75C Display Contrast Adjustment?

[cruff]

I've just acquired a 75C, it would be nice if there was a way to adjust the display contrast a bit. I didn't find anything in the manuals. Has anyone figured out a method for a hardware adjustment?

[Sylvain Cote]

(10-06-2014 11:25 PM)cruff Wrote:  I've just acquired a 75C, it would be nice if there was a way to adjust the display contrast a bit. I didn't find anything in the manuals. Has anyone figured out a method for a hardware adjustment?

The LCD of the 75 cannot be software adjusted like the 71B.

The only way to change the contrast is to disassemble the unit and follow the procedure of the HP-75 Service Manual.

LCD electronic design - Service Manual Page 2-29/30
LCD contrast adjustment procedure - Service Manual Page 5-11

I have not tried the procedure, so I would be cautious doing it.
Like any electronic tampering, you can potentially damage your unit.

Best regards,

Sylvain

[cruff]

(10-07-2014 03:36 AM)Sylvain Cote Wrote:  LCD contrast adjustment procedure - Service Manual Page 5-11

Excellent, I had not downloaded the service manual yet. It mentions the adjustment of a pot, R23. I was afraid it might involve the replacement of a hard soldered resistor or two in a voltage divider. Thanks for the pointer.

[Dave Frederickson]

I would be very careful disassembling your 75C. Many 75's suffer from broken posts, like the common 41 problem. I think that the 75's posts may be weakening over time. When I opened mine I found ten broken posts and I think some of those broke when I was removing the screws despite being very careful. My other 75 exhibits signs of broken posts and is being held together by Kapton tape in the mean time.

Dave

[cruff]

(10-07-2014 10:55 PM)Dave Frederickson Wrote:  I would be very careful disassembling your 75C. Many 75's suffer from broken posts, like the common 41 problem.

Mine has two or three broken posts, I'll know more when I open it. I have experience replacing the lower posts in my 41CL with metal threaded standoffs.
Has anyone tried using something like Plast-aid to do this?

[robert rozee]

i'm sure someone must have thought of this before - repairing broken posts by slipping a suitable diameter ferrule over the post to force it back together. ferrules (of one form) are short thin-walled brass metal tubes with a flared end. attached (with a bit of luck) is an example photo.

   

diameters range from less than 0.5mm up to 8mm or so. the length is maybe 1/2 an inch, but easily trimmed. the flared end is to allow slipping over a stranded wire, after which the ferrule is generally crimped in place with a tool. the idea is that it protects the individual strands of the wire from being broken when placed in a screw terminal. ferrules may come with a plastic coloured boot attached, or without (as in the photo).

perhaps something to add to geoff quickfall's book if not already there?


rob :-)