Replacing Classic II ROMs

[MacGyver]

Hey folks,

I just received a Classic II with the common "thick vertical bar" problem. So far, so good, same procedure as every year. Wash the logic board thoroughly...

Whoops? No Chime, still vertical bars? Okay, let's replace the caps with tantalum ones. What? Still no chime? Grrr!

Then I swapped the ROM with the one of a working Performa 200... ka-ching, it chimes and boots fine?!?

I tested the suspect ROM in the donator Performa 200... No chime, vertical bars.

The frickin' ROMs are dead? I've never seen that one before.

Now, how do I obtain new ROMs? Of course they don't need to be original Apple ROMs, I would be very happy to find pin-compatible EPROM or Flash types and flash them.

Does anybody have an idea what kind of ROMs (pinout?) Apple used and maybe what types could be used to replace them?

My poor Classic II thanks in advance...

Macgyver

 

[gobabushka]

have you looked real good at the roms? make sure that the contacts are clean and all tht

 

[PowerPup]

If your interested I could sell you my Classic II motherboard, it had been working up until the capacitors finally gave out, (if you look at it long enough it kinda resembles bomberman!) Then I lifted a couple solder pads in attempting to fix it. >.< It had been working just fine before all that.

Hopefully I can find a new motherboard for it this Summer, would like to try getting back to programming with it.

 

[bibilit]

Looking at this post i just discovered that apparently two logic board were issued for the Classic II.

One with 2 big chips for Rom and another with 4 small ones:

Mine has the 2 chips alternative.

 

[jruschme]

The frickin' ROMs are dead? I've never seen that one before.

I've only ever seen it once before... on a Plus which had been left on with the vents covered for a long time. It was amazing that the one of the ROMs was the only thing that had failed.

 

[APE]

If the ROM IC itself is MASK ROM it is unlikely to have had a problem but very possible. Thermal expansion/contraction can break joints, corrosion could eat something without it being visible, etc but again unlikely. Though you seem to have beaten the odds here.

If the Woz was involved I'd imagine it conforms to JEDEC spec and you can probably drop an EPROM in place of proper size. Been googling for a few minutes and can't seem to find anything beyond how large the ROMs are in the first place. If anyone needs one written out I can handle it but it sounds like you've already got the hardware for it.

 

[MacGyver]

Hi there,

I checked the ROM pins and the sockets, the contacts are fine. It is really one of the ROMs (can't remember which one right now) which is dead. I even tried to warm it up with a hairdryer, no change at all.

Is there any service document which shows the ROM pinouts? They seem to be mask programmed ROMs, no erasing window, no "standard" type. I tried googling the numbers, didn't find any information on these ICs. Ah, by the way, it's the logic board version with only two ROMs.

I wouldn't mind using a different type of ROM/EPROM/Flash memory with an adaptor socket in between, but I'd need the pinout to build one. Programming the new ROMs is no problem.

PowerPup: Thank you very much for your kind offer, but the problem is: I'm located in Germany, so shipping would be quite expensive, I guess. By the way, the screen looks exactly the same as my Classic II with dead ROMs. Did you check them? ;-)

Thanks a lot

macgyver

 

[PowerPup]

Guess I'd have to remove them and take a look.

 

[Dennis Nedry]

Is it possible that the chips are reversed? Try swapping them, it can't really hurt anything.

The Performa 200 and Classic II should have identical ROM. You could swap one chip at a time to narrow down which one is bad.

Also:

is that a picture of this very logic board that you posted? Because it looks like the left ROM chip is installed wrong in the socket. I see an extra set of holes on the top. That doesn't jive with the marking on the logic board. That could have damaged the chip, I'm not sure.

 

[MacGyver]

Hi,

no, the photos aren't from my logic board, and I didn't post them. I believe they've been taken from another thread where there was a question about how to insert the roms correctly, hence the single misplaced ROM.

