazon, under a tenner and worth every penny! Hope you can fix your traces as well, post progress please :)

rubber_jonnie wrote: 21 Jun 2020 17:29 I got mine from Amazon, under a tenner and worth every penny! Hope you can fix your traces as well, post progress please :)

Mine showed up yesterday. I sat with my STE board for a while last night. The board is surely ruined. If it were one or two traces, maybe. I'll need a great close-up photo of a similar board to put all the traces back (both sides, near the TOS ROM area, as I ruined it as I tried to desolder the ROMs.) I've my fingers crossed for other methods. Or it's me, the headpiece magnifier, and a lot of time.

tony.reynolds wrote: 26 Jun 2020 02:53

rubber_jonnie wrote: 21 Jun 2020 17:29 I got mine from Amazon, under a tenner and worth every penny! Hope you can fix your traces as well, post progress please :)

Mine showed up yesterday. I sat with my STE board for a while last night. The board is surely ruined. If it were one or two traces, maybe. I'll need a great close-up photo of a similar board to put all the traces back (both sides, near the TOS ROM area, as I ruined it as I tried to desolder the ROMs.) I've my fingers crossed for other methods. Or it's me, the headpiece magnifier, and a lot of time.

I would say, I don't know how well this would work on a physical PCB, and I think it is only intended for very small repairs. I used it very specifically on a mylar film flexible keyboard matrix, if you're trying to fix a PCB, it's unlikely to work because you can't solder to it. From what you've described you would need to solder after repair, so this paint won't work, though I suspect if you had track that was just cut through and all you needed was a bridge, it would probably work just fine.

I'd suggest that what it needs is cleaning up properly and reviewing/comparing to a good board and then where tracks and vias are damaged, they need to be replaced by soldered on bodge wires. Be careful you don't add insult to injury by further damaging things.

You may want to try speaking to @Steve as I know he's fixed some STEs for people of late, so he may be able to help.

Well that was a bit of a to do!

About 16:50 ish, my wife starts shouting me and talking about fires, thinking she’s talking about next door burning garden rubbish, I shout down asking what’s up, and notice a burning smell hanging in the air.

At which point she says the dishwasher looks like it’s on fire, but she’s turned it off.

Turns out that she went into the kitchen and smelled burning, opened the DW up and sees the heating element glowing red and out of control and the DW emitting acrid smoke. Shortly after, despite the DW being turned off, the power tripped.

The smoke was acrid, and has given us both headaches an sore throats, but thankfully feeling better now. Missed karate though as felt pretty ill.

DW is now disconnected from the mains just in case, but as is bloody typical, it was plugged in behind the washing machine!! if it starts glowing again, then it’s the devils work!!

So now we need to buy a new DW, so not impressed since we had hoped it would last until we sell and move out.

Stupid white goods.

Dawns washing machine is stupid.. Bad design where the drum actually repeatedly pulls on the wiring loom disconnecting wires! I rerouted that no longer after we purchased it. I had to dismantle it a few weeks ago as it stopped working pretty much totally.. Dawn managed to get a safety pin somehow out of the drum into the pump motor which was actually jamming it behind the impeller ?! I think the laws of physics need to be rewritten as to how that even managed to get into that position in the first place it is just physically impossible!

Yeah, it seems to me that if a little thought was included in the design process, things like that shouldn't happen.

And as for the element in our dishwasher glowing red, it shouldn't happen full stop, there should be additional cutouts or thermal safeties before the element can get to the state of 'burning pits of hell'.

Surely even if the current being drawn exceeds a certain amount then it should cut out, or a separate last resort thermal switch. Just bad design, safety should be top of the list.