.. _hp5065_back:

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:

.. image:: A12A2-1.png

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:

.. image:: A12A2-2.png

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:

.. image:: A3C37.png

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

*phk*