386DX just won't hold a charge

[Zap!]

So I got a 386DX 33 last year, and at the time it had an old barrel battery in it (see pic 1). Well, I removed it and installed a nice battery pack (uses 3 AAA batteries) to jumper J2 (see pic2). It worked great, and all was well for over a year, but then I had to open up the computer case for something. When I powered the computer back on, it stopped holding a charge. So I replaced the batteries. Nothing. I made sure the battery pack switch was "On" (it was). I cleaned the terminals that the positive and negative go to. Still nothing. When I power on, it says "CMOS battery state low."

What do you think it could be? It's weird because it worked perfectly before, until I had to open the case. I don't think I could have hit something. Battery pack looks like this, except the leads are female. Anyway, here is my motherboard:

https://stason.org/TULARC/pc/motherb...386-MB-35.html

 

[Ruud]

The machines I have that need a battery pack always needed FOUR batteries. But you mentioned that it did work, so that cannot be the problem.

What you can do is to measure the voltage directly on the board, starting at the pins where the batteries are connected and ending at the IC itself. Should be more than 2 Volt at the end.

 

[Zap!]

The machines I have that need a battery pack always needed FOUR batteries. But you mentioned that it did work, so that cannot be the problem.

What you can do is to measure the voltage directly on the board, starting at the pins where the batteries are connected and ending at the IC itself. Should be more than 2 Volt at the end.

I'm no expert on this, but I do have a good multimeter. When testing the two pins on the board (where the battery pack plugs in), I am getting around 5.4 volts. The battery pack itself is getting about 4.7 volts, so that appears to be working. Not sure if you wanted me to test it this way, I was a bit confused with the IC comment.

Normally I would say maybe I have to run the computer for several hours to let the battery charge, but these aren't chargeable batteries, they are normal AAA's.

 

[GiGaBiTe]

Normally I would say maybe I have to run the computer for several hours to let the battery charge, but these aren't chargeable batteries, they are normal AAA's.

DO NOT use normal alkaline batteries in a rechargeable battery circuit. You risk the batteries leaking or exploding, they're not designed to be recharged. Unless you're using a blocking diode for charge protection, you need to change those batteries to rechargeable NiMH types.

 

[Zap!]

DO NOT use normal alkaline batteries in a rechargeable battery circuit. You risk the batteries leaking or exploding, they're not designed to be recharged. Unless you're using a blocking diode for charge protection, you need to change those batteries to rechargeable NiMH types.

3 things:

1) Thanks for letting me know. I was told it was ok by some pretty knowledgeable guys, but I could have been given bad info.

2) I am running it from JP2 (outermost two pins), not from where the original barrel battery was located. My motherboard is this one.

3) Do you think it has anything to do with why it's not holding a charge? If yes, why did it work for a year and a half before? I can order rechargeable AAA's if you think it will help.

 

[Chuck(G)]

No, you're fine. I have a 386DX in nearly the same configuration (no blocking diode) that's been happily running from 4 AA batteries for the last 30 years. It's actually pretty surprising how far the batteries can be discharged for the clock to quit working--I recently replaced the (alkaline) AA cells that I last installed around 2000. A couple were down to 0.5 V no-load and the other two were less than 1V. The moral is that the batteries may start leaking before you notice it.

So get that DMM out start measuring to see where you're losing the supply.

 

[Zap!]

Ok, I believe I've figured out the problem, just not the reason. When connecting the battery pack to J2 (see motherboard here), the multimeter reads about 00.11, not 5.4 as I previously said. When connecting to where the barrel battery was, it reads 4.8. I may have to solder the battery pack there, unless anyone can think of a solution as to why I'm no longer getting much power from J2. Remember, it used to work fine from there.

Here's a pic of my MoBo. The (former) barrel battery is toward the bottom right, directly under J42 and to the right of KBD.

 

[Chuck(G)]

Look for cold solder connections or broken traces off of J2. If you get frustrated and have to resort to the barrel battery connections, be sure to use a blocking diode in series with the battery pack. Keep in mind that an ordinary silicon diode will drop somewhere around 0.6-0.7V.

 

[Zap!]

Look for cold solder connections or broken traces off of J2. If you get frustrated and have to resort to the barrel battery connections, be sure to use a blocking diode in series with the battery pack. Keep in mind that an ordinary silicon diode will drop somewhere around 0.6-0.7V.

