So, where were we ?

As you may or may not know, we spent the last couple of years building a new house and moving into it.

Now that things are settling down, I’m slowly getting my lab organized and the HP5065 got on the table a couple of days ago because showing zero photo-current is important.

After some rudimentary debugging which always ended up at the dead end “Replace A12” in the manual, I consulted Corby who told me to check the lamp.

I don’t have a good macrolens, so it is hard to see on this picture, but neither wire to the coil on the bulb were in solid contact with the rest of the circuit:


I’m not sure what the fundamental cause of failure is, it has run fine for a couple of months in our new house, but I managed to get contact established and I now have light again:


The lamp current is a bit on the high side according to Corby, and I’m pondering what to do about that.

A12A1R3 is a 10R nominal resistor which can be used to adjust the working point of the lamp exciter, and increasing it a couple of Ohms would be the traditional cure.

Another option is to decrease the lamp voltage and that made me wonder if there is a potential for stability improvement here, for instance by stabilizing the lamp supply based on the photo-I signal out of A7.

First I am going to measure photo-I, A12A1Q1 temperature and maybe S/N as function of lamp supply voltage/current.

More on that later.

Along the way I found and replaced this beauty in the A3 assembly:


A3 got a tune-up while I had it open and the AGC voltage came up quite a bit higher as a result.