I have been modifying my Howtek 4500 and have run into a spot where my lack of attention in Electrical Fundamentals class has caught up with me.
The main change I have made is to use some more modern PMTs. The 931b units that come stock in the Howtek really fall off in sensitivity in red. Going to a multi-alkali style fixes that and provides a lot more signal overall to work with.
The problem is the new PMTs are too good, I lose the bottom half of my histogram. Scans (Velvia included) bunch up so far to the right that even with DPL Pro I can't get full use of the 12 bits available to me. The endpoints will not adjust far enough.
I think the best way to fix this is to reduce the gain in the front end where the current to voltage conversion takes place. Some other fixes that seem less desirable (or harder) include reducing the available light or modifying the circuit that adjusts the lower endpoint of the scan.
I have included a schematic of the front end that I came up with by inspection and a multi-meter, I think it is accurate. I also have a photo of the circuit board included if that helps.
The op amp is an OPA 602 and I have a machine where I swapped those out for OPA 827s. I don't know if the 827s helped because I swapped the PMTs at the same time.
I would like to shoot for a 5x reduction in gain as a starting point. I think the best way to do that would be to change resistors but with my limited experience I am unsure where to start. Can anyone help me figure this out?
Andy
Bookmarks