VOGONS


First post, by RogrWilco

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Hi folks. My 486DX2/66 build is only posting intermittently. I would appreciate any advice, since this is my first retro build. Happy to provide info and pics re components, too, if it'll help.

I'm using a SOYO SY-25N mobo and a 486 DX2/66.

When powered on, the power supply fan always starts and the hard disk, if connected, always spins up. Good signs.

With just the CPU installed and no other components, I'll almost always hear a series of beeps. Again, good sign. However, on occasion I won't get any beeps even with just the CPU installed.

When I go ahead and install RAM and the graphics adapter, the system will either beep once and successfully post or there will be no beeps at all and no video signal. When there are no beeps, I have to repeatedly power cycle and it will always eventually post. Once it successfully posts once, I can usually restart or power cycle and it will continue to successfully post (not always, however). But if I power off and leave it for a few hours then come back, it will usually fail to post again (ie. no beeps). I then have to power cycle repeatedly again to get it to work (which it always eventually does).

I've tested the power connector and, with no load, the 5V rail is out by 7% and the others by less. I've pulled the BIOS chip, cleaned the contacts, and reseated it. I don't know what else to do. I wonder if it's a problem with a capacitor in the power supply or on the motherboard because the computer, once working, fails to post again if left off for some time. However, inconsistent with this theory, it sometimes (though more rarely) will fail to post even if it had successfully posted a few minutes ago.

I don't have any spare parts to swap in to test which components might be the problem, though I'm happy to buy a replacement once I figure out the source of the problem. I really like this motherboard though with its ZIF socket and 3 VLB slots and would like to save it if I can. I have a multimeter and would really appreciate it if someone could suggest further trouble shooting steps. I'm happy to post pictures of any of the components, too, if it'll help. Thanks a bunch!

Reply 1 of 20, by canthearu

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I'd try contact cleaner in ALL the ISA, VLB memory and maybe CPU socket as a first thing.

Second, would be to change the PSU. If the capacitors in the PSU are marginal, warming them up will often get them working normally.

Motherboard capacitors are unlikely to be bad ... I simply don't see this fault with 486 level boards. If there are any large electrolytic capacitors, it wouldn't hurt to change them, but I really doubt that this would be the problem you are suffering.

Edit: You really need spare parts to switch and swap if you are going to have ANY hope of working out what is wrong. A multimeter usually just isn't enough to work out what is really going on.

Reply 2 of 20, by RogrWilco

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Thanks canthearu. I'll get some contact cleaner and give it a good once-over. I've also ordered a replacement PSU in case that's the culprit. There are no electrolytic capacitors on this board. I think they're all tantalum. I've attached a picture of the board below. If neither of those things work, I guess I'll source a different motherboard and try that (good VLB boards are just so expensive!). Thanks for those suggestions. Would be happy to hear from others, too!

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Reply 3 of 20, by chinny22

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1/2 my retro rigs do this, It's part of their character 😉
yeh ok, really its because I don't have the time, skill, tools to troubleshoot component level issues.

Most of this stuff is over 20 years old, your going to get some little gremlins. Personally I'd just live with it then go and spend a small fortune on another VLB board that's just as old and just as likely to have it's own set of issues.
I would keep a very close eye on that battery though. Looks pretty good in your picture, but I'm sure you know the damage they can do if left unchecked.

Reply 4 of 20, by RogrWilco

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Oh man, Chinny, I like character as much as the next guy, but this gremlin is a little too much to put up with. Thanks for the reminder regarding the battery. I'll keep an eye on it. I plan to replace it with an external battery once I get the rest of the system stable.

Reply 6 of 20, by snufkin

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Have you checked the voltage across the battery when the computer's been off for a few hours. I've read that some boards can act funny if the CMOS battery is low. So if that battery is original and not storing much charge then maybe the voltage goes low enough to start causing problems, but is sort of ok if the power supply has been running for a few minutes. Might be worth attaching an external battery as part of trying to get the board stable.

Reply 8 of 20, by RogrWilco

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I've measured the voltage across my Varta CMOS battery and it's 2.7 V when the power supply is off and the full 3.6 V when it's on. Are those the expected values or should it also be putting out 3.6 V when off? Would it be worth removing the battery entirely to see if that's the problem?

