Dell Latitude D620 laptops

    I bought a couple of Dell Latitude D620 laptops and they have given me quite a few problems.  One of them has a bad external microphone connector which would cost way too much to repair and I resolved the issue with an external USB audio device which works fine but takes up a USB port.  It seems the connectors on Dell laptops are generally not very good and give many problems as we shall see later with the power supply. 
    The other D620 suffered from the infamous Nvidia Quadro NVS 110M GPU and I had to replace the GPU chip even though this is probably only a temporary solution as it will eventually fail again.  Initially Dell denied any responsibility, then, under pressure, they extended the warranty period and later, after a class action lawsuit, Dell agreed to compensate those affected.  I submitted my claim for the cost of the repair and I am waiting to see if I finally do get a decent check or if they tell me that after prorating the available funds I am entitled to the value of a first class postage stamp.  (Update: I was reimbursed for the full cost of the repair.)
    The whole thing was complicated by the fact that the (some?) moderators of the Dell forums are clueless jerks who deleted my post and threatened me with banning for posting the same information about the NVidia settlement that is posted by themselves elsewhere in their own Dell site and even in posts in the same forum.  My post was deleted and I got a chastising email for "talking about legal action against Dell".  Probably some clueless part-time mod but if the rest of Dell is similarly well coordinated it goes a long way towards explaining the crappy quality of their products. 
    The Dell Latitude D620 has some nice features like being able to remove the 2.5" SATA hard drive by just sliding it out.  This allows you to have a backup mirror drive ready to go if the main one has a problem or even to have two different users of the same computer, each one with their own hard drive.  I always liked this feature which is also useful if you have to leave your laptop in hotel rooms.  I secure the laptop with a steel cable using a system of my own invention but I also remove the hard drive from the computer and take it with me or put it in a safe box. 
    The Dell Latitude D620 has an internal wireless WiFi adapter which works just fine in the intended office setting but no matter how good an internal antenna can never even come close to the performance of an external wifi adapter so every one of my laptops has an external USB adapter which allows much better connections over longer distances.  With a reasonably long USB cable of 2 - 3 m I can place the antenna where it better receives the wifi signal and there is just no comparison in performance. 
    The D620 does not have a built-in webcam and I think I prefer it that way.  A built-in webcam is nice but when I am doing videoconference I like to be able to move the webcam around and point it to things I want to show (Look!, I'm in the park!).
    One thing I do not like about the D620 is the need to use specific Dell power supply adapters and not be able to use generic adapters.  The Dell adapters with their electronics, cables and connectors are prone to problems so each laptop has two adapters so there is always a spare.

 

Dell Latitude Power Supply Unit (PSU)

    Dell laptop power supplies often have problems, often caused by bad connectors, either on the cable from the PSU to the computer or the connector on the motherboard itself.  I found I could not use generic power supply units (PSU) because these DELL computers have a special connector with the two power supply contacts but also a third signalling connection.  So I decided to investigate.
    The first interesting thing is that the computer itself says it requires 19.5 V at 4.62 Amps (90 W)

Dell power supply
   
    but the power supply pack supplied with it says it supplies 19.5 V at 3.34 Amps (65 W).
   
    Dell power supply

    The computer says it requires adapter model pa-10 or pa-12.  The PSU says it is "model pa-1650-05d2, 65 W AC adapter PA-12 family".  Another similar adapter I have says it is model hp-OQ065B83.  It seems these power supplies are used with many different models of Dell laptops including Latitude, Inspiron, Dimension, etc. 
    Searching online I find that many people have problems with the 65 W power supply which are resolved by using the 90 W PSU.  It seems the way it is supposed to work is that the computer knows what PSU is connected and will limit its power usage.  If the more powerful PSU is used then it will run at full blast and charge the battery at the same time but if the less powerful PSU is used then it will not charge the battery while the computer is in use and will wait until the computer is turned off.
    The computer and the PSU communicate through the third conductor and if you connect a generic PSU with no third cable then when the computer starts up it will display an error message saying "The AC power adapter type cannot be determined. This will prevent optimal system performance". It seems the computer assumes a very low capacity PSU and will not charge the battery and will throtle back the speed of the processor to conserve energy.
    I decided to do some tests of my own even if very limited due to the limitation of the means and tools at my disposal. Maybe someone with better tools can do better.
    I determined that
  • The signal line gets voltage (3.26 V) from the computer, not the PSU.
  • Computer and PSU exchange data only once when the PSU is first connected, even if the computer is off, not during normal computer use.
  • The signal line is pulled down by the computer for a number of pulses (about 25) and immediately after each pulse the PSU pulls the signal line down for a shorter duration. This is very difficult for me to see on my 'scope so it is mainly guesswork.  I would need a 'scope with memory or a data analyzer to determine this with any certainty.
Dell power supply
    It seems there is some intelligent circuitry at both ends, computer and PS, and both need to be working correctly for the computer to recognize the PSU model.  If the PSU has a problem it might not be recognized but if the computer end has a problem then it will not recognize any PSU.  The following photo shows where the three wires are soldered.  The black clip is negative, the red clip is positive and in the center is the signal wire (apparently with a 130 Ohm to negative). 

