View Full Version : 286 oscillator swap (overclock)

August 3rd, 2010, 07:30 PM
Hi, I've got a 12 Mhz 286 board here, with a socketed 286. Looking on the board, there is the 4-pin crystal oscillator at 24 Mhz, which after divided, produces the 12 Mhz clock signal. I would like to step this machine up to 16 Mhz by replacing the oscillator with a 32 Mhz part. I realize my particular CPU is rated at 12 mhz, but since it is in a socket, I can replace it with the higher rated 286-16. The only other component I'm concerned about is the large surface mount chip (C & T brand) It is labeled "F82C235-12". I suppose this part is specced for 12 Mhz as well. Does anyone have any experience with C&T chipsets, and know how accepting they are of higher than spec frequencies? If C&T parts are iffy at higher speeds, I'll just leave the board alone. I might still decide to leave the board alone; I'm just curious about what could be done. But if someone else has successfully overclocked one of these boards, I'd be pretty temped to give it a go.

August 3rd, 2010, 07:46 PM
*jokingly* Throw a heatsink on the c&t chip and it will live *unjokingly*

August 3rd, 2010, 08:47 PM
The usual way this thing was done was to get a selection of oscillators (your "4 pin crystal") and try them one by one. So you might want to try a 28MHz XCO before you try the 32MHz one. Watch for heating. You probably won't ruin anything if you don't let things get too hot.

August 7th, 2010, 08:40 PM
This was semi-successful. i removed the original oscillator, and put little single-pin sockets in the 3 useful holes (pin 1 is NC). That way I can easily remove/replace oscillators without risking damaging the board from repeated desoldering.

Well, I didn't have a 28 Mhz osc, but I had 28.3, so I tried that. I tried it first with turbo jumpered (slow speed) and then took the jumper off letting it go full speed. Well, in full speed it ends the memory test early (counting up to 1600k or so), and at low ('turbo') speed, I sometimes get a gate A20 error.

So here's my thoughts on this: Using in odd-speed was a bad idea. Putting in the original 24mhz oscillator, everything is back to normal. I have a 32 Mhz oscillator on-hand, but not a 28Mhz one. Since 28.3 'almost worked', I'm thinking 28.0 might work. Im hoping that the issue was caused by a hopelessly strange mismatch in bus speeds, OR the 12-mhz rated CPU not taking the higher speed: i can solve the former by picking a better speed, and the later by replacing the socket CPU with a 16Mhz version. If the issue is caused by the C&T chip not taking the higher clock speed, then I am stuck at 12 mhz.

But... I am already very close to throwing in the towel and living with 12Mhz. That's a good speed for a 286 (Many are only 10), and I'm afraid of breaking it. But the thought of overclocking a 286 is so tempting. I just need a nudge in either direction...

August 7th, 2010, 09:23 PM
You didn't say what your DRAM speed was--if you were headed for 16MHz, you'd probably want to use 80nsec DRAMs.

At any rate, overclocking 286s got pretty elaborate. Have a look at the Xcelx (http://books.google.com/books?id=yDsEAAAAMBAJ&pg=PA78&lpg=PA78&dq=xcelx&source=bl&ots=1wucVfAB3r&sig=GQ1XrH5NkXvnTLLiHMYKwI-_gVo&hl=en&ei=Cz1eTPSHC4fevQPd0cWZDA&sa=X&oi=book_result&ct=result&resnum=5&ved=0CC8Q6AEwBA#v=onepage&q=xcelx&f=false) from 1987.

Anonymous Coward
August 8th, 2010, 06:39 AM
You might also want to check your board layout to see if there is a jumper for adding 1 wait state to the main memory. In later 286s this option might be available as a setting in the BIOS.

August 10th, 2010, 01:39 AM
There are a lot of variables that determine the max overclock, but an "odd" speed is not one of them. You can literally place any oscillator you want as long as the rest of the system can handle the speed.