Sometimes you are your own worst enemy.
Case in point. The Tek 2430 DSO. I showed the other day a successful repair by replacement of the entire analog board from the parts mule. It was fine until the very next day when I went to power it up and it started taking solid channel 1 gain and offset fails again. Same fail as from prior to the board replacement and the same as the week before when I had the same condition occur. Repaired but next day upon power up it failed same as before. The fail points squarely at the CCD hybrid module and initially replacement didn't fix it but now I know why.
So in this latest fail I decided once again to change the channel 1 CCD hybrid with the channel 2 CCD hybrid from the board I removed. It worked. Clean boot, cal done. Works fine. Put it back in the case and powered it up and let it burn in for about 6 hours sitting on the bench. No issues. Powered it off and let it sit for a while. Powered up and again clean boot and works fine. This is really baffling. So I go to put the 2430 back in it's original location and that's when the light bulb came on and I realized I was a jerk.
The 2430 normally resides stacked on top of a 2465. After each time I repaired the 2430 and I was satisfied that the burn in was OK I would put it back on top of the 2465 then run it more. It was from this point where upon the next power up it would fail again. The 2430 is about 2 inches longer than the 2465 so it over hangs in the back and is not supported by it's rear feet. So towards the rear the bottom of the 2430 case is directly against the top of the 2465 case. What I didn't realize was there are vents in the bottom rear of the 2430 case directly below these hybrids. With it being stacked as described the vents were blocked. Obviously this caused the hybrids to run hot. I surmise that they were fine, although probably near their thermal limit, as long as powered on but as soon as power was turned off and the fan stopped their temperature spiked before cooling which resulted in damaging them. But you wouldn't know this until the next power on. Mystery solved. But unfortunately I damaged 2 precious hybrids and the last good one is currently installed. I've since rearranged things so the 2430 is now up on it's bail and all underside vents clear. It's been operating fine for 3 days now with multiple power cycles and no fails. I think I can call it fixed....finally.
The hybrids that were damaged are the 2 large ones with the full vertical fins. Channel 2 top with channel 1 below it. Tek 165-2074 version 3 and 4. It's obvious just looking at their design that they run hot. I'll bet with the vents blocked they were smoking. I did some research and they ain't cheap when available. Starting at $45 USD and up. And I also found out that they are a known fail point. So there's no guarantee if you buy one it's going to work. So I have to make absolutely sure that this 2430 is kept cool. And now I understand why it has such a large and somewhat noisy fan.