I am working on an HP EPM 441A which seems to be in good shape except that it doesn't want to turn off. Pulling the plug out and then plugging it back in right away just turns it back on again. The switch acts like it is dead, except I know it is not: If I hold it down while plugging in the ac power, the unit will enter "stand by" mode as it should, and then the power switch turns it on when pressed. The switch is described in the Service Manual (it is a "soft" switch which turns the 3 power supplies on or off). Also,. if left unplugged for a longer period of time (like overnight) the switch works normally the next time...but only once! The problem comes right back the next time I use the switch. I thought it might be a backup battery problem, but the battery reads 3.1 volts, which I think is OK. The service manual has no schematics, and I am hoping that someone on this forum is knowledgeable about how this circuitry was designed and how it can be troubleshot. For example, there is the word "reset" screened on the Processor Board near the battery, but the manual does not mention it and I can't tell exactly which circuit pads are associated with the reset function!

Hope someone can help! Thanks

Vic

Peter, good idea! But no, I looked and don't see any mention of power on options in the configuration menus. That is how it acts, though, and these were used in an untended system rack type of situation, from what I have been told. But I just dont see any settings for anything involving turn-on or standby.

Too bad! ?I wonder if there is a hidden menu or only settable over GPIB. Seems more likely a fault of some sort though.?

I have one of these so if it gets to the point of internal measurements I can help.?

Peter

On Jul 29, 2019, at 8:56 PM, victor smith <vcs3@...> wrote:

Peter, good idea! But no, I looked and don't see any mention of power on options in the configuration menus. That is how it acts, though, and these were used in an untended system rack type of situation, from what I have been told. But I just dont see any settings for anything involving turn-on or standby.

_._,_._,_

The battery seems to be fine, over three volts. And besides:

I made some progress on this thing. Knowing that I was looking for a logic
shift whenever the power button was pushed, I found the line on the front
panel connector that showed a state change whenever the button was pushed,
and after several minutes of probing around, discovered the Schmitt
Trigger IC on the main board that was toggled by that line. I am lucky,
because I have a second, working power meter of the same type. So I was
able to determine that the problem was that the "bad" unit is only
shifting the Schmitt input between 0 and 1.6-1.7 volts. A good unit shifts
between 0 and 4 volts. I can turn the unit on and off at will with a 1k
pull-up resistor applied to the Schmitt input pin. That tells me the main
board part is probably ok. So it looks like I will probably need to pull
the front panel off to investigate why the level coming back to the main
board is so low. Resistances all seem the same comparing good and bad
units, which is puzzling. But since I really am working blind, and there
are more than one power circuits (with separate ground/return paths), I
may just not be measuring at the right places. With the panel off, I can
try to trace the paths for that button and figure out if it's a bad
contact (I assume carbon rubber pad to circuit trace type, just based on
the feel of it) or some other issue. The cable and connector look OK.

Wish I had a schematic.....

Vic

?

Tom,

Thank you so much. You have confirmed what my own investigation led me to suspect. Long Story Short: I did take off the front panel (always fraught with ways that things can go wrong, I always think something is going to crack) and then managed to carefully remove the LCD Display subassembly, revealing a really hard to disassemble metal shield covering the switches. At that point I decided that rather than break a good power meter trying to repair the front panel, the better part of valor would be to put everything back together and start looking for a replacement front panel!

The good news is that I did manage to reassemble everything without damaging the delicate ribbon connectors, etc.

And now you have confirmed with your experience, thanks telling me about the irreparable switches! Now I don't feel so bad!

As far as that manual, (the E4418) The ones I see on the Keysight site do not have sub-assembly parts lists or schematics, just the final assembly parts. Is there a manual somewhere else with schematics?

I am now in the market for a 441A front panel, (or a 4418 panel), if any group members know of one!

In the meanwhile, I have a nice power meter that needs to be unplugged after use!

Victor,

The front panel?€?s on those meters are very easy to take apart and
re-assemble. It only takes a few minutes. I do it almost every day as I
work on these meters. If you need a step by step directions let me know
and I would be happy to advise.

Dave W6TE

From: [email protected]
<[email protected]> On Behalf Of victor smith
Sent: Tuesday, July 30, 2019 9:30 AM
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] Need help with HP 441A Power
Meter not turning off (or sometimes not turning on)

Victor,

The front panel?€?s on those meters are very easy to take apart and
re-assemble. It only takes a few minutes. I do it almost every day as
I work on these meters. If you need a step by step directions let me
know and I would be happy to advise.

Dave W6TE

From: [email protected]
<[email protected]> On Behalf Of victor smith
Sent: Tuesday, July 30, 2019 9:30 AM
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] Need help with HP 441A
Power Meter not turning off (or sometimes not turning on)

OK Tom,

I'll work up a little word document today and post it with some photos.

Dave W6TE

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Tom B
Sent: Tuesday, July 30, 2019 9:40 AM
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] Need help with HP 441A Power Meter not turning off (or sometimes not turning on)

Hi Dave,

If there is an easy way to take the front panel apart I would like hear about it. I have found it to be a real pain. Please send me your instructions.

Tom Bryan

Yes, Dave, Please provide that info, it would be of great interest!

Tom, I guess the fact that you were able to find a replacement panel means that you never found the actual cause of the switch failure, am I right? If they are metal contacts, I wonder how/why the voltage is coming back to the schmitt trigger but at a level which is just below the threshold voltage to toggle it...What would reduce that voltage, I wonder?

_._,_._,_

OK Tom,

I'll work up a little word document today and post it with some photos.

Dave W6TE

-----Original Message-----
From: [email protected]
<[email protected]> On Behalf Of Tom B
Sent: Tuesday, July 30, 2019 9:40 AM
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] Need help with HP 441A
Power Meter not turning off (or sometimes not turning on)

Hi Dave,

If there is an easy way to take the front panel apart I would like hear about it. I have found it to be a real pain. Please send me your instructions.

Tom Bryan

?

This is a great discussion, and I am grateful to each of your inputs!

I would very much like to see a procedure for taking apart the front panel of the 441 / 4481 power meters. I am not at all confident I can take it apart to replace that switch and get it back together as a working unit again! Dave, I am looking forward to your step by step guide.

Tom, I have had more experience (and am fairly comfortable) taking apart the 34401a front panels, if you want to contact me off list we could perhaps have a talk about where you are getting bogged down. Shoot me an email.

And lastly, as I said, i ended up discovering that in my case, disconnecting the front panel ribbon cable allowed the switch supply voltage to come up to 5 volts. Connecting the panel pulls it down to 1.6 - 1.7 volts. My contacts seem to be ok (based on ohmeter readings) but there is not enough voltage (measured going in to the front panel) to switch the schmitt trigger to turn the unit on (or off). As you say, Tom, there may well have been some liquid involved, but in my case it's not just the power switch. I noticed that pressing other buttons caused slight changes in the switch supply voltage, even though they really should not.

By the way, those schematics really helped me with this.
By the way a friend of mine told me this afternoon that he also has a power meter with a faulty power switch, and he agreed to send it to me to take a look at it. So more data to come!

?Vic