Fluke 2100A Thermometer shaky digital display

[enut11]

I have a Fluke 2100A digital thermometer which has a shaky display.
It seems to display the correct temperature with a K type thermocouple and responds when I touch the the TC tip.
However, the display is not stable, jumps around a lot and sometimes all of the indicators light up '88888'.

No search hits so this may be the first time it has been flagged on the Forum?

I have a service manual and checked the 5.2Vdc, 10.5Vdc rails and all is good and within tolerance. The 6.200Vdc Ref voltage is also good.

I re-seated the digital display modules on the front panel. Further inspection showed up several de-laminating ribbon cables so I did not disturb any more boards or components.

[enut11]

The de-lamination on the ribbon cables appears to be cosmetic only and not a problem if they are not disturbed. Makes it difficult to troubleshoot the inside of the instrument though.

Still searching the net for clues on possible causes and fixes for the changing display. Even tried a shorting link on the TC input terminals.

[lowimpedance]

One possible area to check would be oxide buildup in the range push buttons, a little deoxit would improve noisy contacts.
The ribbon delamination is something I have seen in other vintage Fluke gear that use this PCB interconnect method.
Perhaps replace with standard ribbon cable.

[enut11]

I flushed out the push-button switches and left the unit over-night. This morning it still played up with jumpy temperature display. Letting it run for a while to see if it changes.

[lowimpedance]

 Could be a bad solder joint or component package lead (old age oxidization creeping into the packaging), check for dry joints. A little light flexing and twisting motion may show up any bad joints ie make it worse or stop the noisy display. Also careful prodding areas with a plastic stick to localize the flexing. ANy PCB interconnects would be suspect until properly inspected/cleaned.
Any IC's in sockets ?, remove and re seat etc. Also check the supplies for excessive ripple.
Does the issue seem to be thermal ?, see if it improves or gets worse with applying warm air from a hair dryer to selected areas.

[enut11]

Thanks @lowimpedance. When I opened the case again and dismantled the PCBs I found a very sus ribbon connector that literally fell apart when I touched it.

It is from the power supply to the inner PCB and I am hoping this is the cause of the flickering display. The problem is how to test it because the strip socket on the board is a loose connection.

I could try and use sticky tape to hold the metal strips from touching each other or, as @lowimpedance suggested, solder a new ribbon cable.

The second picture shows more original ribbon cables where the insulation is peeling away so this will be an involved fix with 5 in all to be replaced.
enut11

[lowimpedance]

Those ribbons are well past it, best to improvise a new soldered in  ribbon to bypass any dodgy connectors as well.

[Gyro]

As those are (nice) Panaplex displays, you don't want to be taking any liberties with those ribbons - You have a 200V (ish) power rail in there, that you really don't want coming into contact with low voltage signals (especially as the symptom is flickering display!).

[enut11]

Thanks @Gyro, I did miss seeing that HV on the power supply.

The original inter-pcb connectors consist of flat metal strips supported by clear adhesive tape on both sides (branded Ansley Flexstrip).
The adhesive has dried out and the tape is peeling away on all 5 connectors. I have temporarily secured some with ordinary sticky tape until I can deal with them.

[enut11]

These pin cables are a possibility as the pins fit the main PCB power socket. Clearance between PCBs may be an issue.