Author Topic: Agilent Infiniium 54853A DSO won't power on  (Read 8209 times)

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Offline JackM

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Agilent Infiniium 54853A DSO won't power on
« on: April 13, 2016, 12:22:57 am »
At my place of work I have an Agilent Infiniium 54853A DSO which does not power on after pressing the front panel soft power button. The oscilloscope has been sitting unused for quite a while, for possibly more than a year, but I don't know when it was last confirmed working.
It was actually sent to Agilent/Keysight for RMA, and after paying the $300 repair assessment fee, Agilent said that it needed a replacement power supply as well as a calibration afterwards. The repair and calibration cost was quite exorbitant (roughly $5300 CAD), so it was decided that we would just ship the scope back to us without having Agilent perform the repair.

Here's where things get interesting. I decided to open up the scope myself and have a look at it. I tried searching for a service manual for it, but I could only find a somewhat helpful troubleshooting guide, and one other service guide that appears to be same guide, only published more recently. I removed the power supply (after removing the whole PC motherboard first!) and checked the output voltages. Luckily there was a label on the power supply which showed the pinout for the various power headers, plus the aux header which had remote inhibit pins plus 5V standby power on it.

With nothing else connected to the power supply except AC input, the fan on the power supply spins up at a slow speed when AC power is applied. I checked the output DC voltages on the power headers at this point, and got no voltage outputs at all. I took a guess and shorted the "INH-" pin to the aux return pin and the "INH+" pin to the +5V aux pin, then applied AC power once more. This time the fan did not even spin up, which I assumed meant that I successfully inhibited it from powering on! After I pulled out the jumper I added between the INH+ and +5V aux pin the power supply's fan spun up, but this time at a much higher speed. Now when I checked the output DC voltages on the power headers, all were present and correct as described by the pinout label. Now I'm starting to think that the power supply is perfectly fine and not the issue at all!

After some more tinkering I discovered that I can power up the whole oscilloscope if I manually turn on the power supply by first inhibiting it and then un-inhibiting to initiate power on. The front panel soft power button still does not power on the oscilloscope. However, I found out that if I manually turn the oscilloscope on, then power it off normally and reconnect the internal power supply aux header cable while leaving AC power connected, the front panel soft power button now works! So it would appear that as long as the oscilloscope has standby power, the soft power button works as it should, but once you unplug AC power, you must manually power on the scope otherwise the soft power button does nothing. I can hear a quiet mechanical hum as if the soft power button is trying to power on the oscilloscope, but can't.

Here is a thread on the Agilent test equipment Yahoo newsgroup that pretty much describes the exact same thing that I was seeing, but I have not yet tried leaving the oscilloscope plugged into AC power for an extended time to see if that allows it to power on.

My next course of action would be to check the wiring leading to the soft power button, to see if something is not making a good connection. Does anyone out there have some experience with these older Agilent Infiniium oscilloscopes and how the soft power on works? The service guide PDFs I found online do not go into detail about how this works.
 

Online tautech

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #1 on: April 14, 2016, 10:06:43 am »
Interesting fault....well worth bumping IMO.
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Offline CJay

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #2 on: April 14, 2016, 10:20:11 am »
It's not entirely clear to me if the 5V standby is actually working or not but personally I'd be looking for dried up low value caps and/or defective high value resistors in the PSU primary, plenty of PC style power supplies are actually two power supplies (or more) in  one box and will only start up successfully if the standby one is working properly.

The 'leaving it on AC power' adds to my gut feeling that it's going to be a capacitor causing an issue.
 

Offline JackM

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #3 on: April 14, 2016, 06:46:27 pm »
Theory of operation when the scope is working correctly:
I've confirmed that when the scope is powered off, INH+ is pulled up to the +5V aux power. As long as the power supply has AC power, the +5V aux pin is always on. When the soft power button is pressed to turn the scope on, INH+ is pulled down to the aux return (aka ground) and the power supply is allowed to turn on. When the soft power button is pressed to turn the scope off again, the scope allows the OS to shutdown, then after the OS is finished it pulls the INH+ line back up to the +5V aux power in order to turn the power supply off (chassis fans stop spinning at this point). I did not see any changes on the INH- pin at all during power on or power off, it was always pulled down to ground each time I manually checked.

It's clear that the soft power button is not just a simple momentary switch to short out the INH+ line, and I'm fairly positive that it is not a fault with the power supply unit itself but rather a problem with how the scope is supposed to enable the power supply to turn on.

