HP 8642B Backlight Mod and Easter Egg

[AlphZeta]

I know that there was already a thread (https://www.eevblog.com/forum/testgear/teardown-hp-8642b/msg409957/) where @idpromnut showed some pictures of the internals of his HP 8642B signal generator. Since this one is mainly concentrated on the backlight, I decided to start a new thread.

As always, if you want to see all the details and the high-res photos, please see my blog post (http://www.kerrywong.com/2014/09/01/hp-8642b-backlight-mod/). I have some additional teardown pictures there.

Anyway, since the backlight of my HP 8642B had long burned out, I finally decided it was time to replace them -- not with the incandescent ones but with two white LEDs instead. Here is a picture of the backlighting after the mod:


I also found what appeared to be an "Easter egg" in side the firmware of the HP8642B. Basically if you hold down MSSG button and then power on the unit (with MSSG remain depressed) you will be greeted with the following message after a few seconds:


Not sure if this Easter egg is present on other models (e.g. HP8642A) or whether it is firmware version dependent. The firmware version of mine is 871028.

[AlphZeta]

John Miles from (http://www.ke5fx.com/) just informed me that

"AEL = Albert Einstein Lassiter, one of the firmware authors. See the articles in the December ’85 HP Journal: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1985-12.pdf"

Very interesting indeed.

[CatalinaWOW]

Thanks for the info on an LED replacement.  I just completed mine and it works great.  It is really a pleasure to be able to read the display easily.  I connected to the original pins so the ON and OFF commands still work.

Some additional information for those who choose to do this on their 8642, and who don't have access to the full service manual.

a)  The five screws which need to be removed to hinge open the panel are in holes 1 and 13 on the top, and 3, 8 and 12 on the bottom, both top and bottom counted from the right.
b)  There is really very little clearance for the bulb/LED.  As stated careful sanding is required, and you must get quite close to the header inside.  It would be worthwhile to start from a 3mm size instead of the 5 mm size that the original author used, and that I used.  I have found one LED in my junk pile that actually fits as manufactured, but haven't located a source to get more.
c)  The end caps can be trimmed internally to buy that last fractions of a mm needed to fit the LEDs
d)  Lamp common (on the side of the board which has the cut-outs to clear the MSSG and Local switch) is the positive side at zero volts.  Lamp1 and Lamp2, the other end is at -5V.  This tidbit will save you once through the replacement process if you assume, like I did, that the common is the low side.