I've had a 1A2 system with a 620A modular panel and a 641A modular panel with a 79B2 power unit using 4 CO lines on the 620A and one MOH on the 641A), which has been working perfectly for several years. Without warning the lamps on all four lines quit working. Additionally ringing on inbound calls rings continuously. I have dial tone on all lines and all telsets dial out correctly (without illuminated lamps) and all sets receive calls (with continuous ringing and no illuminated lamps). Hold does not work (disconnects inbound call after depressing hold button). There is a Melco ICM on the 5th line which continues to work correcting including buzzing appropriately and the lamp lights.

I have checked all fuses multiple times including with a continuity tester even though the pop-out is not out on any of the fuses. All the power unit fuses are tested with continuity tester and don't appear blown). I have rotated all the line cards with no difference in performance. I have replaced the interrupter with others of same model from two other power units. I have moved the interconnect cable from the 620 panel to each of the four jacks on the power unit to insure that the jack is okay. The interrupter motor does not make a sound, although I'm not sure it actually does on this unit. My experience with interrupters is only on a 551B shoebox.

I've exhausted all the tests I know how to perform.

Bottom line: None of the lamps illuminate and the ringer on inbound calls is continuous rather than interrupted.

Any suggestions on how to proceed? 10vac supply fried? Interrupter fried?

Bill Bentsen
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Without even thinking further, did you check the voltages to see if they are all there?

-Hal

Your 10 volts AC output from the 79B2 has failed. This voltage is used for all lamps and the interrupter motor. If you can locate the ST terminal in the upper left quadrant of the 620 panel, touch a known good ground to it. If the motor starts running, then it is you lamp ground that is missing. Again, this is an issue within the power unit itself. You might want to try to common all of the ground terminals on the power supply together just in case.