I have an Apple IIe P.A.L, board number 870-0073A dated 1982. Its also marked B607-0664.
I am having some problems with 80 character mode in that if I type in PR#3 it goes into 80 column mode, but after about 10 minutes the machine becomes unstable and I get on screen graphics corruption, or missing columns (Every other one) and from time to time the card just become unavailable.
It seems to be thermal as powering off for a short while allows everything to work as normal again, even if only for 10 minutes.
I have 2 cards, a RAM2E II from Garrets Workshop, and a standard remake of the 80 column/64K card. The problem occurs with both cards so I'm confident it's the machine and not the cards.
I'm trying to find a decent schematic for my specific machine, but not having any luck (Mine has the CPU bottom left corner, and the AUX slot in the middle(ish) of the board in front of slot 3) so if anyone can point me a a schematic for this specific machine I'd be grateful.
BTW I do have a number of schematics and PDFs of books, but all seem to be different revisions to mine and don't show the aux connector and what's connected to the socket.
TIA
My motherboard is identical to yours. This has all the schematics that you need: click!
Here is a side-by-side of the two layouts:
SideBySide.jpeg
I think that's about the only book I haven't downloaded!!!
Thanks again for your help, much appreciated.
EDIT: Seems to have exactly what I am looking for, thanks.
This isn't a diagnosis, but components E1 and E4 would undoubtedly be capable of causing the symptoms you describe if they corrupted EN80 and/or RW80.
E1 is a 74LS125, so it might be sensible to swap in a spare, or borrow one from another device.
E4 is the MMU, so it's not an off-the-shelf part.
Maybe some capacitors went bad. It is a common problem with old computers. I may be wrong but I think the only reliable way to test these is to desolder and test. An ESR meter will tell you if it's leaking current, but testing for capacitance when in-circuit won't give you reliable readings.
Since there are often hundreds of electrolytics in a monitor or computer, using an in-circuit ESR tester saves a lot of time. The exact capacitance usually isn't important.
The author just recapped his PSU and he doesn't have any electrolytics on the motherboard, so I don't think this issue is due to bad capacitors.
Thats good to know, and I had already figured that it could be the MMU, but am secretly hoping it isn't!!
I don't have a spare 74LS125, but I have a TL866 II Pro so I can test it, and I'll probably order some just in case.
I'll let you know the outcome.
Thanks.
@Frozen signal
@Robespierre
The board has no electrolytics and I've recapped the PSU as CVT mentioned, so I doubt it's power or bad caps.
I pulled the LS125 and it tested just fine in the TL866, and so far after reinserting it, it seems to be running ok.
Seems to be...
Usually it takes a little while to fail, so I'll go back in a minute, but will also order some 74LS125s in case this one is marginal like the TCA 650 was.
Of course it could be something as simple as a dirty contact, but I'll replace it anyway and keep my fingers crossed that's the fix and not a bad MMU!!!
Incidentally, the LS125 is not at E1 on my board, it's more like C12.
You can also remove the plastic body of the IC sockers to examine and clean the contacts. More details here: click!
Also if you live in Europe, you should get a hold of this: F-02 Flux Remover.
This is pure isopropyl alcohol in a spray and it evaporates completely leaving no spots behind. I use it extensively.
F-02.jpg
Hey CVT, thanks, I'd already used IPA on the socket and used some Electrolube EML contact cleaner, but I think I'll do it again whilst I'm waiting on the replacement IC. I am a bit wary of disassembling sockets in situ, I usually end up with a failed socket, but I can always replace it if needs be.
I'll run a proper test tonight when I get home from work to see if removing and reseating the LS125 did actually work and I'll take it from there.
It's probably not the MMU. You said you have graphics corruption or missing columns. The MMU is not involved in the video generation, you don't even need to have a CPU to have video signal. The IOU is the one doing most of the job. But it's as difficult to replace as the MMU. Probably even more difficult in your case as it's the PAL version of the IOU.
It's really strange that it works fine for 10 minutes. If I were you, I'd try to find if a IC is hot and place a heatsink on it to see if it takes longer before the problems appears.
