First post, by Blavius
- Rank
- Newbie
I'm trying to turn a IBM PS/2 Model 76 into something I can play dos games on. I added the new MCA soundblaster and some memory, now its time to boost the speed a bit.
The unit came with it's stock 486DX2 - 50MHz. I've replaced it with a DX4 (3x multiplier) and later with a am5x86 (4x multiplier). Although the clock speed has increased, the actual performance (DOOM benchmark) didn't: there was only a minor increase going to the DX4 (about 20%), and no further increase with the 5x86. Looks like the system is bottlenecked elsewhere, so the obvious candidate would be to increase the bus speed.
The motherboard in my unit came in either 25MHz or 33MHz, presumably to offer a budget DX2-50 next to the DX2-66, so it would be reasonable to assume that I could increase the bus speed to 33MHz without even stressing the components. I removed the 50MHz SMD crystal and put a socket in its place, in which I stuck a 50MHz 4-pin crystal I had lying around. This worked, the system performed normally (so, at 25MHz)
Next step was to put in a faster crystal so I got a 66MHz from the local electronics store, and...nothing. No boot, nothing on the screen at all. I also ordered a 60MHz from aliexpress, same result. I put an oscilloscope on the cyrstals and found that the 50MHz one that works has a 4Vpp amplitude, and the 66MHz has only 2Vpp. So, I assume that is too low. I don't have a signal generator that goes up to these frequencies to test things out, so I'm stuck to trying crystals.
This is where I get stuck. Checking around on Aliexpress I noticed that most sellers do not provide any information besides the clock frequency. Some talk about the supply voltage (3.3v or 5v), but nowhere can I find anything on the output amplitude. So, where do you get you get your crystals? What do you consider reasonable specs?