What's wrong with my 1984 Macintosh?

[giolicious]

it's been while since the last time i used this old Mac. A few days ago i tried to power it on to take some pictures and surprised to see my screen like this and startup sound distorted:

Do you think its a problem with the logic board or memory upgrade board or the crt monitor(analog board)?

more pictures can seen at http://www.flickr.com/photos/91379880@N00/sets/72157623523929099/with/6242620937/

i posted about this mac here before at viewtopic.php?f=7&t=13137&hilit=giolicious

It's actually an original Macintosh with RAM upgrade.

Here's the Mac board without the RAM upgrade:

and here's the memory board:

One more thing, do you think i can get the two 74LS244 chips from the memory upgrade board and placed it on the Mac logic board?

Anyone knows where i can buy cheap 128k or 512k macintosh board to replace those missing chips?

will highly appreciate any replies and suggestions.

 

[H3NRY]

The video problems are probably on the analog board. Check out the electrolytic capacitor at the top of the board - I forget what it's labeled, but it's the one which always overheats and fails. A search here will probably turn up repair advice. There may be other analog problems if sound is distorted. It's possible you have digital woes, but analog is much more likely for these symptoms.

It's possible to run the digital board without the RAM upgrade. Buy a couple of 74LS244s. They are still available through most electronic parts suppliers, and new ones are much preferable to ones desoldered from circuits. You will also need a pair of 10-pin, 5-resistor, 47-Ohm SIP resistor packs to plug into the two sockets at the head of the RAM array. Thus outfitted, your Mac will appear to have 128K.

Your Mac was originally 128K, and has had the 64K chips replaced with high quality ceramic 256K chips. The upgrade RAM board appears to be well designed, too. It's a copy of a Beck-Tech upgrade. Good luck!

 

[giolicious]

i tried it on a different analog board but i still get the same symptom with or without the upgrade RAM board. Probably a problem with the logic board?

 

[Mac128]

The board will not function correctly without the RAM upgrade as it is missing 4 key parts which are instead placed on the upgrade board, per H3nry's observation. Since it exhibits the exact same symptoms in two different Macs, but also with or without the RAM upgrade, my guess is that this is something to do with the upgrade. I say this because the board without the upgrade exhibits the same symptoms as with, which suggests that this is default behavior without the 4 components missing from the board. Since it exhibits the same problem with the upgrade board, it appears the upgrade board is not functioning properly, if at all. Check to make sure the connections are all being made properly, and are tight and secure. Possibly clean all the contact pins looking for corrosion.

Of course it is possible there is another problem, separately or in addition to this one, causing the issue you are seeing, but the fact it persists both with and without the upgrade board suggests the problem is on the upgrade board ... To me anyway ... Definitely worth a thorough check.

 

[giolicious]

i forgot to mention that i have done what Mac128 and H3NRY have suggested before on my first post here. I removed the RAM upgrade board and bought & placed the 4 missing parts and the Mac started just fine but with the same problem as my first post in this thread.

i will try to post some pictures later.

 

[WhoaMan]

Alrighty, lets pretend we are showing a res of 512x384... and at 1bit per pixel, this leaves us with a video buffer size of 24kilobytes...

lets divide that buffer by 3 (since you have three sections showing on your screen) , this will give us an 8kilobyte chunk...

assuming we were running with 128kilobytes of ram... our primary video buffer would start at address 0x1A700

0x1A700 - 0x1C6FF 1st third of video buffer

0x1C700 - 0x1E6FF 2nd third of video buffer

0x1E700 - 0x20700 3rd third of video buffer + primary audio buffer

since the 2nd third and the 3rd thirds are repeating and you have distorted sound... it looks like either VA13 or AX13 is stuck low causing the 1st third of the screen to show part of the stack.... the second third of the screen to show the correct area, and the 3rd area of the screen to repeat the 2nd third's buffer... and to generate audio from the end of the 2nd thirds buffer data.

Check the following:

VA13 stuck low on:

pin 11 of U1G, 74393

BMU0

AX13 stuck low on:

pin 4 of U3G, 74253

BMU0, pin 14

pin 10 of U2F 74257

 

[giolicious]

I'm sorry but I'm not sure how to locate these ones.

Check the following:

VA13 stuck low on:

pin 11 of U1G, 74393

BMU0

AX13 stuck low on:

pin 4 of U3G, 74253

BMU0, pin 14

pin 10 of U2F 74257

 

[WhoaMan]

U1G, U3G, U2F, and BMU0 should be the silkscreen markings on the logic board.

VA13 and VX13 are the signal names, i am guessing that one of the two will be stuck low ( 0 volts ). You might want to check the pin numbers mentioned on those parts for any loose or bad solder joints or shorts (an open is more likely in this situation).

 

[giolicious]

still no luck.

Are those suspected parts easily replaceable? I mean are they available on local electronics store?