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Dallas RTC chip replacement results in video card not found

RetroSpector78

Experienced Member
Joined
Dec 30, 2018
Messages
153
Location
Belgum
Hi,

I have an old Philips 286 laptop that had a dead Dallas RTC chip.
The CMOS tests failed and I couldn't setup the system (always started with default settings and couldn't persist anything in the BIOS).

Replacing the Dallas RTC chip was a bit of a botch job ... It was very difficult to remove, and I think I damaged some pads / vias along the way.

Initially the result was a dead laptop. Knowing really well I screwed up on the Dallas RTC I decided to look at the traces.

I had put in place an IC socket for the Dallas RTC chip, and I get the same behavior with the original Dallas RTC chip that was in there.

The Dallas RTC chip is getting power (5V) .

The PCB is multi-layered so I cannot trace al of the connections, but I did notice that on the Dallas RTC A01 and A03 were not connected.

where-as I could verify the following connections (continuity check):
  • Dallas RTC A00 was hooked up to Bios EEPROM D0
  • Dallas RTC A02 was hooked up to Bios EEPROM D2
  • Dallas RTC A04 was hooked up to Bios EEPROM D4
  • Dallas RTC A05 was hooked up to Bios EEPROM D5
  • Dallas RTC A06 was hooked up to Bios EEPROM D6
  • Dallas RTC A07 was hooked up to Bios EEPROM D7
So I hooked up
  • Dallas RTC A01 was hooked up to Bios EEPROM D1
  • Dallas RTC A03 was hooked up to Bios EEPROM D3
Progress ... When I turn on the laptop now, I get orange lines on the LCD, and

- 1 long beep, followed by 3 short beeps
- 1 beep
- 1 long beep, followed by 2 short beeps

The BIOS is an Award BIOS. So this seems to suggest an issue with the video card.

Now most likely there are still some broken traces on the Dallas RTC.

For example I cannot find the connections for
  • pin 14 AS
  • pin 15 RW
  • pin 16 RESET
  • ping 23 SQW
I also don't know if hooking up A01 -> D1 and A03 -> D3 is correct (seemed to make sense and I did get something on the screen and some beeps).

How would I go about debugging this further ? Is there some kind of reference design how these Dallas RTC chips are wired into the BIOS chips, and where the above pins might end up going ?

Do these beeps give any hint on what the issue might be ? I only touched the pads of the Dallas RTC so I wasn't anywhere near the video card.
 

RetroSpector78

Experienced Member
Joined
Dec 30, 2018
Messages
153
Location
Belgum

Yes, but I never put it in backwards. I did read about some incompatibilities with the DS12887A chips on older motherboards. My first attempt was with such a DS12887A, but with my DS12887+ and. the original DS1287 chip the computer always does the same thing.

Don't know that much about how such a Dallas RTC is wired on the board, with what other components it interacts with, or how I could further diagnose this.

With this multi-layer board there are so many options, and although I did manage to trace down the address lines, the VCC and GND, where are still some pins that I cannot find.

Got some pictures here (seems to be a 256kb (??) file size limit here for picture) : https://drive.google.com/drive/folders/1xMmg5xNaZU6EHqbII0PzSAOa7zo9i37C?usp=sharing
 

Chuck(G)

25k Member
Joined
Jan 11, 2007
Messages
38,881
Location
Pacific Northwest, USA
Without a schematic for your system, it's going to be hard to figure this one out. However, you can at least get a clue from the IBM 5170 schematics here. See sheet 19 for how the MC146818 is hooked in. The Dallas 1287 has the same pinout--it just contains the battery backup under that epoxy.
 
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