Remake of the Hewlett Packard 11456A Read Out Test Card

The HP 11456A Read Out Test Card is a nifty lit­tle test board that plugs into the HP 3470 series dis­play mod­ules. I real­ly wish that I had had one of these, before I start­ed work­ing on my faulty HP 34740A dis­play mod­ule.

There are a few old pic­tures of the orig­i­nal 11456A Test Cards to be found on the inter­net, but find­ing an actu­al phys­i­cal one seems next to impos­si­ble. But to check and trou­bleshoot a stand­alone 3470 series dis­play mod­ule with­out a plug-on mod­ule attached can be very dif­fi­cult with­out one.
It can be done with a mul­ti­tude of jumpers, but even then many of the test-points still are not easy to access. Many of the trou­bleshoot­ing tech­niques in the ser­vice man­u­al are based on uti­liz­ing the 11456A Test Card.

I decid­ed to build my own ver­sion of the HP 11456A test card just in case I need­ed it for future repairs, and also because I was curi­ous about how the card worked to oper­ate the dis­play mod­ule in stand-alone mode.
I used EagleCAD V7.7 for design­ing the board, and had it fab­ri­cat­ed by OSH Park.
In all actu­al­i­ty, it is a fair­ly sim­ple board with fixed jumpers for pow­er and sig­nals, test points, four switch­es, and a resistor.

I was unable to find the exact val­ue used for the resis­tor R1 used between the Direct Input and the Leakage Test Input. So I guessed at a val­ue of 10 Megaohm for the resis­tor. The ser­vice man­u­al states “A dif­fer­ence of more than 2 counts in the zero read­ing when these pins are con­nect­ed to GND 1 indi­cates a leaky FET.” and when test­ing the 34740A dis­play using the 10 Megaohm resis­tor I saw a dif­fer­ence of 1 count, jump­ing to 2 counts infre­quent­ly. So I guess that my FET is most like­ly not leaky, if the resis­tor is correct.

Edit: Thanks to Jim Adney, after check­ing his orig­i­nal 11456A board, I now know that the val­ue for R1 is 3.9 MOhm (orange/white/green/silver).

The com­po­nent list for the 11456A Test Card is short, and includes:

• 4 each ATE1D-2J3-10‑Z SWITCH TOGGLE SPDT 50MA 48V
• 30 each 2110–2‑00–80-00–00-07–0 TERM TURRET SINGLE L=5.84MM TIN
• 1 each 112–050-213R001 CONN SCSI RCPT 50POS R/A SOLDER
• 1 each MBB02070C3904FCT00 RES 3.9M OHM 0.5W 1% AXIAL

To keep costs down I designed the lay­out to min­i­mize board size, while group­ing test points by their func­tion, and allow­ing just enough spac­ing for con­nect­ing an oscil­lo­scope probe. It is not an exact repli­ca of the orig­i­nal test card as far as lay­out and size, but pro­vides all the func­tions of the orig­i­nal.
I did get the switch label­ing invert­ed for High and Normal from the actu­al phys­i­cal switch posi­tion on the Rev. C ver­sion, but that is easy to fix with a stick-on label.
The HP 3470 dis­play mod­ules can be worked on with­out this card, but the 11456A sure would have made trou­bleshoot­ing and work­ing on these dis­plays so much easier.

Eagle CAD V7.7 board and schemat­ic ZIP file for TestCardRevD
With updat­ed switch and test point labeling.

OSH Park shared project page for order­ing the Rev. D 11456A Test Card