Now, once the scope loses AC power (physically unplug the unit and wait a few seconds) and I measure the voltage on the +5V aux pin, I get about 4.912V which is the expected +5V rating. However when I measure the INH+ pin, I get 4.75V - 4.79V which is lower than what I saw before when the scope is working properly and can power itself on. Still measuring the INH+ pin, if I press the soft power button in I see the voltage on the INH+ pin drop down to about 3.3V with a bit of jitter. This is also where you can hear a faint mechanical hum. If I hold the soft power button in for more than 5 seconds, the INH+ goes back to up to 4.75V and stops humming.
My guess is that something is partially shorting or loading down the INH+ line, causing a tell-tale voltage drop, which may also be preventing it from being pulled down to ground to allow the power supply to turn on. As to why this is happening only after losing standby power and not after I manually power up the scope and then power it down again, I am at a loss.

For reference, I've attached a picture of the power supply label which describes the pinout. What's important is the AUX power connector pinout (5 pin header).
« Last Edit: April 14, 2016, 07:32:34 pm by JackM »
 

Offline simmconn

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #4 on: April 15, 2016, 06:20:13 am »
This may be a long shot.
I have a 54852A which uses the same Motorola PC motherboard as the 16900A and 16902A logic analyzers. Both of my LAs exhibit the same problem of having difficulty powering up after shut down. Initially I thought it was a battery problem, because if I remove the battery and clear the CMOS, the unit would be able to power up.  Interesting thing is, the battery is good, but I have to remove it, not just clear the CMOS to let the board power up. That leads me to tracing the standby and battery related circuits on the motherboard and compare them to the Intel 815 reference design. It turns out there are some minor differences between the Motorola PC motherboard and the Intel reference design, which made the Motorola PC motherboard more sensitive to the tolerance of the +5VSB rail from the power supply. The power supply must have deteriorated over time that the +5VSB drops to ~4.7V with about 200mA loading. The voltage drops further on the motherboard and must have put the south bridge chip into a weird state which continuously draw more power than it should while refusing to power up. 

The workaround was simple, a silicon diode on the +5VSB is bypassed, so it has enough voltage to power the standby circuit on the motherboard even with the deteriorated power supply. The power draw on the battery is now in order (~3uA when AC is unplugged) and the unit is able to power up every time. The real fix would be to fix the power supply +5VSB circuit to be within tolerance under load, but the simple workaround works for me well enough.
 

Offline Jwalling

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #5 on: April 15, 2016, 10:31:03 am »
As a work-around rather than an actual fix, have a look in the BIOS setup and see if there's a setting for power on after power fail.

Jay
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Offline simmconn

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #6 on: April 15, 2016, 06:34:23 pm »
That's what I thought and did at the beginning, but it didn't work for the normal usage pattern. After I enabled the 'always on after power failure' in BIOS CMOS setting, if I unplug AC when the OS is still running, the unit would power back up when I reconnect the AC; but if I press the power button or let the OS shut down the system, it will not turn on again by the power button, even if I unplug and plug AC. So I figured the problem is in standby mode (S5). As long as the system goes into S5, it locks up and will not recover until you remove all the power (including battery) to reset everything.
Later voltage/current measurement on the motherboard also proved it.
 

Offline free_electron

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #7 on: April 15, 2016, 08:04:01 pm »
pop the front panel, use q-tip dipped in alcohol and wip the golden contact fingers behind the power button. that will fix it.  known problem. some gunk gets between the rubber and pad. carbon particles get in it and the button becomes a few kiloohms instead of open and close..
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Offline JackM

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #8 on: April 15, 2016, 08:12:31 pm »
I tried removing the CMOS battery entirely, but it didn't seem to help; I still could not power on the scope using the soft power button. I haven't yet measured current draw from the CMOS battery with AC unplugged.

I'm not so convinced that it is a problem with the PC motherboard though. The power supply remote inhibit lines route to the power distribution board first, which I believe is where the front panel soft power button is also connected to. The acquisition board and the rest of the oscilloscope interfaces with the PC motherboard via a ribbon cable to a PCI interface card. It is also this PCI interface card that controls the PC motherboard power button header lines (emulating a PC case power button). So I'm thinking that is something wrong with the power distribution board and/or the soft power circuitry, such that it is unable to pull the INH+ pin low enough to power on the power supply.

pop the front panel, use q-tip dipped in alcohol and wip the golden contact fingers behind the power button. that will fix it.  known problem. some gunk gets between the rubber and pad. carbon particles get in it and the button becomes a few kiloohms instead of open and close..

