386SX-40 Motherboard Not POSTing

Discussion about old PC hardware.

386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-23 @ 10:51

I have this motherboard:

Image

It's a nice little compact motherboard, built in 1994 so long into the 386SX's production. It also has onboard cache, so I would like to get it to work. However, it doesn't POST. With a CL-GD5401 ISA VGA, when I turn it on I get nothing on the screen and no beeps for a good while, maybe 5-6 seconds, then I get a string of very rapid beeps, well over 10, maybe even 20 of them.. Then nothing for a second or two, then the same beep string again. It's almost like ringing a phone. I've tried moving ram to slots 1-2, 2-4, populating all 4 of them, nothing changed. I've removed, cleaned the contacts on the sockets and reseated the BIOS chip and the Cache chips, and the same behavior. I have tried it with the onboard battery enable jumper (which I believe is JP6) closed and open, I've tried connecting a battery pack to what I think is the external battery header (JP5) and that did not help.

This board came to me with some battery damage. I removed the battery and cleaned the damage, neutralized it with vinegar etc. There is basically no visible damage to the board that remains, except one trace that I think may be suspect:

Image

So maybe I should wire a jumper under the board for this trace, could this be it?

Also, I am not very familiar with pre-486 hardware and can't really tell what the other jumpers on this board may be for. Does anyone recognize it and/or could point me to a jumper configuration page for it?
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-23 @ 10:59

Update: I have re-reseated the BIOS and Cache chips and replaced the RAM with 30-pin sticks with 9 1Mx1 chips and now On boot I get a continuous string of short beeps after 20 seconds of nothing..

Update2: The behavior reverts to the previous long continuous beep, long pause, long continuous beep cycle when I remove the ISA VGA (which worked when I used it in a 486 about 6 months ago, so I have no reason to think it's faulty)..

This is just bizarre..
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Re: 386SX-40 Motherboard Not POSTing

Postby Deunan » 2019-5-23 @ 11:26

appiah4 wrote:On boot I get a continuous string of short beeps

I had that too when I tried bad BIOS image. It's actually a string of beeps then pause but the pause at 40MHz seems to be so short it's hard to distinguish. I only found that out in emulator (trying the same image) when I downclocked the CPU a lot. So it might help to de-turbo the mobo with a wire/switch on the header?

Anyway, I think the issue would be the BIOS (corrupted image thus wrong checksum, or just bad chip contact), or the L2 chips bad or cache jumpers set wrong.
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-23 @ 11:31

Deunan wrote:
appiah4 wrote:On boot I get a continuous string of short beeps

I had that too when I tried bad BIOS image. It's actually a string of beeps then pause but the pause at 40MHz seems to be so short it's hard to distinguish. I only found that out in emulator (trying the same image) when I downclocked the CPU a lot. So it might help to de-turbo the mobo with a wire/switch on the header?

Anyway, I think the issue would be the BIOS (corrupted image thus wrong checksum, or just bad chip contact), or the L2 chips bad or cache jumpers set wrong.


If it's the BIOS chip having gone bad I'm fucked because I can't even find the make and model of the board.

If it's the L2 chips, I can try to find replacements..

If it's the cache jumpers - well which ones are those and how do I set them? The cache chips are the only things that get warm when I try to boot. Actually, only one of them does.

JP1 and JP2 are right below an IC labeled AVASEN AV9107-05 which is on top of a solder pad labelled OSC2A so I suppose it is a variable rate oscillator of some kind, probably oscillates at 50/55/80MHz depending on JP1/JP2 settings. Looking at other motherboards, this is likely the way it works:

JP1 JP2
Cl Cl 40MHz<- Suspect
Cl Op 50MHz
Op Cl 66MHz
Op Op 80MHz

JP6 is probably battery enable
JP5 is most likely the BAT_EXT. Pin 2 is connected to Battery + so it's plus, that means Pin 1 is almost certainly also +, I guess Pin 4 is - then. So:

JP5:
Pin 1: +
Pin 2/3 Close for Battery
Pin 4: -

That leaves JP3/JP4 as the mystery jumpers, maybe cache jumpers? CPU Type?. I've tried every possible configuration and it still did not boot :(
Last edited by appiah4 on 2019-5-23 @ 12:33, edited 1 time in total.
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-23 @ 12:17

I have found one exact same board that got sold on eBay: https://www.ebay.com/itm/386-Motherboar ... _cvip=true

But the photos are gone..
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-23 @ 12:59

I've stumbled across this: http://www.os2museum.com/wp/the-isa-osc-mystery/

There actuall is an oscillator for the ISA bus on this motherboard, but maybe it's possibly the same issue? The only other ISA VGA cards I have are two Trident TVGA9000Bs and a TVGA8900D which I also know for sure work. I will try with those tomorrow..

