Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-25, 23:12

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Oh, now I see the other 8253 is not socketed. Too bad but I'd advise against desoldering it. Why risk damaging a working mobo. Unfortunately the 8253 is required for RAM refresh to work. Or the speaker. And yes, it's mostly only XT mobos that have it, all later ones integrated the DMA and timers …

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Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-25, 21:15

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Welp, then I guess we now have to find out what else is connected to this data bus. Assuming this is also similar to the 5150, it would be the 8253 timer and 8255 I/O. Remove both, see what happens to the voltage. If that still gives you the same result (high voltage when cold) then you'll have to …

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Re: Is this Pic Programer any good?

Posted on 2019-04-25, 18:24

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Do you you have the software? The hardware and it's age aside, without the software to operate it I'd say it's worthless junk.

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Re: format b: /4 - not working?

Posted on 2019-04-25, 11:02

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

AFAIR unless the drive is HD only (and it could be - especially the 3.5" ones that were manufactured just as floppies were all but phased out) it should be possible to format both to HD and SD/DD. But on drives that auto-detect the format it might not be possible to force it via software, but then …

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Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-25, 10:51

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

If you don't have a freeze spray, stack a few coins (the thicker the better) in a plastic bag and put that in a fridge or freezer. After a while the stack will get cold and touching it to the U54 chip should rapidly cool it down. The bag will prevent shorts and don't worry about tiny water …

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Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-25, 07:41

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Check this out: https://youtu.be/rE0bbTAkncs Look how it starts rising, then breaks down and strarts falling... I observed higher voltages on these pins (upto 3,8V I think) when mobo was cold but it quickly fell down to ~1,4V. Hm. Still, why would that one be special - can you test if any other 2, …

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Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-24, 23:22

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Hm. Just to confirm, this was tested with no CPU, no 8288 and no BIOS ROM? There's something wrong still with bit 4. It's weird that you get 2.2V on U58 pin 15, and yet only 1.52V on U54 pin 13. These should be connected so why is it different? Can you double-check the schematic, these should be the …

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Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-24, 22:10

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

F and S are very close in terms of speed but I think F uses less power. In most cases F will work fine as S replacement. If replacing U58 doesn't cure the mobo, try with the resistors again and check the voltages - I'm curious if the bit 4 will still show different readings. In fact if you get odd …

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Re: My new 486-386 junk boards [IT gore]

Posted on 2019-04-24, 19:00

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

I always thought that any 486 mobo that had space for the regulator would have it installed, but apparently that's not always true. There were some "budget" variants that could only accept 5V CPUs and the regulator section was not populated. So, perhaps this one was a budget mobo that someone tried …

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Re: mitsumi d509v - settings?

Posted on 2019-04-23, 17:56

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Does this help? https://www.embeddedsw.net/EMUFDD_MITSUMI_Jumpers.html

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Re: What's the best CRT monitor for DOS gaming?

Posted on 2019-04-23, 12:41

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

That focus issue is probably CRT being almost gone, too many hours clocked and cathodes no longer emit enough electrons to keep the brightness high enough. The electronics will detect that and attemp to compensate by raising high voltage (up to a certain, safe point) but this will have the side …

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Re: Upgrade from 386 to 586?

Posted on 2019-04-23, 10:32

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

A BIOS option "Slow refresh" will usually bring the DRAM refresh rate to 1/4 of the original value - some 64ms vs 16ms. Note that the performance improvement is single % at best (depends on the CPU, I suppose it would matter more for slower CPUs with no cache) while the risk of glitches raises …

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Re: Upgrade from 386 to 586?

Posted on 2019-04-22, 22:37

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

The problem with CPU-local cache memory is how to invalidate the entries when DMA is moving data (from FDC/HDD, sound card maybe) to RAM. There's obviously no protocol for that on a 386 since it's not supposed to have cache at all. Cyrix assigned some unused 386 pins, both on PGA and SX PQFP …

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Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-21, 09:08

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

No need to trace pin 19 any further. Actually it's another mistake on my part, I thought you tested U58 B-side rather than socket. But with this last test I've re-read everything and it falls into place. Pin 16 that was wonky on last test is O4, and here again it's different value than the rest ( …

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Re: Delay line modern replacement?

Posted on 2019-04-20, 17:49

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

retardware wrote: As the signal gets double inverted each stage, wouldn't this even out? Yes, it should - but inverters are not the only gates that can be used as a delay line.

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Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-20, 17:22

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

The test posted on 2019-4-18 @ 10:25 where I measured U23 U58 and U66? Well yes, but those tests were performed with BIOS EPROM in socket and once I put it back right now - then voltages again dropped on the pins 2-9 to around 1,4V and pin 5 is close to 0,1V. Once EPROM is removed from it's socket …

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Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-20, 12:20

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

A small update: 1) Forgot to to clarify that for tests without the pin 7 resistor in 8288 socket the test on U58 should be reversed. That is, in this scenario you measure voltage on the BIOS socket and apply resistor H/L signal on the A side - so, effectively, on the ISA slot because of how it's …

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Re: Delay line modern replacement?

Posted on 2019-04-20, 09:13

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

I'd advise agains LS chip as delay lines. The H->L transition is much faster for TTL tech than L->H. HCT is much more uniform, better idea I think.

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Re: Delay line modern replacement?

Posted on 2019-04-19, 23:46

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by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Not sure if better but you could probably "stack" a pair or two of '04 inverters for each 20ns range. A 74HCT04 has typical propagation of 8ns per gate, so two would add to 16 or so. The problem is, it might be too temperature dependant and only work properly if cold/warm. Worth a try though - HCT …

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Re: XT clone motherboard repair advice/help needed (KT 10 M/B)

Posted on 2019-04-19, 23:34

by

by Deunan

in Marvin \ General Old Hardware

in Marvin \ General Old Hardware

Hm. So the test with the resistor has different result now than it had last time when you measured voltages on 3 of the '245 chips. And the only thing different is the voltages on B side were a bit higher now (you said those slowly go down so chances are last time it took you more time to measure …

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