HPAK 8642B Synthesized Signal Generator: backlight replacement

The 8642B is an excellent generator, very clean, at least at offsets >1 kHz, hard to beat. It is also very heavy, thanks to a special modular concept that HP was pushing at the time. Their intention was to make the unit more serviceable, with the result that the generator is super heavy, and so expensive that it never was a real commercial success for HP. Frequency range is from below 100 kHz, to 2115 MHz. Pretty useful, with amplitudes from -140 dBm to 20 dBm.

The generator has a rather large (for the time) LCD display – fully story can be found in the HP Journal, December 1985.

backlight assy

backlight detail

The backlight has a very thoroughly designed light diffusor, which directs light from two 5V axial bulbs evenly to the LCD. Sure enough, these bulbs can burn out.

Some webpages claim that such bulbs would only last a few 100, maybe 1000s of hours, but such statements are incorrect. The bulbs used, 5 V, 115 mA, will typically last about 40000 hours, much longer than common household light bulbs.

t-1 axial lamp

These little bulbs have 1.9 Lumen each, not bad. To replace with a LED, 14000 mcd, at 25 deg angle, are about 2.2 Lumen. Close enough.

Found some 3 mm superbright white LEDs:

ligitek superbright 3 mm white

0.04 USD each!! Amazing!

Also these won’t least forever, white LEDs do lose intensity over time, like, 50% remaining intensity after 20000 hours.
I decided to run them below the rated current, at about 16 mA (120 Ohm series resistor with 5.2 V supply). Maybe this will make them last a bit longer.

Note that the backlight is software-controllable (special functions 134 and 234). This is how it is implemented (let me know if you need to full schematics of the 8642A or 8642B):

8642b backlight driver

The resistors (1.8 Ohm) make sure that the lamps operate at 5.0 Volts, not 5.2 Volts, and don’t interfere with the operation of the LEDs. Quite amazingly, running at 5.2 V vs. 5.0 V would reduce the life span from 40000 hours, to 25000 hours!

5-0 volts bulb
5-2 volts bulb

Agilent sold these bulbs for about USD 18 each!!

No mechanical modification of the 8642B at all, the LED and the resistor fit well into the cavity (don’t worry about the lens of the LED – the light will find its way.

8642b backlight mod

The result:

8642b backlight uneven

– not to my full satisfaction (non-uniform brightness, looks a bit dark on the left hand side, albeit, very easy to read).

After a bit of head scratching – turns out I soldered in the left LED with incorrect polarity – so it can’t work.

With this little mistake corrected, all is good:

8642b backlight

Now, let’s hope that these LEDs will last. Never mind, I have a bag of spares!

Note: the display assembly is said to be rather ESD sensitive. Make sure not to damage it!

2 thoughts on “HPAK 8642B Synthesized Signal Generator: backlight replacement”

  1. Nice and easy,
    Any body make this “upgrade ” on HP8904A (synthesizer), backlight on this 8904A is in fact a kind of neon supplyed with high voltage …. beuuuh 🙁
    In my 8904A this backlight is dead !
    So anny experience is welcome 🙂 i want to keep my device in live with a nice vision of the display …
    Regards, Phil VR

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.