Silly question is the bios battery good and have you tried a clear cmos?

chrismeyer6 wrote on 2022-05-26, 15:57:

Silly question is the bios battery good and have you tried a clear cmos?

Battery is good, it does hold the voltage well enough that it shouldn't be a problem. (2,7V, while rated at 3V)
I did try to clean the cmos - no effect.

maybe try remove cmos chip, computer should work without it, at least does it on presario 433.

That no life + pop in the PC speaker sounds like a similar issue I have had on an Olivetti before, although I can't confirm since I didn't have a POST card to work with at the time. Have you tried checking the oscillators and the capacitors yet?

tabm0de wrote on 2022-05-26, 21:48:

maybe try remove cmos chip, computer should work without it, at least does it on presario 433.

Woah, really? You mean BIOS ROM? I cannot think how could that ever work, but it wouldn't hurt either, so i tried it out - dead. No post codes or anything. Reset and pc speaker pop does happen properly.

Strahssis wrote on 2022-05-26, 22:00:

That no life + pop in the PC speaker sounds like a similar issue I have had on an Olivetti before, although I can't confirm since I didn't have a POST card to work with at the time. Have you tried checking the oscillators and the capacitors yet?

Thankfully this board has no electrolytics at all. Everything is ceramic. And if there would be any bad tantalums, they would blow into my face, which has not happened, so seems to be all good.
How could i check the oscillators without oscilloscope? Only thing i can currently say is that the CLK led is properly lit on POST card, but the board has 2 oscillators and maybe one of them is not good...

Ah that's very nice! I should've known that, since most of my Compaqs don't have electrolytics either. I don't have an oscilloscope, so I can't tell you how to do that. I have sadly always been limited to just replacing the oscillators and see what happens.

Checked the power rails (when th computer is on) and see if something is shorted?

Strahssis wrote on 2022-05-28, 19:08:

Ah that's very nice! I should've known that, since most of my Compaqs don't have electrolytics either. I don't have an oscilloscope, so I can't tell you how to do that. I have sadly always been limited to just replacing the oscillators and see what happens.

I'm really tempted to go on that route - replace just to see what happens and if something changes... I might do that at some point if we get truly stuck.

Joakim wrote on 2022-05-28, 20:27:

Checked the power rails (when th computer is on) and see if something is shorted?

Haven't done that in the way you described. As the POST card shows all voltages with fully lit LEDs, i assumed there can not be a problem with that. Otherwise one of the LEDs would be dimmed a bit. I would also hope that with shorted rails either of those two would happen: 1 - something becomes so hot it explodes (like tantalum caps) or 2 - my relatively modern ATX PSU (with AT adapter and all voltages truly present, checked) would trigger some protection and would not run.

Checked the voltages right now with multimeter and all is perfect while the machine/PSU is turned on.

So i took out the large oscillator and replaced it with another one that was laying around in my parts bin. Unfortunately it didn't make any difference - the machine still behaves as described before.
There is another one, near the left end of riser card slot. It doesn't have any markings on it, so i was not able to try something different there.

I once more went through the board inch-by-inch and tried to find any traces with any damage. But this board is truly in pristine condition... except it doesn't work. 😜

Only thing I can see is one slightly bent pin on the isa slot. Probably nothing.

does removing the jumper from P7 change anything ?

One way to debug this board without an oscilloscope would be to compare voltages with a similar known good board. Of course this requires a second board 😁

So, I've received the PLCC to DIP adapters for my TL866II. Here's the bios from the compaq board.

Just quickly looking into it, it seems okay. I can read some human readable strings out from it like "COMPAQ" and the date of "07/02/93" plus bunch of error messages the bios can show.
Just in case, i also tried to just reflash the same bios image back to the eeprom to make sure it succeeds and has a "fresh copy" in it. It did succeed, but the machine still doesn't boot.

Unfortunately i can't find any other usable bios images from the internet. I did find some kind of file, which made a bootable floppy that uses rompaq to flash the bios it has internally, but my machine does not boot, so i need the raw image. And i couldn't find it from the image. The floppy image is attached - maybe some of you can extract the bios rom from it?

To have this information archived as well: The bios rom chip is TMS27c010A. 😀

weedeewee wrote on 2022-05-29, 09:22:

does removing the jumper from P7 change anything ?

Nothing... 🙁

GigAHerZ wrote on 2022-05-27, 06:53:

tabm0de wrote on 2022-05-26, 21:48:

maybe try remove cmos chip, computer should work without it, at least does it on presario 433.

Woah, really? You mean BIOS ROM? I cannot think how could that ever work

it wouldnt, OP meant old style RTC/CMOS/battery combo chip which your board doesnt have because its more modern integrated design

GigAHerZ wrote on 2022-05-27, 06:53:

The POST debug card shows 4241, last code therefore is 42, the previous was 41.

does it use standard port 80h? http://mrbios.com/techsupport/award/postcodes.htm lists non standard ports 85 86
http://www.bioscentral.com/postcodes/compaqbios.htm same codes

41 Check RAM refresh
42 Start write of 128K RAM test
43 Reset parity checks

stop at 42 would indicate bad ram? no idea if desoldering onboard will automagically switch to simms

The ram is a highly suspected indeed. you need an oscillator to test them and the soldering skills you can probably replace them.

I watch Adrian Black doing this all the time but it's way out of my own league.

Unfortunately the closest thing i have to oscilloscope is a salea logic analyzer clone. 🙁

But i do have a reasonably good soldering station with hot air gun and some kapton tape, aluminium foil, etc. Probably will need to desolder the existing ram and see, if it starts behaving differently...
Good to know that there are others with same suspicion, gives me a bit of comfort to go into it. 😀

NB! I also watch Adrian's channel amongst other things. Currently is playing an Amiga 1000 repair video. 😁

So i desoldered the ram chips. It went amazingly well - the kapton tape held beautifully. I recently just bought another roll as the previous one melted very easily and you never know really, what is coming from aliexpress. This one was way better now. Probably my best ever desoldering job i've done. 😁

But unfortunately the behavior didn't change. It seems completely dead.
The debug card doesn't show anything and didn't show anything before desoldering either. I'm afraid it was a hiccup of the debug card itself earlier.

I also noticed the U101 (near floppy connector) becoming a bit warmer than other similar chips around. I desoldered that, too, but still - dead.

Only warm things are VGA chip and CPU. And if i try another chip in the socket, then the soldered cpu stays cool and the added cpu becomes hot. (With P7 jumper also changed when additional cpu is used)

Anything else i could try?

I was more thinking of replacing the ram chips.

GigAHerZ wrote on 2022-05-26, 17:44:

chrismeyer6 wrote on 2022-05-26, 15:57:

Silly question is the bios battery good and have you tried a clear cmos?

Battery is good, it does hold the voltage well enough that it shouldn't be a problem. (2,7V, while rated at 3V)
I did try to clean the cmos - no effect.

FYI, 2.7V is close to empty for a button cell. Though I'm guessing that an empty battery likely isn't your problem.