Reply 9 of 20, by debs3759

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That might be the problem. If it's only giving 2.7V when powered down, it's on it's way out, as it's a 3.6V battery. Removing it will probably just wipe your CMOS when powered down, and it will need replacing. Some people replace them with a CR2032 (3V but non rechargeable) or ML2032 (same but rechargeable), which seems good enough.

See my graphics card database at www.gpuzoo.com
Constantly being worked on. Feel free to message me with any corrections or details of cards you would like me to research and add.

Reply 10 of 20, by snufkin

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RogrWilco wrote on 2022-01-02, 03:08:

I've measured the voltage across my Varta CMOS battery and it's 2.7 V when the power supply is off and the full 3.6 V when it's on. Are those the expected values or should it also be putting out 3.6 V when off? Would it be worth removing the battery entirely to see if that's the problem?

I'd probably go with remove the existing battery, since that's best done before it leaks, and it will eventually leak. Then attach a battery to the external battery via the motherboard connector. Fairly common way is to get a 3xAA battery holder (so 4.5V, which is fine) and put a 4-way connector on it to match the external battery header.

Reply 11 of 20, by RogrWilco

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A final update: I tried everything suggested, but none of it worked. In the end, it seems it was a jumper configuration issue. A jumper on the motherboard not referenced in the manual was set incorrectly and I only discovered it when going through the configuration tables for my motherboard from Ultimate Retro. Still don't know what that jumper does, but the settings from Ultimate Retro worked and now it posts consistently. Thanks to everyone who tried to help!

Reply 14 of 20, by snufkin

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Interesting, compared to the MicroHouse sheet, the manual CPU diagrams show slightly different settings for that jumper. The MicroHouse sheet lists just one CPU (P24D) as JP14 1&2 + 3&4, and the P24T (plus a couple of others) as 2&3. But the manual diagrams on page 8-12 look to show most CPUs, including the P24T as 1&2 + 3&4, with just two CPU types with JP14 on 2&3.

Given the two CPU settings that put the jumper on 2&3 are the Cyrix 486S and the 486SX, maybe it's something to do with the floating point unit.

Reply 16 of 20, by aspiringnobody

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RogrWilco wrote on 2022-01-07, 19:27:

JP14. Again, unreferenced in the manual, so I have no idea what it does!

If I were a betting man I might say that it sets the CPU to WB cache mode. I've got a couple of boards that behave similarly if I try to enable write-back cache. If I can get them to post they work fine, but they don't always post.

Is it an &EW 486?

Reply 17 of 20, by RogrWilco

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snufkin wrote on 2022-01-07, 20:56:

Interesting, compared to the MicroHouse sheet ...

Didn't know about MicroHouse. Thanks for introducing me to another resource!

Reply 18 of 20, by RogrWilco

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aspiringnobody wrote on 2022-01-08, 02:05:
RogrWilco wrote on 2022-01-07, 19:27:

JP14. Again, unreferenced in the manual, so I have no idea what it does!

If I were a betting man I might say that it sets the CPU to WB cache mode. I've got a couple of boards that behave similarly if I try to enable write-back cache. If I can get them to post they work fine, but they don't always post.

Is it an &EW 486?

I'll have a look to see if this chip has WB cache or not. Probably not. What's &EW 486?

Reply 19 of 20, by aspiringnobody

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RogrWilco wrote on 2022-01-09, 01:06:
aspiringnobody wrote on 2022-01-08, 02:05:
RogrWilco wrote on 2022-01-07, 19:27:

JP14. Again, unreferenced in the manual, so I have no idea what it does!

If I were a betting man I might say that it sets the CPU to WB cache mode. I've got a couple of boards that behave similarly if I try to enable write-back cache. If I can get them to post they work fine, but they don't always post.

Is it an &EW 486?

I'll have a look to see if this chip has WB cache or not. Probably not. What's &EW 486?

On the 486 it will either say &E or &EW. &EW means the chip is capable of running in writeback cache mode.