Hello:
This is long. Hopefully it won't bore anyone. Maybe someone else will find it interesting...someone else who doesn't accept the frequent 'no such thing as a free lunch' answers...
Thanks for humoring me.
I have a Tektronix J16 photometer that came with a J6502 (I think) probe, the 8 degree luminance probe.
When I got it prices were still out of my league for the J6523, the 1 degree luminance probe, but I recently acquired one.
If you don't have time to read a long post, what I am MOST interested in finding out is what device is used for the photodiode in the J6523, if you have that information (TEK or other p/n).
Someone got me a J16 option 2 (metric?) manual copy, and it leaves me with a few questions. I'm hoping someone here is familiar or has relevant manuals.
The J16 option 2 manual has a schematic that shows The J6501 through J6505 probes and their respective pinouts. The unit 'detection' for each probe appears to be selected by which probe 'unit circuit' is connected. The photodiode in called CR310 in all the probes.
My J16 displays ft-lamberts rather than metric (cd/m^2?) that the Option 2 one does.
I do not have the 'normal' J16 schematic and wonder if there are different gain resistors or different scaling resistors for the DMM portion to accomplish displaying in different units.
I have no J6523 manual. I asked someone selling a J6523 manual on eBay if it had a schematic or part number for the photodiode, and he said it had neither.
I am trying to find a Tektronix or OEM part number for the CR310 photodiode to learn more about it's characteristics (if published anywhere).
I have been thinking about how I might get another decade of 'sensitivity' out of the J16. I think it reads down to 0.1 ft-lambert.
The simplest thing I am going to try is a 6-1/2 digit DMM (Fluke
8506A..or maybe 8605A) connected to the access point used for calibration and see how stable the additional digits are. Maybe I can trust 0.10 in stead of 0.1 ft-lambert. The meter has averaging and filtering...no harm in trying.The next thought was to switch in a different gain resistor in what I assume is a transimpedance amplifier the photodiode connects to. Unfortunately the lowest range uses a 100M resistor and parallel 2200 pF cap. All but one range resistor and one range capacitor scale by a factor of ten, so I assume 1Gohm and 220 pF. Assuming I can find a suitable resistor, I'm leery about switching such a high resistance, leakage, etc. Maybe a TO-5 relay...
I haven't quite figured out the J-16 input circuit yet...it's a little more exotic than textbook. It's surprisingly simple...a 741C with a dual JFET at the front end. One gate is grounded. It looks like half a diff amp but I don't know how that interacts with a photodiode transimpedance stage. It's not anything I've ever seen before. I should post a sketch of the input. Both the jfet and op amp are connected to dual supplies.
My guess is they just made a discrete jfet-input op amp...it must be really old if there were no suitable op amps (nor anything more exotic) available.
I am not in a hurry to change the J16 input circuit...just understand it and what it might take to get more sensitivity out of it...a change may come later if there is a solution that drops in with minimal butchery...it ain't broke, so I'm not ready for 'fix' it yet.
Thank you
Murray Leshner Holland MI