[MENDED] E4418B Power meter doesn't turn on

[nfmax]

There is a long-standing issue with these power meters that eventually they fail to respond to the front panel power on button. See, for example https://www.eevblog.com/forum/repair/breadcrumb-for-agilent-e4418b-repair-power-button-not-working/msg3468522/#msg3468522. It is associated with the front panel keypad FPCB, (https://www.eevblog.com/forum/repair/hp-agilent-e4418b-power-meter-keypad-flex-circuit-e4418-20007-needed/) and also, curiously with the memory backup battery (https://groups.io/g/HP-Agilent-Keysight-equipment/topic/31675429?p=Created%2C%2C%2C20%2C1%2C0%2C0)

After a year or two of normal operation, it struck my instrument too 

Guided by the various posts above, I dived into it. I found the CLIP, including circuit diagrams, posted online. I have excerpted some relevant parts in the attachment. I found that the power on/off state is maintained by flipflop U34-A, powered by VBACKUP. This is maintained by the backup battery, so the intention is that the instrument will turn 'on' when the mains is reconnected if it was 'on' when the mains was removed. In fact, the whole power switch circuit seems to have been designed to mimic exactly a mechanical switch. You can turn the instrument on & off with the mains disconnected (but why?) The power button momentarily connects VBACKUP, through a 51.1kΩ resistor R51, to the input of Schmitt trigger U33-A, which toggles the flipflop. U33 is also powered from VBACKUP.

With no mains applied, VBACKUP measured 2.86V, rather low. Interestingly, the voltage at ONSWITCH_1 (with the power button not pressed) was only 0.77V, too low to reach the threshold of U33-A, explaining why it wouldn't turn on. In fact, when the power button was pressed, it dropped down further to 0.72V, owing to the additional load from R52. This means that a current of about 41μA is permanently flowing from the battery through R51! Disconnecting the keypad* reduced this to zero, confirming a problem with the front panel. All voltages were measured with respect to the rear panel chassis ground connector, which takes a 4mm plug.

Connecting the mains supply increased VBACKUP to 4.64V, and ONSWITCH_1 to 1.19V, still not enough to turn it on, but the battery voltage, measured at its +ve terminal, rose to 3.02V. Momentarily jumpering a 10kΩ resistor between VBACKUP and ONSWITCH_2 caused the instrument to power up normally (with the battery self test passing!). In this state, VBACKUP measured 5.00V and ONSWITCH_1 measured a few mV higher. Now, the front panel button would turn the instrument off normally.

My diagnosis is that there is a leakage resistance, in the front panel FPCB, between ONSWITCH_1 and some circuit which is ~0V in the off state, but approximately +5V (or maybe more?) in the on state. Probably something to do with the LED indicators. With the instrument off, this gradually runs down the backup battery: there have been reports that replacing the battery can 'fix' the problem. In my case, this obviously isn't enough, and anyway it won't stop the discharge current.

What I did was to abandon the idea of being able to change the on/off state with no mains connected, in order to eliminate the battery drain. I removed R51, and connected a 4.7kΩ resistor between ONSWITCH_1 and +5VPSU (see attached image), which is the 5V supply from before the FET power switch controlled by U34-A: it is present whenever the mains is connected. The voltage at ONSWITCH_1 with mains connected was now about 3.46V, and the power on & off operation was restored 

This fix should also have removed the excess battery drain. The battery current can be measured by measuring the voltage across 1kΩ resistor R168, and in my case this was a satisfactory 110nA, low enough not to bother replacing the battery, which now measured 3.07V. The voltage drop across this resistor explains most of the voltage drop in VBACKUP observed above, when the mains was removed, the remainder arising presumably from the battery internal resistance.

This may not be the universal solution to the problem, but it should give some clues to anyone else in the same situation: much cheaper than replacing the front panel FPCB anyway!

Good luck!

(*) J14 is a latching connector: lever up the two ends of the top plastic part (with difficulty) to release the FPCB

[eb3frn]

Hi
same problem here, battery down and not turn on anymore also replacing the battery for a new one.l

After drop R51 and add the 4k7 resistor, it has worked again!

Thanks a lot!

73! Iban
eb3frn

[dxl]

Interesting. I had exactly the same problem a year ago. I lowered the resistance a bit, which made the voltage go above the threshold. This will decrease battery life in my unit, as i didn't made the modification like you did.

My unit still has the problem that it doesn't reset on power after it was switched of (it hangs in the bootloader). Luckily it resets if i do a quick power-off-power-on sequence (less than 2s). Maybe i'll try your modification and see whether it goes away.

The Reset issue is not caused by the Supervisor circuit - i replaced the MAX710, but that didn't help.

Thanks for your hint, i'll try it.

[nfmax]

Hi
same problem here, battery down and not turn on anymore also replacing the battery for a new one.l

After drop R51 and add the 4k7 resistor, it has worked again!

Thanks a lot!

73! Iban
eb3frn

You’re welcome!