I will definitely try this! Now to figure out how to take off the front panel assembly. I really hope it doesn't involve removing the acquisition board or power distribution board, that would be a real pain.
Can I ask how you knew about this? I could find very little information online about troubleshooting these Infiniium oscilloscopes, considering their age and that they are still a high-end oscilloscope.
 

Offline Jwalling

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #9 on: April 15, 2016, 08:17:05 pm »
I tried removing the CMOS battery entirely, but it didn't seem to help; I still could not power on the scope using the soft power button. I haven't yet measured current draw from the CMOS battery with AC unplugged.

I'm not so convinced that it is a problem with the PC motherboard though. The power supply remote inhibit lines route to the power distribution board first, which I believe is where the front panel soft power button is also connected to. The acquisition board and the rest of the oscilloscope interfaces with the PC motherboard via a ribbon cable to a PCI interface card. It is also this PCI interface card that controls the PC motherboard power button header lines (emulating a PC case power button). So I'm thinking that is something wrong with the power distribution board and/or the soft power circuitry, such that it is unable to pull the INH+ pin low enough to power on the power supply.

pop the front panel, use q-tip dipped in alcohol and wip the golden contact fingers behind the power button. that will fix it.  known problem. some gunk gets between the rubber and pad. carbon particles get in it and the button becomes a few kiloohms instead of open and close..

I will definitely try this! Now to figure out how to take off the front panel assembly. I really hope it doesn't involve removing the acquisition board or power distribution board, that would be a real pain.
Can I ask how you knew about this? I could find very little information online about troubleshooting these Infiniium oscilloscopes, considering their age and that they are still a high-end oscilloscope.

If the scope will *shut down* when it's running using the soft power button, then I doubt that's the problem.

Jay
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Offline simmconn

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #10 on: April 15, 2016, 09:29:55 pm »
I tried removing the CMOS battery entirely, but it didn't seem to help; I still could not power on the scope using the soft power button. I haven't yet measured current draw from the CMOS battery with AC unplugged.

I'm not so convinced that it is a problem with the PC motherboard though. The power supply remote inhibit lines route to the power distribution board first, which I believe is where the front panel soft power button is also connected to. The acquisition board and the rest of the oscilloscope interfaces with the PC motherboard via a ribbon cable to a PCI interface card. It is also this PCI interface card that controls the PC motherboard power button header lines (emulating a PC case power button). So I'm thinking that is something wrong with the power distribution board and/or the soft power circuitry, such that it is unable to pull the INH+ pin low enough to power on the power supply.

The 16900A LA has a similar architecture. The soft power button goes through an interface board designed around a Xilinx FPGA, and the power supply unit (actually two of them) are connected to a power distribution board then to the PC motherboard and the plugin modules.

In my case I started looking from the PC motherboard because it has to come up first (or at least at the same time as the rest of the system), and my previous attempt trying to fix a 16700A power supply didn't go very far due to lack of service information. So I try to avoid the custom power supply until I have exhausted other possible causes. IMO the problem with the motherboard is a design margin issue, it just happen to come in play due to aging of the power supply. Keysight is right in that the PSU needs to be replaced, but the cheap workaround is worth a try.

The users manual for the motherboard is available online. You can find pinout for all the major connectors on the motherboard and measure the voltages on the ATX power connector such as +5VSB, and also simulate the power button by shorting the pins on the headers. I also heard (from free_electron?) that the south bridge chip tends to go bad on those motherboards. If that's the case you'll have a bigger problem to deal with.
 

Offline JackM

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #11 on: April 18, 2016, 05:02:51 pm »
Today I took the front panel off and then took apart the whole front panel assembly to get at the small PCB for the cursor keyboard and power button (had to remove the LCD screen first!). I cleaned the graphite dome contacts for the soft power button, and then checked the resistance after pressing the button. When closed, it reads anywhere between 200 to 45 ohms, depending how hard I press it. As Jwalling has said, if I am able to turn the scope on and off via the soft power button after first manually powering it on, the chances of the problem being solely the soft power button contact resistance is pretty slim.

I wonder if it is worth connecting an external 5V supply to the 5VSB rail, bypassing the 5V output from the oscilloscope's power supply?
 

Offline JackM

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #12 on: April 18, 2016, 05:25:48 pm »
I've confirmed that when I use an external power supply connected to the 5VSB rail on the power distribution board, effectively bypassing the 5VSB rail from the oscilloscope's power supply, the oscilloscope can power on right away after being left completely unplugged from AC power for a few minutes. So this would appear to confirm that the oscilloscope's power supply cannot provide enough current to the 5VSB rail causing a voltage droop when the oscilloscope tries to power on. Still a bit of a mystery as to why this problem doesn't occur once the power supply has been completely powered on at least once.