I double-checked the position of the ROMs in the sockets and double-checked the bad ROMs in a known-good Performa 200. No luck, the ROMs are toast. I didn't try to find out which one exactly is dead, as I figured I'd like to replace them both to get the same timing etc., to avoid other problems.

Is there any place on the MoBo I could continuity-check the ROM sockets against to get a pinout of the ROMS?

Where are the hardcore, low-level hardware guys in this forum? I need you badly ;-) .

Thanx a lot,

MacGyver

 

[techknight]

yes, trace them up to the CPU. the pinout of the CPU is known. This way you can discover the address and data pin locations.

 

[dougg3]

I'll throw what I know into this discussion If you're thinking about replacing the ROMs with new chips, this might be helpful:

Since the 68030 has a 32-bit data bus, I'd suspect that the two-chip variant of the motherboard is using a couple of 16-bit chips and interleaving them. It appears that the Classic II has a 512 KB ROM, so each chip is going to be 256 KB, or 2 megabits. So a valid replacement is going to be a 40-pin, 128K x 16-bit chip. I would assume that they used chips with a standard pinout, but we can't really know for sure unless you trace them like techknight said. At the very least, it would be worth it to figure out which pins are connected to VCC and ground on the motherboard -- if they match up with the standard pinout, you're probably good to go. When I did my IIci hacking with the DIP ROMs, I discovered that the four chips on the IIci motherboard use a standard JEDEC pinout. I'd be surprised if the Classic II chips don't.

The JEDEC pinout for 40-pin, 128K x 16 EPROMs is:

1     = VPP (programming voltage, probably not connected on a mask ROM)
2     = chip enable
3-10  = D15-D8
11    = ground
12-19 = D7-D0
20    = output enable
21-29 = A0-A8
30    = ground
31-38 = A9-A16
39    = program strobe (probably not connected on a mask ROM)
40    = VCC

Assuming it is a standard pinout chip, I'm thinking the chip you will be looking for is a 27C2048, such as the AT27C2048. I'm having a hard time finding any place that still stocks the 40-pin DIP version of that chip.

You might be able to get away with using a pair of 256K x 16 EPROMs instead. They are twice as big, but you could ignore the upper half of each chip. The only pinout difference is that pin 39 is now A17 instead of program strobe, so you could tie that pin to ground to force it to always address the lower half of the chip when reading. Digi-Key stocks the AT27C4096 (although it ain't cheap! Almost $9 per chip) Also, beware, they are one time programmable...you get one try and then you have to buy more blank chips if you did something wrong. Edit: Mouser has it for a better price.

Programming the replacement chips might be tricky. My Willem programmer only works with 8-bit chips. I'm sure there are 16-bit programmers out there somewhere...

If you do this, you should also make sure that the EPROM's VPP pin is tied to ground or VCC -- in the case of the AT27C4096, either one is ok. That just makes sure the chip is in a defined state. It's possible that they already wired it up that way on the motherboard--if they didn't, you will have to do it manually.

I can help you figure out the ROM interleaving if you do this...for every 4 bytes in the ROM, 2 bytes will go into one chip, and the next 2 bytes will go into the other chip.

All in all, as long as it's a standard JEDEC pinout, I'd say it's definitely doable if you can find a programmer device that can burn 16-bit EPROMs!

 

[uniserver]

you could have the guy that offered you his board, pop his romms out press them into some foam,

send it in a envelope, i'm sure postage would be cheap for a smaller padded envelope.

i personally do not know how a ROM can go bad, Maybe all this solar activity is cookin em?

i know the roms that have a window, if you remove the sticker, sometimes they can get erased by nature

 

[uniserver]

did you ever find resolution with this?

 

[macgyver84]

Sorry for a 'slight' delay in my answer. The mask ROMs seem to be pincompatible with 27C400 EPROMs used in Amigas. The TL866 programmer needs an adapter to deal with those, that's why I put the project aside and forgot about it. I will try it out as soon as I get the adapter from china - might take some time