First posted to TekScopes at groups.io
The 5D10 Waveform Digitizer plugin, essentially a 1MS/s DSO, gets some credit for keeping the 5000 series oscopes interesting. While a 5D10 can digitize one or two channels from another plugin, only the "slow" 51xx scopes supported that capability. 5000-series docent Christian Weagle tried to adapt a "fast" 5440 with initial success that didn't pan out. I fell down this rabbit hole and eventually climbed out with a good result for my 5441. The same should work for a 5440 also - i.e. 5403 mainframes.
To enable LEFT plugin access for a 5D10 in a 5403/5440/5441 scope:
add three connections between plugin connectors on the interface board.
1. left A13 <---> center A17
2. left B13 <---> center A16
3. horizontal A19 <---> adjacent DIP14 7473 pin 6
The two left <---> center connections give the 5D10 access to the differential signal from the left plugin bay. That alone allows acquisition of a single LEFT channel when the left plugin DISPLAY switch is OFF. A pair of 10cm F-F "dupont" jumpers made this connection trivially and reversibly.
The horizontal A19 <---> 7473 pin 6 connection lets the 5D10 keep control of the display when the left plugin DISPLAY switch is ON. Then the 5D10 can cycle the channel select logic to sample both channels from a dual channel plugin without chopping up the display.
Mr. Weagle offered a "reasonable guess" that Tek didn't support this configuration because adding extra load to the signal path would blunt high frequency response of the left plugin bay in normal operation of the faster 54xx scopes and later attributed disappointing results to hitting that problem hard. After much pondering how to deal with that, I eventually got around to trying it to assess the magnitude of the problem and found… no problem. By not-very-careful measurement, the added load attenuates a 6div 50MHz signal by a fraction of a trace width at most and a 100MHz signal by approximately not much. The mild loss of far-above-spec bandwidth from the left bay is good to know but I won't worry about it.
Beware. While the added jumpers themselves didn't bog down a 54xx-speed left plugin, Tek cautions that a horizontal plugin in the center bay certainly will. The manual says "nondestructively". The manual also only contemplates plugins compatible with 5100 series scopes. I haven't tried to work out if any of the faster plugins could be more vulnerable.
The procedure for modifying elder 51xx mainframes advises adding some resistors. 54xx scopes already have them.
Schematics for 5116 and 5111A (after the first 206 units) show ferrite beads on the left <---> center connections. Considering that no ferrites are advised for updating older scopes and Tek apparently shipped a couple hundred 5111As without them, I suppose that their potential benefit is less than compelling.
I have an Option 1 scope without the readout board. That allowed easy access to the back of the plugin interface board. For scopes with readout, removing the readout board may be helpful or necessary. The manual for installing the Option 3 custom readout wiring describes how to pull the readout board from a desktop scope by partially separating the back ends of the display and mainframe modules.
Doing this with a 5444 might be as simple as adding the two signal jumpers and turning the left beam intensity down. ?.
Working this out involved some side trips that might become future posts, but that will be for another day.
Please tell us if you verify this working in another 54xx scope.