I'm now measuring the current draw through the 5VSB rail when the oscilloscope is powered off, and I am getting 173 mA. After the oscilloscope has successfully powered on, the current draw drops down to 30 mA. When the oscilloscope is unable to power on (after AC power loss event), current draw goes up to 182 mA - 198 mA. 173 mA does seem a bit high, could this mean that maybe it is also the motherboard that is drawing too much standby current, like what simmconn and/or free_electron has mentioned? I am not sure what the normal 5VSB rail current draw should be for this oscilloscope.
 

Offline Jwalling

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #13 on: April 18, 2016, 05:31:33 pm »
I've confirmed that when I use an external power supply connected to the 5VSB rail on the power distribution board, effectively bypassing the 5VSB rail from the oscilloscope's power supply, the oscilloscope can power on right away after being left completely unplugged from AC power for a few minutes. So this would appear to confirm that the oscilloscope's power supply cannot provide enough current to the 5VSB rail causing a voltage droop when the oscilloscope tries to power on. Still a bit of a mystery as to why this problem doesn't occur once the power supply has been completely powered on at least once.

I'm now measuring the current draw through the 5VSB rail when the oscilloscope is powered off, and I am getting 173 mA. After the oscilloscope has successfully powered on, the current draw drops down to 30 mA. When the oscilloscope is unable to power on (after AC power loss event), current draw goes up to 182 mA - 198 mA. 173 mA does seem a bit high, could this mean that maybe it is also the motherboard that is drawing too much standby current, like what simmconn and/or free_electron has mentioned? I am not sure what the normal 5VSB rail current draw should be for this oscilloscope.

Have you looked at the 5VSB both with a scope and a DMM? Perhaps it's dipping  at the moment when you press the soft power button?

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Offline JackM

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #14 on: April 18, 2016, 07:37:59 pm »
Have you looked at the 5VSB both with a scope and a DMM? Perhaps it's dipping  at the moment when you press the soft power button?

Unfortunately I don't have another decent oscilloscope on hand for this. I took a look at the rail and only saw a large peak-peak voltage spike the moment I press the power button, but no large voltage dips. It went from 1.4V peak-peak to a short spike of 4V peak-peak. I had to use a handheld Fluke Scopemeter instead, which isn't ideal.
 

Offline Jwalling

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #15 on: April 18, 2016, 08:07:00 pm »
Have you looked at the 5VSB both with a scope and a DMM? Perhaps it's dipping  at the moment when you press the soft power button?

Unfortunately I don't have another decent oscilloscope on hand for this. I took a look at the rail and only saw a large peak-peak voltage spike the moment I press the power button, but no large voltage dips. It went from 1.4V peak-peak to a short spike of 4V peak-peak. I had to use a handheld Fluke Scopemeter instead, which isn't ideal.

With that, I think you may be seeing the root problem. How about putting a largish (1000uF to 2200uF?) capacitor on the 5VSB rail? You may need to experiment a bit here...

Jay
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Offline CJay

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #16 on: April 20, 2016, 12:43:52 pm »
Have you looked at the 5VSB both with a scope and a DMM? Perhaps it's dipping  at the moment when you press the soft power button?

Unfortunately I don't have another decent oscilloscope on hand for this. I took a look at the rail and only saw a large peak-peak voltage spike the moment I press the power button, but no large voltage dips. It went from 1.4V peak-peak to a short spike of 4V peak-peak. I had to use a handheld Fluke Scopemeter instead, which isn't ideal.

With that, I think you may be seeing the root problem. How about putting a largish (1000uF to 2200uF?) capacitor on the 5VSB rail? You may need to experiment a bit here...

Jay
Are we sure this PSU doesn't work in a similar fashion to the ATX PSON scheme, I.E. the 5VSB is pulled down to signal the main PSU to start up and that there's a failure within the PSU relating to startup of the main supply?
 

Offline Jwalling

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #17 on: April 20, 2016, 05:39:56 pm »
Are we sure this PSU doesn't work in a similar fashion to the ATX PSON scheme, I.E. the 5VSB is pulled down to signal the main PSU to start up and that there's a failure within the PSU relating to startup of the main supply?

