Crystal for MEGATRENDS MEGA-KB-H-WP \ VOGONS

Crystal for MEGATRENDS MEGA-KB-H-WP

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First post, by Astralix

Posted on 2023-09-05, 17:36

Astralix Offline

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Joined: 2023-08-12, 13:36

Hi!

We are restoring a Rohde & Schwarz PSM7 based on an intel 486DX ISA board. The board has some brutal corrosion around the battery and the crystal was corroded too. There are no manuals around the keyboard controller so we have to guess the value of the crystal. I too have the same board, but the frequeency is not the question, but the capacitive load.

There are types from 6pF up to 12.5pF available, what is the right one?

The crystal is connected to the chip with 2x 4.7nF in series to ground and 1kR. But type code on the crystal and no application note for the MEGA-KB-H-WP.

Thanks for any help!
Astralix

Reply 1 of 5, by pentiumspeed

Posted on 2023-09-05, 23:17

pentiumspeed Offline

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Location: Great Northern: Canada.

Standard 12MHz crystal for keyboard controller ICs (40 pin) IIRC, most of them are based on clones or Intel 8042 microcontroller IC with different firmwares but they tend to work on most boards with any of compatible keyboard controllers.

Cheers,

Great Northern aka Canada.

Reply 2 of 5, by Astralix

Posted on 2023-09-06, 10:25

Astralix Offline

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Joined: 2023-08-12, 13:36

Oh, my fault!

I thought the RTC was part of it, but the 12MHz crystal is still in perfect condition. We need to trace down, where the RTC crystal is routet to. I could not find any datasheets for the chpset...
SAMSUNG KS83C206
OPTi 82C392
OPTi 82C493
I could not identify any separate RTC / BIOS RAM chip. Still possible, that it is integrated in that MEGA-KB-H-WP keyboard controller and both crystals are routed to it. Need to beep it out, as on the photos no traces are visible from the little crystal.

A little progress is made in cleaning and re-routing broken traces, but the board still does 6x beep, so Gate A20 function is still defective.

Reply 3 of 5, by pentiumspeed

Posted on 2023-09-06, 14:14

pentiumspeed Offline

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Joined: 2017-05-17, 23:17
Location: Great Northern: Canada.

Any 32KHz crystals will do, easily found on the modern motherboards.

Cheers,

Great Northern aka Canada.

Reply 4 of 5, by ChrisXF

Posted on 2023-09-06, 15:02

ChrisXF Offline

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Joined: 2021-01-29, 14:21

Some datasheet goodness for the Chipset:

http://www.bitsavers.org/components/opti/data … pset_199108.pdf

I noticed a lot of even the big-name motherboards of the era are pretty much identical to the suggested build in those manuals.

Hope that helps!

Reply 5 of 5, by Astralix

Posted on 2023-09-06, 17:58

Astralix Offline

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Rank: Newbie
Posts: 8
Joined: 2023-08-12, 13:36

Hey guys!

@pentiumspeed: Thanks, will pick some out of the spare parts box.

@ChrisXF: That sure helps a lot to track some connections and understand the signal flow.

Thank you!

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