The 74LS112 spec sheet quotes maximum clock frequencies of 25MHz or 30MHz depending on the loading of the outputs. It is going to be decidedly marginal at 50MHz.
The 74HCT112 is only guaranteed for 24MHz clock. The 74S112 is rated for 80MHz minimum. The 74F112 is rated for 100MHz min.
Agreed and thank you for pointing that out. I will not be using the "LS" variant as replacements then. However, the 74HCT112 seems to be rated at 66MHz at a Vcc of 5V?
The "F" variant looks interesting and plenty available from China, but the 74S112 seems to be the available as genuine NOS parts and perhaps the cheapest option on eBay at present.
Digikey and Mouser have both types in stock.
True and thanks for pointing that out, but the drawback for us in the UK is that both Mouser and Digikey have a ?12 shipping cost. Not unreasonable given they ship from the USA, but nevertheless dis-proportionally high for a small order under ?30.
I managed to remove the crystal without any damage to the board using the SMD heat gun. I tested is as suggested with a 470¦¸ load resistor. The output was around 3.5V and stable for several minutes. Without the load resistor the amplitude was over 4V. Frequency is just a smidgen under 50MHz (49.999xxx) on the FC. I hooked it back up to the board using jump leads, powered up the 5V rail but there was no output. I hooked up the sig gen again and got output from both 74S112s. Could it simply be that the sig gen is handling the sink current better than the crystal? Or does the almost 0.8V voltage drop under load suggest that the output from the oscillator is weak? I'm still uncertain whether to replace both ICs and the crystal for good measure?
