content/hp8662a_2.rst

===============================================
 HP8662A Synthetized Signal Generator - Part 2
===============================================

:Author: David Douard
:Category: Electronics
:Tags: HP8662A, repair, test equipment, HP, Generator, RF
:series: HP 8662A
:series_index: 2

This is the part 2 on my series about my HP 8662A Signal Generator.
After fixing the PSU, several more problems needed to be addressed:

- a few sloppy keys on the keypad,
- the know did not work at all,
- and more importantly, some very unpleasant spectrums at some frequencies
  (covered in `part 3 <{filename}/hp8662a_3.rst>`_),
- some output level accuracy problems.


Keypad
======

The keypad was, as allways with HP devices from this period, very hard and
"clunky" for most keys but 2.

No surprise: these keys were missing their famous (well, for some at least) spring:

.. image:: {static}/images/hp8662a/key_missing_springs.jpg
   :alt: Picture of the 2 missing springs in the kayboard
   :class: image-process-large-photo

As you can (not) see, the "Status" key as well as the "Increment Set" key have
no spring at all.

While I was cleaning the unit, I've been half-lucky and found one of the 2
missing springs. But I had to figure a solution for the other missing one.

I thought about:

- buying a replacement key: the ones I found are ridiculously expensite,
- buying a front panel for an HP gear from this era: might be a reasonable approach,
- looking for ready made springs: could not find,
- asking some custom made ones from a company making springs: got a few
  responses, one of them quite reasonable even for small quantities (between 3
  to 5€ each).

While I was looking for a solution, I also gave a try to a simple solution: cut
a replacement "spring" from a plastic wrap. I tried several wraps (of several
widths), and I found one that seems to work just fine, and the feeling is about
the same as original keys:

.. image:: {static}/images/hp8662a/key_hack.jpeg
   :alt: Picture of the spring replacement hack
   :class: image-process-large-photo

.. image:: {static}/images/hp8662a/key_hack_anim.gif
   :alt: Animated picture of the spring replacement hack

I cleaned all keys using some deoxit contact cleaner which also lubricated
those noisy springs making the keys nice ans smooth again.


Rotary encoder
==============

The rotary encoder was not working at all, and once again, a classical failure
for those old HP gears: the small bulb used as light source for the optical
encoder was dead.

I tried to replace it with a 3mm LED I had in my junk box, but it did not work,
so I bought a few replacement bulbs on ebay, and sure enough, the encoder is
now working ok (not perfectly, there are some missed steps: there are two small
screws that look like adjustment screws, but I'm not sure how to tune them).

.. image:: {static}/images/hp8662a/encoder.jpeg
   :class: image-process-large-photo
   :alt: Pictures of the rotary encoder