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    It has been a while since I changed an RTC module on a vintage motherboard so when one came in for repairs here, I thought it a good chance to take some pictures and do some pages documenting it for those who are interested.  

     This board was sent to me recently by a vintage PC collector in Southern California. His early Pentium board is a fine example of the clone boards available in the mid to later 1990s, and like many the Real Time Clock module was not holding its settings due to the age of the battery that is internal to the RTC module.  

  These were typically 24 pin Dallas Semiconductor DS1287 or DS12887 RTC modules soldered in and present a QUITE A BIT of work to remove on the typical 3 or 4 layer motherboards of the time.  Removing a older style battery with two pins is fairly easy for most people with some soldering experience as you can heat up one lead, and then pull it free, then heat the other and pull it free.   As you can imagine it becomes difficult when you have more than two pins, and trying to get all 24 pins on an RTC up to 700 plus degrees and pulling them all at once without damaging the board or surrounding parts is a difficult task if you don't have one of those large specialized soldering rigs with a tip that fits the 24 pins at once and lots of heat.

   Those of us who can't spend the big bucks for a special tip and monster soldering rig just to remove one size chip, will use normal pin by pin desoldering techniques where you use a vacuum desoldering iron, or a standard iron and some desoldering wick.  I fortunately many years ago got the chance to buy a good used PACE digital soldering station that has vaccuum or pressure air flow, and controls two irons with full digital temperature control so that you take less chance of damaging a board by over heating it. 

    After the first pass with the vacuum desoldering iron, the RTC mounting site looks like this below - most of the pins are desoldered and free moving but two still are partially stuck.


The trick here is to get the pin fully heated and then add a bit of new solder to help loosen up the old, then hit the vacuum button to pull the melted solder out. Then I rock the lead with the tip of the soldering iron a bit back and forth to be sure its free.  Then go on to the next pin - or if you are worried about overheating skip to a pin on the other side, going back and forth so one side does not get too much heat in the same zone in a period of time.


Below we have a picture of the RTC mounting area on the back of the board, and chip has been successfully removed, and the holes are now all clean.  This required going back a few times to get the empty holes fully cleaned out.


Below we see the top side of the board where you can see two holes have some solder left in them which would make installing a new chip or socket for a new chip a problem.


This picture is the older RTC module  - which I cleaned up the leads some to make it easy to plug into the new socket so you can test it if need be at some later time.


Below we have cleaned out the holes well.  This usually takes putting enough fresh solder into the holes with the hot iron till it fills the hole smoothly, then vacuum (or wick) it back out.  This leaves the inside of the hole (which hopefully still has its plating) ready to solder easily to the new part.


One easy way to check the holes is to shine a light thru them - as seen in the next two pictures showing the open holes.  The first picture is from the top, with the light behind it.


The next picture is from the bottom with the light coming from the top.


Now we insert a new 24 pin socket - doing a socket will make it far easier to test this part in future or to replace it next time - if needed.   Below is a picture of the bottom of the board after we soldered in the socket.  Again to keep heat to the board a minimum, I soldered in an alternating pattern from one side to another and skipped every other pin.  Then came back and did the ones that were not yet done, again alternating from one row to the other so you dont do two or three side by side which might heat stress the board more or overheat the new socket's plastic frame and allow a contact or two to shift position.

Note you can see some flux residue here, slightly yellow or brownish clear puddles around each pin - which will be cleaned off after checking connectivity.


And here another pic from the side for a better view


And here below is a pic from the top, showing the new socket.  Be sure to put the keyway of the socket toward the end marked on the board.  Usually a small indent marks the end you want.  (actually with this socket it would work either way - but it helps to have it right as when you put in a chip, it can go two ways and one would be dangerous - :-)


Next we test with a low current low voltage continuity test meter, to be sure each pin connects to the traces underneath.




And then we use some strong alcohol or other flux remover and an old tooth brush to clean the flux residue off of the pins underneath and now they look good - like factory but a bit more shiny usually.  This pic was a bit blurry and dim - sorry.


And then I inserted the new part, and put the old one in the protective foam for return to the client.  While the way they put date codes on these varies - it appears the old one was made in 1997 and the new one in 2016.  The old chip shows a 9709 code (ninth week of 1997).  The new one is blurry but looks like 1612  - so the 12th week of 2016 is a likely guess.  The battery inside should last at least 3 years - most did well after 5 years and a few made 10 years, but this board appears to have been built in 1997 so its 20 year old give or take a bit and really other than the RTC failing, looks pretty good. 


      Last but not least we test and then carefully package the board and return to the client and await word from them about the success with their original problem of the system not saving date, time and cmos settings (all stored in the RTC's memory area). 

 Update 2/15/2018   I found a great article on modifying the RTC chip to use an external coin battery on one of the best vintage computer websites - check this link for it if you would rather try this method which I found on http://www.classic-computers.org.nz

 Update 4/16/2018   I have placed a datasheet for the DS12887 RTC on the site so those who are interested can grab it and learn more about this interesting chip.  Click http://www.pcfixes.net/files/datasheetDS12887,pdf  to download.

Update 6/14/2018   I have placed the Dallas Semi / Maxim datasheet for the DS12885, DS12887, DS12C887 and DS12C887A  clock module on the site so those who are interested can grab it and learn more about this interesting chip.  Click http://www.pcfixes.net/files/DS12885-DS12C887A.pdf to download.



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