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Macintosh Classic Ram problem


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Hello Folks

I recently acquire a Macintosh Classic for parts not working , logic board was pretty clean without battery explosion, as usual I washed Logic board and recapped with new radials capacitors . at the first start up no chime and distorted checkerboard pattern , I then replaced VIA and Classic start up with chime and apparently works but required at start up more ram even if expansion board was installed ( System 7.0.1)

At this point I started up with system 6.0.7 floppy and discovered that only 512k ram were "detected" with or without ram expansion board installed  ( at least the onboard 1 mb should be detected ) According with Ferrix I replaced the two multiplex MUX , no success .

Floppy drive works , SCSI ok , adb ok , start up with Rom ok .

Any suggestion ? thank you!

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Hello Macalle,

you have to check the connections from the pins of UH6 all the way to the CAS PAL  UL1

and also to VLSI UI4

The must be a broken connection there

In my case the connection was broken from UH6 to the via, but in your case may be somewhere else on the line.

 

 

Edited by marcelv
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Hello Marcel 

thank for your suggestions 

i have also tried replacing UH6 ( no success)

i have checked all connection as following:

pin UL1  to   pin UH6 

9    to   2   ok

3    to   5   ok

4    to   7   ok

5    to  10  ok

 

UH6 to U14 BBU

pin 13 14 1  R69 to pin 17 U14 BBU  ok also to SCSI pin 10

pin 3  UH6 to UF3 pin 15 ok

 

UH6 pin 9 to UI6 pin 5 ok

 

UL1 CAS PAL to ram expansion board ok 

UI1 pin 19 to UI14 BBU pin 81 ok

UE3 pin 1  to UI14 BBU pin 82 ok

UE3 pin 15 to UI14 BBU pin 81 ok

 

I only found an error in the schematic concerning UL1 to UI4  but I have another faulty MOBO and are correct as I write here following 

UL1 pin 6 to UI4 pin 15 ( schematic say 18) 

UL1 pin 7  to UI4 pin 18 ( schematic say 15)

 

Will update when I'll found issue ( I hope !)

 

Alex 

 

 

    

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does pin 4 connect to to pin 26 UI4 ?

 

could you complete the following list so all pins are checked (don't have a schematic myself)

UH6 
pin 1,13,14 to pin 17 UI4 BBU
pin 2 to pin 9 UL1
pin 3 to pin 15 UF3
pin 4 to pin 26 UI4
pin 5 to pin 3 UL1
pin 6 to
pin 7 to pin 4 UL1
pin 9 to pin 5 UI6
pin 10 to pin 5 UL1
pin 11 to 
pin 12 to
pin 15 to


these seem to be correct (schema is incorrect)
UL1 pin 6 to UI4 pin 15
UL1 pin 7 to UI4 pin 18
 

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  • 7 months later...
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@macalle69  Did you ever have any luck sorting this out?  I have a Classic that is having the exact same issue.  I checked the via noted by @marcelv (from UH6 pin 4) and couldn't find any broken traces (without a schematic) from UH6 to elsewhere.

 

I removed UH6 and tested the vias underneath and didn't notice any issues.  I also pulled out Q1 thinking some capacitor goo may have impacted that chip (not sure if it has any tie-ins to the memory system).

 

 

Edited by superjer2000
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Sorry, I'm going back and trying to find continuity between the pins noted earlier in the thread. For the numbering, is pin 1 at, for example, the chip notch, or at the little white dot on the logic board?  For example on UL1 (a square chip) there is a notch on one corner and there is a white dot on the logic board by one of the pins in the middle of one of the sides. Do I go counterclockwise to count the pins?

 

Sorry for what seems like a very elementary question but I'm not getting the readings I would expect trying to countpins either way. 

Edited by superjer2000
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Ok - the white dot on the logic board is pin 1. I had the same good checks as macalle69 except for UI1 pin 19 to U14 pin 81, but that test also failed on a known good board I have. 

 

I am soaking the the board in IPA again to see if there is some residual capacitor goo, but I washed it pretty well the first time so I don't think that's the case. 

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Well, I ponied up the US$17.00 for the Classic logic board schematics from Uniserver - I checked every single connection I possibly could on the sheets from the processor to the RAM chips and socket and I had connectivity on every single line.  I am guessing I have a problem with either UH6 or UI6 as those interface with RAM and would have been potentially subject to capacitor goo.  At this point, I think I'll order a few of these readily available chips and replace them one at a time and see what happens.

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1 hour ago, superjer2000 said:

At this point, I think I'll order a few of these readily available chips and replace them one at a time and see what happens.

 

Where do you order that type of chip from?

 

Also, I should probably contact Uniserver - I could use a few schematics. Good to know that is still an option. I remember reading a thread about the service that was selling schematics and wasn't sure if it was still around.

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10 hours ago, tanaquil said:

 

Where do you order that type of chip from?

 

Also, I should probably contact Uniserver - I could use a few schematics. Good to know that is still an option. I remember reading a thread about the service that was selling schematics and wasn't sure if it was still around.

From Mouser.  Digikey would have the same.

 

These are the chips I have put in my cart that aren't proprietary and interface with the memory subsystem.  I figure I will try one at a time to pinpoint the failed one.  If that doesn't work, I'll just satisfy myself running Airborne on my 512k Classic, or I'll put it on eBay as a "512k Mac Classic Prototype??" and make big bucks.

 

 

Mouser.JPG.5c9ac0dfa6e078856ff6cb6724903c61.JPG

 

 

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  • 1 month later...
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@macalle69 @marcelv

 

I was able to fix my Classic that was only recognizing 512k of RAM.  I had previously bought the Classic schematics from maccaps and checked every memory related trace and couldn't find any problems.  I ordered the chips above from Mouser but figured I'd start with UH6 (74LS174) as that was the chip that was related to the exact same issue Marcelv had.  

 

Replaced that single chip, booted up and all 2MB of installed memory was recognized.  Thanks for pointing out your original fix thread Marcelv.  Now I have extras chips of various types but they might come in handy for something at some point.

 

It was funny, when I first recapped the motherboard and the machine booted up, I was getting all kinds of weird system crashes.  I thought there were potentially more issues with the system until I realized the system was only recognizing 512k.  Now ClarisWorks runs like a champ.  

 

I had mentioned in another thread how much I love Quikchip SMD1 for desoldering SMD components.  It's literally a 3 minute job.

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11 hours ago, superjer2000 said:

@macalle69 @marcelv

 

I was able to fix my Classic that was only recognizing 512k of RAM.  I had previously bought the Classic schematics from maccaps and checked every memory related trace and couldn't find any problems.  I ordered the chips above from Mouser but figured I'd start with UH6 (74LS174) as that was the chip that was related to the exact same issue Marcelv had.  

 

Replaced that single chip, booted up and all 2MB of installed memory was recognized.  Thanks for pointing out your original fix thread Marcelv.  Now I have extras chips of various types but they might come in handy for something at some point.

 

It was funny, when I first recapped the motherboard and the machine booted up, I was getting all kinds of weird system crashes.  I thought there were potentially more issues with the system until I realized the system was only recognizing 512k.  Now ClarisWorks runs like a champ.  

 

I had mentioned in another thread how much I love Quikchip SMD1 for desoldering SMD components.  It's literally a 3 minute job.

Happy you solve the problem!!

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  • 2 years later...

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