Re: 286 motherboard repairs

Posted on 2022-02-19, 17:03

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

For the most part, yes. Unfortunately on most 286 mobos (except the very late ones with advanced chipsets) the KBC does have several more functions than just keyboard I/O. It controls A20 gate/mask, it can reset the CPU (and NPU), and some input port pins might be connected to jumpers, things like …

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Re: 286 motherboard repairs

Posted on 2022-02-19, 00:15

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

02 code for your BIOS seems to be RTC/CMOS related - do you have a speaker/buzzer connected? Your POST card should have one, and a 2-pin connector + short cable to hook that up to mobo. Does this make any beeps, because a hang on 02 code should beep. Please check if the RTC chip is getting power, …

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Re: 286 motherboard repairs

Posted on 2022-02-18, 23:32

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

In the mean time, I did drop to a 1K resistor, no change. It is is always 02,01. From my understanding of these POST cards, that means it's halted on 02, 01 having been completed. That makes sense about the keyboard controller. Mine is an erasable variant. So; 8742 installed, 146818 removed - No …

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Re: 286 motherboard repairs

Posted on 2022-02-18, 22:52

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Ah, so you do get something on the POST card without the KBC chip? You see, the BIOS code first tests what KBC has to say about the reason for reset, is it cold or warm one. It will not show any codes before that and if the value it gets is warm reset, but it was actually a power-on, BIOS will fail …

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Re: 286 motherboard repairs

Posted on 2022-02-18, 22:45

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Give me 5 minutes, I'll try it. Just had another thought, I'd tried the 8042 keyboard controller out of my 386SX in the 286 with no joy. But what about the untested 8742 keyboard controller from this dead 286 board in the 386SX board. Tried it, it works perfectly, at least enough to boot into the …

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Re: 286 motherboard repairs

Posted on 2022-02-18, 22:31

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Well, we have developments, but unfortunately not good ones. I tested both 74ALS245's near the keyboard controller, both test fine. I also swapped in an Intel 8042 in place of the 8742 that was there. Interestingly, the PCB is actually marked 8042. But now it won't throw any post codes out at all …

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Re: 286 motherboard repairs

Posted on 2022-02-18, 16:55

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Deunan, I shall pull that 74ALS245 and test it again, although I have been through and checked all 74 series chips before. Do I recall that keyboard controllers are for the most part interchangeable? Would it be worth trying a different one? I'd really appreciate a more advanced test BIOS. I'm …

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Re: 286 motherboard repairs

Posted on 2022-02-18, 07:21

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Right... I have traced pin 27 on both EPROMS to a jumper next to them, with nothing installed. On pin of this is connected to VCC, so I have added a jumper here. There is also a jumper next to it that ties pin 20 Chip enable to ground. I presume this needs adding as well, as CE is active low. I have …

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Re: 286 motherboard repairs

Posted on 2022-02-18, 04:48

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Modded the BIOS as you said and now 1234 appears on the post card. Good, that means the CPU is working and fetching instructions, at least it goes this far. I looked at your BIOS image and spotted 2 things right away, it's an early Phoenix BIOS which might not output any codes, I'll check that …

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Re: 286 motherboard repairs

Posted on 2022-02-06, 05:04

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

I have also managed to trace the PC speaker output. Pin 59 on the P82A203, which is SPKRDATA, goes to U63, a NAND gate. The other input for the NAND gate comes from pin 55 on the P82A203, which is labled OUT2. The output from the NAND gate then goes to an input of a 74LS244 buffer. The output of …

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Re: 286 motherboard repairs

Posted on 2022-02-06, 04:41

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

These old PCBs can have subtle damage like cracked traces or vias that are rather difficult to find. It's possible, and a good project for someone who has the patience to go slow and can switch to other things when it's not going well. Otherwise it can be pretty frustrating. What I do with such …

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Re: KVM Switches and 1 Keyboard/Mouse to rule them all

Posted on 2022-02-03, 18:21

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

I've used a couple of different "Adderview" branded KVM's with my retro setup. They do serial mouse emulation perfectly. I've just had to make up the correct cables to go from the PS/ port on the KVM to the serial port. Most the adapters sold on ebay seem to have the incorrect pinout. I was using 2 …

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Re: 286 motherboard repairs

Posted on 2022-01-28, 17:23

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by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Thanks, thats interesting. I haven't had a chance to do that yet, but will over the weekend. The chips removed were 27C256's, so 32k. I have replaced them with 28C256 EEPROMS for convenience, fitted in an adaptor to change the pinout to 27C256. I've tested these on my other 286 board that originally …

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Re: 286 motherboard repairs

Posted on 2022-01-28, 00:05

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by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

That low level testing sounds good. Could also be a CMOS problem maybe (looks like some Pheonix BIOS use 02 as a CMOS read/write test)? Maybe it needs a battery on the external header (if it has one, J21?) or the BATT1 position? Looks like the CMOS/RTC chip is between the keyboard connector and the …

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Re: 286 motherboard repairs

Posted on 2022-01-27, 23:50

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

These old PCBs can have subtle damage like cracked traces or vias that are rather difficult to find. It's possible, and a good project for someone who has the patience to go slow and can switch to other things when it's not going well. Otherwise it can be pretty frustrating. What I do with such …

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Re: 286 motherboard repairs

Posted on 2022-01-27, 23:44

by

by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Landmark is a good point to start. There will be no output on the POST-card with it. You get a display only on MDA/CGA or you hear output from the speaker. You need to be patient, it may take some time until it starts. Thank you. I was trying with a Hercules card, but I wonder if that isn't …

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Re: Pentium CPU issue

Posted on 2022-01-27, 06:48

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by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

I can't see the photo of the jumpers clearly enough, but is there any indication as to which one is pin 1, often theres a small 1 or a dot, or an arrow printed. Or worst case, if you look underneath, you may find that one has a square pad and the rest have round pads. Once you have identified pin …

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286 motherboard repairs

Posted on 2022-01-27, 05:43

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by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Hi all A friend has asked me if I can repair a 286 motherboard that was in an IBM 5170 case he bought. I've done a bit of work so far, but no success. The board is branded "Western computer 286 turbo". I can find zero information about it online. So far I have; Given the board a thorough wash, as it …

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Re: PC chips M912 cache

Posted on 2022-01-15, 03:34

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by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Hi Sorry its taken a while, I've been very busy All sockets have Vcc and Vss where expected. No sign of broken traces. I'm fairly certain its a BIOS issue now... Andrew

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Re: PC chips M912 cache

Posted on 2022-01-12, 15:23

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by Skip94

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

I assume you did the jumpering like this: m912_cache.JPG According to the manual TAG RAM must be populated with a 256K (32Kx8) chip. Have you tried this? m912_tag.JPG In your picture there´s a 128K = (16Kx8) chip as TAG-RAM, which in the PC-Chips manual is documented in none of the supported …

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