Thank you for the info. I'm pretty sure that I have identified the culprit. Appears to be that capacitor D2 (to the right of that middle screw at the bottom). See how it goes directly to the copper line that connects to the positive terminal (very bottom pin) of JP2? Well, the right side shows very low voltage, while the left is just fine. Do you or anyone else know what kind of capacitor that is so I can replace it?

Also, where the old barrel battery was (bottom left, J2) shows some corrosion from when it leaked a little. While I don't think that has anything to do with this, do you think it could be a problem?

 

[modem7]

Thank you for the info. I'm pretty sure that I have identified the culprit. Appears to be that capacitor D2 (to the right of that middle screw at the bottom). See how it goes directly to the copper line that connects to the positive terminal (very bottom pin) of JP2? Well, the right side shows very low voltage, while the left is just fine. Do you or anyone else know what kind of capacitor that is so I can replace it?

D2 and D1 are diodes. They are wired in series per the diagram at [here].
And like that diagram, I expect that your diodes are also of type 1N4148.
And like that diagram (battery of roughly 5V/6V), expect your D2 and D1 to each 'drop' about 0.5V

Important: Do that voltage measurement with the motherboard powered off.

Notice how the negative end of D1 goes to one pin of jumper block J42.
I suspect that jumper J42 is used to select which battery gets used by the motherboard (i.e. the NiCad, or the non-rechargeable battery on JP2)
Try removing/refitting J42 a few times (in case of a poor connection).

 

[Timo W.]

Also, where the old barrel battery was (bottom left, J2) shows some corrosion from when it leaked a little. While I don't think that has anything to do with this, do you think it could be a problem?

I can see quite a few damage from the old leaked battery. Seems like you removed the battery but did not take care of the damage it did. Over time, the damage spread - which is why it now suddenly stopped working. One trace or via is finally eaten away.

Desolder the keyboard connector, remove the solder mask and check/fix all traces and vias that show signs of corrision.

 

[Zap!]

D2 and D1 are diodes. They are wired in series per the diagram at [here].
And like that diagram, I expect that your diodes are also of type 1N4148.
And like that diagram (battery of roughly 5V/6V), expect your D2 and D1 to each 'drop' about 0.5V

Important: Do that voltage measurement with the motherboard powered off.

Notice how the negative end of D1 goes to one pin of jumper block J42.
I suspect that jumper J42 is used to select which battery gets used by the motherboard (i.e. the NiCad, or the non-rechargeable battery on JP2)
Try removing/refitting J42 a few times (in case of a poor connection).

.
Thanks, I will test it while the computer is off when I get home. J42 is CMOS memory clear, which I read about here.

D2 seems to be faulty, but I can't be certain of what to replace it with. I'd like to try a quick aluminum foil test (obviously just to test, not as a solution) to see if it is for sure.

 

[Zap!]

I can see quite a few damage from the old leaked battery. Seems like you removed the battery but did not take care of the damage it did. Over time, the damage spread - which is why it now suddenly stopped working. One trace or via is finally eaten away.

Desolder the keyboard connector, remove the solder mask and check/fix all traces and vias that show signs of corrision.

Actually, I did clean it up and stop the spread with vinegar, it was worse. While I agree with you that I need to fully repair it, I don't believe it has anything to do with my battery problem. The traces just don't match.

 

[Chuck(G)]

.
Thanks, I will test it while the computer is off when I get home. J42 is CMOS memory clear, which I read about here.

D2 seems to be faulty, but I can't be certain of what to replace it with. I'd like to try a quick aluminum foil test (obviously just to test, not as a solution) to see if it is for sure.

Most likely a 1N4148 general-purpose silicon diode. Insanely cheap--so cheap that the only thing that makes sense when buying them is to purchase 100 just to make shipping costs reasonable. This is a low-current application.

 

[Timo W.]

While I agree with you that I need to fully repair it, I don't believe it has anything to do with my battery problem. The traces just don't match.

Believing is not a good way to rule something out. You need to do it anyway, so why not just do it? The corroded traces and vias are part of what powers the RTC. If one is broken now, it will no longer work.

 

[Zap!]

Believing is not a good way to rule something out. You need to do it anyway, so why not just do it? The corroded traces and vias are part of what powers the RTC. If one is broken now, it will no longer work.

You are right, but I have not ruled it out at all. I am at work now but will thoroughly be testing all the options tonight. I have parts coming tomorrow as well, plus I am off from work. I do want to fix the corrosion as well. That copper line can not stay like that.

 

[Zap!]

Just an update: It was indeed the diode. I removed and replaced it with a 1N4148, and the 386 held a charge 8 hours later. Thanks everyone for all of your help, your responses were greatly informative.