Heat-related intermittent problems can be caused by bad sockets, tarnished leads, or bad wire-bonding inside the package.
Locating these faults is why technicians use freeze spray and/or hot air as diagnostic tools.
That's good to know as I a relative noob to troubleshooting the Apple IIe.
I've checked for hot ICs and none are getting remotely warm and I've gone to the extent of waiting for the fault to occur and pulling suspect 74LS ICs and testing them when a failure occurs, but all have come back fine.
It does seem like the 74LS125 is where the problem centers around as when I pull it and reinsert immediately the problem goes away right away rather than requiring a cooldown period, so I have replacement ICs coming and before that I'll fit a replacement socket too.
I may also fit new sockets for the IOU and MMU anyway while I'm at it.
Unfortunately I don't have any freeze spray at the moment, though I could use hot air.
I'll be honest, I don't trust the single wipe sockets, so will be replacing some of them soon.
I tried the disassembly of the 74LS125 as suggested and it won't come off easily, so I think I'll likely just replace the socket rather than break stuff up.
Was definitely worth a go though.
I hope you are right with the 74LS125. But reading the problem you have vs what the role this IC has in the Apple IIe, you might have more that one problem.
This is a quad tri-state buffer and if you look at this schematics
http://www.applelogic.org/files/IIESCHEMATIC.pdf
on page 1, just under the IC labeled UB3, you can see the first two of these buffers, and a third one is on the bottom-right of that page.
They disable access to RAM during DMA and/or PHASE 1. Or more precisely, the first one disable access to RAM during DMA, the second force the motherboard's RAM to be read-only during PHASE 1 (video generation phase) and the third one force the language card RAM to be read-only if EN80' signal is high.
The last one is on the page 3, on the botton-left near the oscillator. It's C pin is connected to the CLKEN' of slot 1 (pin 19). Bringing CLKEN' HIGH will disconnect the 14Mhz clock from the motherboard. I don't think you will do that by mistake but just to be sure: DO NOT BRING THIS SIGNAL HIGH. It will damage your CPU. It is intended for cards in slot 1 that would replace the motherboard's clock with their own. The CPU will sustain damage if the clock is suspended for longer than 40 microseconds.
Also, if you are interested in videos of someone repairing old vintage computers, check out this youtube channel: https://www.youtube.com/@adriansdigitalbasement
It's really good and might give you ideas about what is your problem.
@frozen signal - much appreciate your input and will bear in mind what you've said and will be aware of the need to avoid bringing CLKEN high on the CPU, however it's unlikely I'll do this.
I am quite the fan of Adrians Digital Basement, it's my go to channel for a lot of stuff :) and I have done a lot of repairs on other machines, such as Atari 8 & 16 bit and quite a few other 8 bit machines, I'm just not familiar with the Apple IIe hence the reach out for help.
I'm going to replace the sockets for the MMU/IOU and LS125 as single wipe sockets like this have caused me issue in the past and I do have replacement LS125's on the way.
I totally agree that there could be more than one issue here though.
Initially I had problems with video and power, and fixed those with a PSU recap and replacing the TCA 650 which seems to have been marginal, even once I got the power sorted (It was being powered at 5v instead of just over 10v) Video is good now, and I was hoping that the PSU recap would fix the 80 col problem, but no, so next is to diagnose and fix one step at a time.
I'm just crossing my fingers it isn't either the MMU or IOU and perhaps just a bad socket.
I just pulled the plastic cover off the MMU socket as suggested by @CVT and forund this:
PXL_20230406_085829442.jpg
The wipe itself was barely attached and fell off as soon as I touched it:
PXL_20230406_085850591.jpg
Is it my problem? I don't know but I'll replace it as a matter of course. The IOU and LS125 sockets had no obvious problems, but they're going too.
There is no need to replace the entire socket. You can just replace the broken contact, if you have a spare socket of this type. If the other contacts feel solid to the touch, they are fine. You can even position the chip on the bare contacts and run the machine this way for an hour to make sure the issue is resolved. Same with the LS125's socket, in case you managed to remove its plastic casing. The smaller sockets are held much tighter than the larger ones.