I HOPE that is the issue here.
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Re: 386SX-40 Motherboard Not POSTing

Postby Deunan » 2019-5-23 @ 13:32

I've never seen a "battery enable" jumper. But I would expect to see a "mono display" jumper somewhere in that area. So my guess would be that is JP6.
As you noted the 40MHz setting might not work so make sure you are using some other one (this is a 40MHz CPU so it should be set to 80MHz but obviously is must also work on slower clocks as well).
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-23 @ 14:04

JP6 is definitely connected to Battery + on one pin and the CMOS chip on the other. If you open it the CMOS chip can not receive voltage from the battery and you must use JP5 as BAT_EXT. I can see no Mono/VGA jumper on thisboard unless it's one of JP3 and JP4.. Which may well be, i.e. one is CPU type (SX/DX) the other is Video Type (Mono/VGA). The board does not POST in any possible founr combinations of JP3/4 though.
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Re: 386SX-40 Motherboard Not POSTing

Postby Deunan » 2019-5-23 @ 14:21

It still doesn't sit well with me. There is no point in having such jumper, you have one on JP5 middle 2 pins for that. This might be CMOS clear then. Which pin on the CMOS chip is this connected to? Not ground I hope? Because powering the mobo with JP6 in place would be a bad idea then.

Also, no point in having any SX/DX switch, this is an SX-only mobo. Even if the chipset was configurable, such connection would be permanent. JP3 and JP4 could be cache related.
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-23 @ 14:51

Deunan wrote:It still doesn't sit well with me. There is no point in having such jumper, you have one on JP5 middle 2 pins for that. This might be CMOS clear then. Which pin on the CMOS chip is this connected to? Not ground I hope? Because powering the mobo with JP6 in place would be a bad idea then.

Also, no point in having any SX/DX switch, this is an SX-only mobo. Even if the chipset was configurable, such connection would be permanent. JP3 and JP4 could be cache related.


Now that I more carefully inspect the board, Pin 1 on JP6 that I thought was connected to the Battery is actually NOT connected to the battery, the trace from the battery is VERY close but not connected - it actually goes to the EXT_BAT Jumper directly so you are right. I can't see any traces leading from that pin though, might be Ground?

The trace from Pin 2 of Jp6 to Pin 33 on a chip labelled:

OPTI
80C42 V1.04
C 1991 MR BIOS TM
9207M5009

Keyboard controller I presume. There is a 80C42 pinout here: https://pdf1.alldatasheet.com/datasheet ... 80C42.html

According to this, Pin 33 is P16 which is in the P10-17 block and described as:

PORT 1: 8-bit, PORT 1 quasi-bidirectional I/O lines. P10–P17 access the 35 –38 signature row and security bit

What could the JP6 jumper be?

JP1/2 definitely are FSB; The jumper block is connected to the IC labelled as OSC and JP1 and JP2 are connected to R20 and R14 so they probably alter the input signal for the clock generator chip..

JP3/4 are connected to the CPU on one pin and the other pin seems to lead to the cache memoryand/or the ICs in the vicinity.. So it probably sets something about the cache?
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Re: 386SX-40 Motherboard Not POSTing

Postby Deunan » 2019-5-23 @ 15:15

The keyboard controller does have 2 I/O port that can be (and often are) used to connect configuration jumpers. If this one is not connected to battery then I still think it's the mono display switch. But the mobo should boot no matter it's setting.
JP3/4 are your prime suspects now, you need to be sure these 2 are set properly before you start looking for other possible sources of problems. I don't recognize the mobo though.
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-27 @ 05:54

Deunan wrote:The keyboard controller does have 2 I/O port that can be (and often are) used to connect configuration jumpers. If this one is not connected to battery then I still think it's the mono display switch. But the mobo should boot no matter it's setting.
JP3/4 are your prime suspects now, you need to be sure these 2 are set properly before you start looking for other possible sources of problems. I don't recognize the mobo though.


Thanks for this input.. It would seem JP5 is indeed some kind of BIOS input, and probably either monochrome/vga or SX/DX. All examples of this board I can see online have it jumpered so I will leave it closed.

As for JP3/4, I can't seem to get it to post no matter what. Now that I am done with reinstalling Windows on my P3, I can focus on this board.. I will get on it with a multimeter and start checking all traces near the battery area; I have a suspicion that some trace or another is dead.
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Re: 386SX-40 Motherboard Not POSTing

Postby treeman » 2019-5-27 @ 07:00

I have a similar board by look that has a 386dx40, it oneday just died. I noticed the battery had slight corrosion so I took it off fixed corrosion etc. I gave up on it then oneday read that alot of the old boards won't boot without a battery. I connected one and it came back to life.

