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Rosemount 400A components get hot on Main Electronics Board

Affected components:

  • 620428 Rosemount 400A Main Electronics Board (schematic 620429)
  • 620433 Rosemount 400A Isolated 4-20 maDC board (schematic 620434) … (item is now obsolete by OEM … replaced by RIGAS 25C0007R0)

Situation: The analyzer appears to operate normally (mostly anyway). U13 seems to get very hot (so does U4 [voltage output buffer amplifier]) and there is a 620433 (V/I option board) installed. Also, when attempting to light the analyzer, the analog display will overrange and remain overranged until the power is cycled on the analyzer (TP-5 will be saturated at about 13 vdc).

Problem: someone has employed the E1-E2 and E3-E4 jumpers and this is causing a nasty feedback loop that U13 is trying to compensate for. When the analyzer goes upscale (it always spikes during startup) this causes the analog signal to spike which, in turn, causes the 4-20 maDC card to spike. The 4-20 signal being fed back to U13 has now locked it railed high.

Resolution: remove the 4-20 maDC board or remove the E1-E2 and E3-E4 jumpers.

Other info: U12 & U13 & U4 are µA714 opamps (a.k.a., uA714); they can be replaced with OP07 or OP77 or OP177.