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Possible memory fault with rare compact 386SX

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Reply 20 of 23, by MikeSG

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1) Landmark displays a higher Mhz for all CPUs that are above an AT.

2) Minimum RAM for win95 is 8MB. Otherwise it uses the hard disk paging file non-stop.

3) Due to oxidisation, any chips that can be reseated on the board should be reseated multiple times.

4) 10uf caps (blue in picture) are almost never good on boards this old, and can cause lockups.

5) 386 CPUs use half the XTAL clock speed, 32 = 16, and can generally be run a speed higher. Eg 20Mhz (40Mhz Xtal).

6) There's all kinds of 30 pin SIPP modules on Ebay. The board may support 1MB (x4 = 4MB), or may support 4MB sticks each. 3 chip RAM will most likely work but don't hold me to it.

Reply 21 of 23, by BurntOutElectronics

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MikeSG wrote on 2024-06-20, 14:14:
1) Landmark displays a higher Mhz for all CPUs that are above an AT. […]
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1) Landmark displays a higher Mhz for all CPUs that are above an AT.

2) Minimum RAM for win95 is 8MB. Otherwise it uses the hard disk paging file non-stop.

3) Due to oxidisation, any chips that can be reseated on the board should be reseated multiple times.

4) 10uf caps (blue in picture) are almost never good on boards this old, and can cause lockups.

5) 386 CPUs use half the XTAL clock speed, 32 = 16, and can generally be run a speed higher. Eg 20Mhz (40Mhz Xtal).

6) There's all kinds of 30 pin SIPP modules on Ebay. The board may support 1MB (x4 = 4MB), or may support 4MB sticks each. 3 chip RAM will most likely work but don't hold me to it.

1. The speed I'm looking at is the "true" frequency and not the AT equivalent.

2. I'm not trying to run windows 95, the hard drive I had with it installed goes to the do's prompt rather than going straight into the GUI.

3. I can certainly do this, though only the keyboard controller and BIOS EPROMs are socketed other than the ram.

4. Tantalum caps generally short. I've never seen them go open circuit. If one was shorted I'm sure that the power supply would go into current protection.

5. Yep I'm aware of this, however the CPU is receiving both 16 and 32mhz now on pin 15. It fluctuates around the two. It's one or the other, like a frequency divider is playing up.

6. Yeah okay. I'll have to check prices

Reply 22 of 23, by BurntOutElectronics

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According to this rather inciteful website:
http://www.righto.com/2023/11/intel-386-clock … ircuit.html?m=1
No, the clock is supposed to be fixed on CLK2 at double the operating frequency, i.e. 32mhz. So it shouldn't be crashing and going between 16 and 32 like it is. I'll have to see where the trace runs to

Reply 23 of 23, by MikeSG

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CLK2 is the pin on the CPU and means half the XTAL speed... 16Mhz should also be written on the CPU?

Edit: Replaced all the 10uf caps on my 386sx board that had freeze issues and did not clear it. - May be traces, like what you said