I don't have any of the same type of socket. Well, I suppose with the IOU socket and LS125 socket, I suppose I technically do, but as it goes I've had so many troubles with older single wipe sockets, so although I understand it's repairable, I'd rather replace the whole socket with new.
As for the LS125, that socket broke, as did the IOU socket, despite gently warming to make the plastic more flexible beforehand. Luck of the draw it seems and I suspect the plastics may have become brittle with age.
Well, I've had a bit of a trial with the IIe over the last weekend.
I found a bad CPU socket, so swapped that, but after using freeze spray on the MMU to see if that was the issue (It still my be the case that it is) and also on the IOU, the machine started behaving like it didn't have a CPU, and it turned out there was no clock going to the CPU.
I gave the board and sockets around where I'd used the freeze spray a good clean using an airbrush and IPA to get into and under the sockets and the base machine is now running again thankfully and is once again stable, having been running a game for a couple of hours just fine.
The LS125 has been replaced, and I don't believe it's that, so I'm waiting on a replacement 74S02 as it's an easy swap and where the clock comes from.
So, no progress on the 80 column/RAM problem (The cold spray did suggest the MMU was the issue before everything went sideways) but at least the base machine is back up and running.
Ok, so I seem to have got to a conclusion with this at lunchtime today, and it's a faulty, or at least marginal, MMU.
If I take the steps to recreate the problem, with 3 small heatsinks added to the MMU, the problem takes longer to appear. If I then freeze spray the heatsinks, the problem clears until such time as the effects have worn off and the problem shows it's face again.
I'll obviously try and locate an MMU for a PAL machine, but am wondering if in the meantime I can add a suitable fan to keep things cool enough for it to work until I can source a replacement.
Really big heatsink in the meantime?
It's certainly under consideration!!!
If it doesn’t work out, because plastic barely conducts heat, I have a pretty effective method of adding a heatsink to DIP chips.
First let’s take a look at the inside of a typical plastic DIP package:
clear_ic.jpg
Notice that the silicone is sitting on a metal plate that is thermally isolated from the pins and there is at least 2 mm of plastic on each side. The goal is to attach a heatsink to the bottom of this plate. For this I use a drill bit for cutting holes in plastic with a 4 mm tip to drill right up to the metal plate from bellow. I do it by hand, so there is no chance of going through the plate itself. Once the tip of the bit hits the metal plate, I switch to a flat tip file to make the bottom of the hole flat:
Step 1.JPG
For a heatsink it is best to use a copper plate, but anything that can be soldered will work. I fill the hole with solder and drill a hole of the same size in the middle of the heatsink:
Step 2.JPG
Then I solder the heatsink to the bottom of the chip thought the hole:
Step 3.JPG
Now for most chips the plate on which the silicone sits is not made from a material that can be soldered, so the heatsink will come off, but it will have a knob that perfectly fits in the hole and touches the silicone plate. In this case I just put some conductive paste in the hole and it's good to go:
Step 4.JPG
If it stays soldered it’s even better. If not, I put some superglue on the sides to prevent it from rotating around the hole and touching the pins.
Step 5.JPG
Now that there is a heatsink connected to the silicone plate itself, if it’s still not enough, larger radiators can be connected to it and it will be able to transfer the heat out very effectively.
That transparent IC is pretty cool.
That's a pretty interesting method, though given that if I kill the MMU I am left with a very dead machine unless I can source a replacement MMU, so I probably won't risk it, since the base machine works if you choose not to use the expanded RAM or 80 Column mode.
It's not getting more than about 29-30C, though instacooling with freeze spray does do the trick.
Food for thought though if I get desperate.
The really dangerous part of the method is the molten solder on the silicon backplate. I never tried it on an already damaged chip - it might kill it completely. But it is something to try instead of tossing this one in the trash once you have secured a replacement.
The silicon die was usually attached using a high-temperature solder such as Pb95Sn5, so that will not be disturbed below 300°C.
You could just use conductive epoxy paste to mount the heatsink instead, without any heating.
I may try if I can find a replacement, it's be better than binning it, though I do usually keep marginal components as a last resort.