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Hello to all. I changed out 2 center bulbs in an HVAC SE control and now I can not get the bulbs to work! I put a 12v load on the PC board and all four light up just fine. When I plug the unit in, the lights will not light!!! Is there anything that anyone knows of outside the HVAC control that would not allow the lights to light? In line fuse? Any help would be appreciated! Thanks

There is nothing outside of the HVAC control that would prevent only two of the bulbs from illuminating. Did you replace them with incandescent bulbs or did you try to convert them to LED?

Thanks for input. Replaced with incadescent. The boards are old and fragile I may have broken a board conductor . I can apply a 12 v source to
The bulb circuits on the back
Of the bulbs and they light up . It seems hard to clean the holes outmatch soles to install new bulbs.

I would have a close look with a magnifying glass and check for both solder bridges and damaged traces. The smaller the trace, the more susceptible they are to damage from heat. Technique, the correct wattage soldering iron, de-soldering braid (or other appropriate de-soldering aids) and the correct solder are all required to consistently make these types of repairs. In most commercial settings an electrostatic discharge workstation is also used to prevent damage to any components sensitive to static charges.