I have a couple of spare Tempest boards I am working on and when I hook my oscilloscope to the X/Y loops I get a real mess. Not readable and not recognizable. The boards will run in test mode however I cannot tell what it is reporting. The X output is stuck at +10v on one and +4v on the other. I traced the circuit back to the DACs however the input to the OpAmps are basically zero. Is there a way to scope the output from the DAC? I get no reading with a DIMM or scope on the input to the OpAmps for X/Y from the DAC however the outputs look a lot like the individual channels. It looks like the output is bad at the first stage where the DAC is sampled and held in the capacitor. I have checked the inputs to the DAC and none appear to be dead or stuck using a logic probe.
Given that the DACs are not cheap and I don't want to start just shotgunning the board hoping I will find the right chip. I want to understand what is going on and how to fix it. The X BIP does nothing however the Y BIP does causes the output to change in the Y direction. Which chips are most likely to fail? DAC, OpAmp, switch or multiplier? Let's not even mention the silicon VDR which seems to be impossible to find.
I have managed to fix one board set however it had physical damage to the R/W line from the CPU. It was fairly simple to trace back from the LS244 to the CPU with a logic probe.
Thanks in advance,
Kirk S.
Given that the DACs are not cheap and I don't want to start just shotgunning the board hoping I will find the right chip. I want to understand what is going on and how to fix it. The X BIP does nothing however the Y BIP does causes the output to change in the Y direction. Which chips are most likely to fail? DAC, OpAmp, switch or multiplier? Let's not even mention the silicon VDR which seems to be impossible to find.
I have managed to fix one board set however it had physical damage to the R/W line from the CPU. It was fairly simple to trace back from the LS244 to the CPU with a logic probe.
Thanks in advance,
Kirk S.
