[Test-Equipment] Question Regarding Fluke 8050A

[Barry]

I have a Fluke 8050A that's showing early signs of LCD disease.  I know there are several options out there for retrofitting this with 7-segment LEDs, programmable displays, etc., but my thought was to use some HP 5082-7032 "dot-style" LEDs (the kind with the built-in decoder/latch that's seen in gear like the 8640B, 8655A, etc.).  Instead of using the existing decoder/latch ICs, these can be driven directly from the 4-bit data lines and strobe them with the existing strobes (just need to invert those) and powering them from the existing -5V line.

I put together a breadboard with the last two digits working in conjunction with the LCD and all was fine.  When I added the third display, the system stopped working.  The LCD began to show all zeros and the LEDs were showing somewhat dim zeros as well.

It turns out these little LEDs are pretty power hungry and adding the third one was enough to drag the Vcc line down to about 3.5V as well as causing that power rail to start oscillating significantly.  Looking at the specs, those LEDs need 170mA in a "worst case" scenario (depends on how many individual LED dots are needed - assuming an "8." is worst case) with typical consumption at 94mA.

The -5V rail is delivered via an LM7905 and my first thoughts were that the additional power required for these LEDs were too much for that (along with everything else it's being called on to provide power for).  Would that be a reasonable assumption?  If so, I'm thinking I could incorporate another 7905 just to supply these LEDs; however, that gets into whether the transformer can handle the additional load (counting on needing about 3/4 amp more for all these new displays) and I don't know the specs on the transformer.  Of course, I can lose the old decoder/latches and the LCD will be gone so there's some power saving there but I think that may be small compared to these LEDs.

I can always go with some of the previously-mentioned options to retrofit this thing but I really like those HP displays and they fit perfectly in the limited space.  I'm thinking I'm approaching the limit of what the transformer can be called on to deliver and so may be wasting my time but not sure.  Any thoughts?

Thanks,
Barry - N4BUQ