IIc with checkboard

[ironborn65]

I got this very nice Apple IIc, finally! I was looking forward for the whole set!!
The pic was taken by the owner, so it's not in my "cave" yet.
About the checkboard ... what should I expect? Anything already known? I searched for "checkboard" in this forum but got nothing

 

[olePigeon]

Could be RAM, ROM, and/or CPU.

 

[stepleton]

Try searching the web (not jut here) for "checkerboard" instead of "checkboard" --- this seems to turn up some discussion threads that might help.

 

[maceffects]

If it has "MT" Ram it probably has a bad memory chip and I would start there.

 

[ironborn65]

The baby is with me now.

I haven’t opened it yet, as I’m already working on the SE/30 project.
The CRT is functioning, but I’m wondering—should I service it by inspecting and, if necessary, replacing the capacitors?

 

[bibilit]

The CRT is small and tight , not easy to work inside. The only thing that can go wrong is the RIFA capacitor, so i will not worry until the board is not working properly.

 

[ironborn65]

Back again (wow two years!!!) with my Apple IIc. Luckily I have a working IIc so I can compare things.

What I checked so far:

• Voltage is 4.87V a little bit low, but I have the same measurement in the working one
• Board looks clean, no scratches or damage
• Swapped CPU, MMU, both ROMs and the IOU from the good machine to the bad one, still checkerboard. They all work fine in the good IIc
• Reset line goes high at boot and stays stable. I did the other way around, those chips are not faulty.
• The ROM is 4x, and as i said it work if inserted in the working one
• The apple close open control reset maneuver does not restart it
• Clock is a clean square wave at 1MHz
• Address lines A0 to A15 all toggle, each one slower than the previous, not perfectly though
• The chips RAM data out bit pins all sit at the same max voltage
• No chip is hot, only the RAM feels maybe slightly warm
• I haven't swapped the char gen ROM yet, the one on the rear side
• I tried piggybacking a good RAM chip on each of the RAM chips, one at a time. Not really hoping it would fix it, just wanted to see any change at all. Nothing changed

One strange thing: if I power off and on again quickly, I see random dots in the middle of the checkerboard. If I leave it off for 10 seconds or so, no dots. I guess some bits stay high in the RAM from the previous run?

Now I'm out of ideas. The RAM is soldered so I'd rather not replace all 16 chips blindly, but it's the only option I see.
Any insight?

Thanks

 

[ironborn65]

here

Thread 'Apple IIc checkerboard'

Friday at 7:07 PM

it better sits here, apologies I'll follow it here in case

• ironborn65
• Replies: 0
• Forum: Apple I, ][, /// & Lisa

 

[bibilit]

Dots are most of the times Ram related.

 

[ironborn65]

hi @bibilit tks.
I was considering cutting and rising the data bit one at a time to see a different behavior and find the culprit. Can it be a way?
In others threads I found the lucky guys were able to boot from the ROM and from there is path is straightforward.

 

[bibilit]

Never done that way, but why not.

You don’t say what model you have, but the common logic board has a main ram set and second set of it.

The main one being the one closer from the keyboard.

Also have you tried booting with the keyboard cable removed ?

 

[ironborn65]

I tried without the keyboard, same result, checkerboard.

About the version, I got this.
And yes .. I am testing it without serial and floppyEMU



I realized that the previous owner added the ROM4X, (W1 and W2 are correctly set), but I guess that if he/she did so, it was probably working with the stock ROM, so some damaged occurred in the "upgrade" .. but it could have failed after a grace period after the upgrade. I need to check for continuity, he/she might have damaged the board even if I did not spot anything so far.

There is a scratch close to W1, but there is continuity

 

[ironborn65]

all continuity test run ok on RAS, Data and Address lines, across RAM IOU CPU and MMU
then I removed the board to look at the back, and I got this surprise:



this was done on purpose, is this a known factory modification?

 

[bibilit]

No, this is not factory

 

[ironborn65]

this is done in the clock/quartz area and it's a precision operation, it's not done by a mistake IMHO or in a basement, there are also bridges.
The SN74LS161ANDS has been replaced by a Quartz and there is a bodge wire and another cut.
Still this is weird thing, it's factory fix, at least to me

 

[bibilit]

Don’t think is a factory modification.

 

[ironborn65]

thanks @bibilit to following me on on this.
Regardless of who made the hardware modification, it doesn’t seem to affect the timing: the 14 MHz and 7 MHz oscillators and the 1 MHz clock all look correct and match the measurements on the working IIc. The previous owner says he was unaware of the modification and that the machine was working until it suddenly stopped… assuming he’s being honest with me.
I’m now focusing on the RAM chips, as the address bus seems to show repeated attempts to read data, and the RAS/CAS waveforms match those on the working machine.
I suspected the fourth RAM chip (DRAM03) in the first 64 KB bank, so I desoldered it, and it did indeed turn out to be faulty, but it will not be the only one.
4164 or 4264 chips rated at 150/200 ns are reportedly suitable replacements for the 200 ns 4864 chips. Are there any concerns about mixing these chips?

 

[bibilit]

Hi,
Not at all, IIRC the part number difference being just a matter of brand, but you can mix them without any issue.
use a socket if you have, always better than direct soldering.