I skimmed the posts and didn't see anything about connecting a battery, sorry if you did then ignore this
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-27 @ 11:26

treeman wrote:I have a similar board by look that has a 386dx40, it oneday just died. I noticed the battery had slight corrosion so I took it off fixed corrosion etc. I gave up on it then oneday read that alot of the old boards won't boot without a battery. I connected one and it came back to life.

I skimmed the posts and didn't see anything about connecting a battery, sorry if you did then ignore this


I did not reconnect a battery but I did connect a 3xAA battery pack to the external battery header, which did not help.

I am hesistant to solder on another timebomb to the board.. Kimd of hoping the External Battery would work I guess?
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Re: 386SX-40 Motherboard Not POSTing

Postby treeman » 2019-5-27 @ 11:35

It should work in theory but you know how these old boards are sometimes, I guess u could solder in 2 wires to the holes then solder them to a battery. The wires are very quick to remove after
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Re: 386SX-40 Motherboard Not POSTing

Postby root42 » 2019-5-27 @ 13:48

Don't give up on this board! It is absolutely beautiful! I love those baby ATs. Ultra compact, elegant, plus it has Cache! By the way: did you try without cache?
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-27 @ 14:04

root42 wrote:Don't give up on this board! It is absolutely beautiful! I love those baby ATs. Ultra compact, elegant, plus it has Cache! By the way: did you try without cache?


I will try but repairs beyond a certain expertise are beyond my capacity.. If I can identify obvious broken traces and whatnot on the power lanes I suppose that would be telling, I will update the thread with my progress and ask for your feedback. Cheers guys!
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-30 @ 06:07

I will have some time to lay the board on a desk, and check all traces I can follow for continuity tonight.

When checking the JP5 BAT_EXT header I realized, the jumper is on Pins 2 and 3, so that means those are + +.. That means the pinout is either:

Code: Select all
NC + + -


or

Code: Select all
- + + NC


But I connected toe battery to pins 1/4 because that's how I saw it work in the past *facepalm* I will try booting with the battery pack connected to pins 3-4 and pins 1-3 and see if that makes a difference.

Could someone walk me through how to measure voltage on the power rails? I connect the AT PSU and turn it on, then check between the trace and a ground pin on the power connector? Is it dangerous to do voltage testing with a multimeter with the power turned on?

Also, about checking the AT PSU for voltage; if I just turn the psu on without any load, can I just test the voltages between the +/3 5/12V pins and any ground pin?
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Re: 386SX-40 Motherboard Not POSTing

Postby Deunan » 2019-5-30 @ 08:10

appiah4 wrote:When checking the JP5 BAT_EXT header I realized, the jumper is on Pins 2 and 3, so that means those are + +.. That means the pinout is either:

That would be weird and unusual pinout. Not saying it's impossible but very unlikely.

How it works (note, the pins can be in reversed order, always check!):
- pin 1 is external battery +
- pin 2 is internal battery +
- pin 3 is connected to RTC/CMOS
- pin 4 is GND (connected to either battery -)

You need a jumper on 2-3 to have the on-board battery power the RTC. Without the battery though the jumper is useless. On some mobos putting the jumper on 3-4 is a way to clear the CMOS, though, because there is usually a diode there between pin 3 and the chip, it's not a good method. Requires many minutes to work.
Pin 1 is not connected to pin 2, well not directly. There is another diode there that goes to RTC as well. In theory you could have both batteries connected and the one with higher voltage would be powering the RTC. Diodes protect the internal battery from being charged by the external one and the external one from being charged at all. Sometimes there is a separate jumper for the on-board battery charging (it allows both rechargeable NiCd and one-time lithium cells to be used) - doesn't seem like this mobo has one. But that means it's always trying to charge the internal battery, through a resistor, from the 5V line.

The parts would most likely be D2, D3 and R43. You should not connect external battery to pins 2/3 since the mobo will then be able to "recharge" it. Not safe, even though the current is small, might heat up smaller cellls like AAA considerably.

Anyway, I think a test card would be good investment at this point. The beeps are never particulary useful in case of actual mobo failure (and not just a problem with RAM or video card).
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Re: 386SX-40 Motherboard Not POSTing

Postby appiah4 » 2019-5-30 @ 08:32

Hold up, I'm confused.

That means on my board:

Pin 1: Connected to Pin 2
Pin 2: Connected to Battery +
Pin 3: Connected to CMOS
Pin 4: Connected to Battery -

So, to use an external battery I would close pins 2-3, connect Pin 1 to Ext Battery + and Pin 4 to Ext Battery -? So my previous attempt failed because I left Pins 2-3 open? Fair enough, maybe the board just did not power on without external power!

But wouldn't that try to recharge the external battery? That means I can't safely use a regular 3xAA battery holder, and would have to resort to something like this?

Image

I guess I could just save myself the headache of trying to figure out the EXT_BAT header and solder headers onto where the battery used to be and connect it to those header like I did on a Pentium board in the past?

Image
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