Hi. I'm reluctant to resurrect an old thread, but I have a 289 here with the exact same HV burn dmg.
As many before me I have difficulties finding the exact same parts as the Fluke originals.
Tried random varistors, nope => crap. High capacitance range zero reading (~0.250nF) and resistances were out too for some reason, despite the varistors measuring over 1 Gohms.
So I'm thinking the originals must be very low capacitance parts.
Without varistors and with 1/4W 1k resistors in place of the PTC-s the cap zero reading w/o leads is 0.085nF. Dunno if that's ok or not. Other than that everything, including cap(at least in uF and up) range is in spec
So my question:
would it be bad idea to use GDTs (like Bourns 2095-80-BLF) instead of MOVs there? Coud it be that the right MOV with the right capacitance could be somehow compensated upstream to cancel out their impact? Like, no MOV is sub pF anyways, right?
On the other hand GDT would be slower and 'clamp' harder. Perhaps not a good idea with the Low Z function.
Thoughts?
Thanks ahead!