Dell power supply

    One possible cause of problems is the signal line not making a good connection because the pin is bent or broken.  This should be relatively easy to fix.  Another, more serious, problem happens when the 19.5 V wire is shorted to the signal wire due to an aging cord.  Then the signaling circuitry at the computer is fried and the computer will no longer recognize any PSU. 
    More information on the power supply, the battery and the signalling can be found here,  here,  here, and here.  Those pages refer to a Dell D610 and the battery pack looks physically different but electronically it looks like everything applies.
   
    Because of the signalling I cannot use a generic 12Vdc power supply and I am forced to use a 12Vdc to 220Vac converter and then the regular Dell PSU. At 12 V it uses about 3.5 A if the battery is fully charged but about twice as much if the battery is charging.
   

    The problem with this approach is that it requires much more power than strictly necessary.  For one thing the two conversions (12 Vdc - 220 Vac - 19.5 Vdc) are wasteful but also the laptop feels it is being supplied power from the mains and goes at full speed and the screen at full brightness whereas when it is working from the internal batteries it saves power by slowing the processor and diminishing screen brightness. 

 

Battery Pack PC764

    The battery pack stopped working so I decided to open it up and investigate.  Cracking the case open was not easy.  It snaps closed and it was also glued.&nbps; As I could not find any easy or obvious way to open it I ended up cutting it open with a utility knife.  I found it has six Lithium - ion cells type 18650 (18.0 x 64.5 mm).  Each cell provides about 3.7 V and they are connected in two parallel strings of three cells each in series which makes a nominal 11.1 V. 
   


    A1 and A2 in parallel form the lower cell with the negative directly connected to the computer negative.  B1 and B2 from the middle cell and C1 and C2 form the upper cell which is connected through a software controlled switch to the + power pins.  I measured voltages and found the two upper cells had 4 V each but the lower cell was dead.  No voltage and infinite resistance.  Now I need to decide whether to replace the entire pack, replace all six cells or replace the two faulty cells.  I am inclined to try replacing just the two faulty cells because there is a risk that the controller circuit might have some problem so the risk is minimal. 
    The battery pack connector has 9 pins numbered beginning at the far end from the battery cells.  Pins 1 and 2 are the + power output which is connected to the battery through a semiconductor switch controlled by software.  Pins 5, 8 and 9 are connected to the computer ground and directly to the negative end of the batteries.  I am not quite clear on the purpose of pins 3, 4, 6 and 7 although the seem to be used for some kind of signalling.  From information gathered from other Dell battery packs I gather it may be that pin 4 might be SM bus signalling and pin 7 may be an on/off signal.  Pin 6 is labeled "PESS" and pin 7 is labelled "alert".  I guess the alert signal may be used by the battery pack to signal to the computer that the battery is low.  In the photo you can see pull-down resistors to ground. 
The test pushbutton and the five LEDs are circled in red.

   
Update
    I replaced the two dead cells but the pack, in spite of having the right voltages, still did not work with the laptop.  I investigated the control circuit and found some damaged components so I think this is probably the cause of the problem.  The control circuit is quite complex.  As you can see in the photo it has some ICs with many pins.  After looking over the circuit very carefully I detected some damaged or missing components which I have marked in red. 

    It looks like two SMD resistors are missing and a black component (diode?) next to them is damaged.  I have no idea how this could have happened because I was very careful when I disassembled the pack and because the components are located in a location quite protected by other components but, at any rate, it seems this is beyond my capability of repair because I do not have the schematic.  It's time to buy a new battery pack.