I believe that +5VSB stands for +5 volt standby which is always present when the power cord is plugged in on any ATX motherboard/power supply. What you are referring to is often called #PS_ON

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Offline simmconn

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #18 on: April 20, 2016, 06:48:56 pm »
Are we sure this PSU doesn't work in a similar fashion to the ATX PSON scheme, I.E. the 5VSB is pulled down to signal the main PSU to start up and that there's a failure within the PSU relating to startup of the main supply?
Yes it does. PS_ON# is pulled low to signal the PSU to turn on the main power. In my case, a collapsing 5VSB put the south bridge chip in a weird state, such that the main board is not able to pull the PS_ON# low to turn on the main power. Although the failure is in the PSU, it is easier for me to apply a workaround on the main board. I guess people tend to look at the problem from different angles and may end up with different fixes.
 

Offline Jwalling

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #19 on: April 20, 2016, 08:05:45 pm »
Are we sure this PSU doesn't work in a similar fashion to the ATX PSON scheme, I.E. the 5VSB is pulled down to signal the main PSU to start up and that there's a failure within the PSU relating to startup of the main supply?
Yes it does. PS_ON# is pulled low to signal the PSU to turn on the main power. In my case, a collapsing 5VSB put the south bridge chip in a weird state, such that the main board is not able to pull the PS_ON# low to turn on the main power. Although the failure is in the PSU, it is easier for me to apply a workaround on the main board. I guess people tend to look at the problem from different angles and may end up with different fixes.

So have you tried using a cap on the rail as I mentioned above?

Jay
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Offline simmconn

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #20 on: April 20, 2016, 09:28:33 pm »


So have you tried using a cap on the rail as I mentioned above?

Jay
No I didn't. JackM may be able to let us know the result of the fix you proposed.
 

Offline CJay

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #21 on: April 20, 2016, 11:03:06 pm »
Are we sure this PSU doesn't work in a similar fashion to the ATX PSON scheme, I.E. the 5VSB is pulled down to signal the main PSU to start up and that there's a failure within the PSU relating to startup of the main supply?
Yes it does. PS_ON# is pulled low to signal the PSU to turn on the main power. In my case, a collapsing 5VSB put the south bridge chip in a weird state, such that the main board is not able to pull the PS_ON# low to turn on the main power. Although the failure is in the PSU, it is easier for me to apply a workaround on the main board. I guess people tend to look at the problem from different angles and may end up with different fixes.

Sorry, if the PSU is at fault then I must be missing the point entirely as to my mind the only satisfactory, safe and reliable fix here would be to repair the PSU so I'll butt out and leave you to bodge it up.
 

Offline Jwalling

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #22 on: April 20, 2016, 11:14:08 pm »


So have you tried using a cap on the rail as I mentioned above?

Jay
No I didn't. JackM may be able to let us know the result of the fix you proposed.

Sorry. I got you mixed up with original poster. Doh!
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Offline simmconn

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #23 on: April 21, 2016, 12:26:07 am »

Sorry, if the PSU is at fault then I must be missing the point entirely as to my mind the only satisfactory, safe and reliable fix here would be to repair the PSU so I'll butt out and leave you to bodge it up.

I agree. In Reply #10 to this thread I explained why I did what I did. "bodge it up" describes it quite well. I just need a quick fix to keep my LAs going until the next catastrophic failure, or another upgrade. Attacking the problem from the load end instead of the source end put the unit back into action when I needed it and it's been trouble-free for a few months now. If I had insisted on fixing the PSU, I may still have the unit open on my bench today and my lost time would be worth more than the LAs themselves. Sometimes there are multiple issues contributing to the same problem and fixing one of them would keep you going, then it makes sense to fix the easiest one.  YMMV.
 

Offline JackM

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Re: Agilent Infiniium 54853A DSO won't power on
« Reply #24 on: April 21, 2016, 08:52:21 pm »


So have you tried using a cap on the rail as I mentioned above?

Jay
No I didn't. JackM may be able to let us know the result of the fix you proposed.

I placed a 1000uF electrolytic cap across the 5VSB and the aux return pins, but saw no improvement in helping the power supply turn on. Measuring the current on the 5VSB rail still shows 174mA of standby current, jumping up to 199mA when the soft power button is pressed. So the real root problem may be that the power supply cannot provide enough current on the 5VSB rail that is required when the oscilloscope is powering up?

simmconn, you mentioned that you were able to find a manual online for the Motorola motherboard in your logic analyzers? I tried finding one for mine but I couldn't find a thing. The motherboard in my oscilloscope is a Motorola brand with a part number "01-W2891E02H".

I also took a look on Agilent's replacement parts store online, but of course the power supply unit as a part is not available to buy separately from Agilent! They just say that you must take it in for repair by them. Maybe I could try a third-party repair service and see if they'd sell me a replacement power supply unit? If it turns out to be not that much cost, I'd probably just do that and fix the root problem here.
 


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