Author Topic: TDS754C Scope turns on by itself. Feature?  (Read 3742 times)

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Offline rx8pilot

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TDS754C Scope turns on by itself. Feature?
« on: February 06, 2015, 12:12:08 am »
Trying to figure out if its a feature or a problem that a Tek TDS754C will turn on by itself. If the master switch on the back is off, it stays off as it is physically disconnected from mains. From the 'standby' mode it turns back on after a few minutes. No sign if this in the manual, but maybe someone knows of a secret setup menu where this can be turned on/off. Possibly a feature that someone would want it to re-boot after a power failure?

If not, I will pull it apart to see what may be wrong. Could be a simple filter cap on the button.
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Offline tautech

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #1 on: February 06, 2015, 12:23:44 am »
Trying to figure out if its a feature or a problem that a Tek TDS754C will turn on by itself. If the master switch on the back is off, it stays off as it is physically disconnected from mains. From the 'standby' mode it turns back on after a few minutes. No sign if this in the manual, but maybe someone knows of a secret setup menu where this can be turned on/off. Possibly a feature that someone would want it to re-boot after a power failure?

If not, I will pull it apart to see what may be wrong. Could be a simple filter cap on the button.
Now if it connected itself to a DUT and gave you the info you need.....that would be a feature.  :-+

Something wrong with the soft switch? Not held low or high?

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Online nctnico

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #2 on: February 06, 2015, 12:42:07 am »
IMHO this is likely a contact problem. I think there is also an on/off push button switch on the main board which could be bad (at least the TDS 500A / 600A / 700A have one).
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Offline rx8pilot

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #3 on: February 06, 2015, 12:47:04 am »
...and out comes the screwdriver. Lets see what we can find.

EDIT:
Took the case off to see if I could get to switch and/or PSU easily. Not really. I don't have a backup scope with 500Mhz so I have to wait until I am able to order my new scope before I try a fix on this one. As long as is can still measure things, I can make money with it.

I will say, the internal construction and PCB layouts are a thing of beauty. It is worth a solid study! Gorgeous.

 
« Last Edit: February 06, 2015, 02:09:48 am by rx8pilot »
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Offline Jwalling

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #4 on: February 06, 2015, 02:08:51 pm »
...and out comes the screwdriver. Lets see what we can find.

EDIT:
Took the case off to see if I could get to switch and/or PSU easily. Not really. I don't have a backup scope with 500Mhz so I have to wait until I am able to order my new scope before I try a fix on this one. As long as is can still measure things, I can make money with it.

I will say, the internal construction and PCB layouts are a thing of beauty. It is worth a solid study! Gorgeous.

I've repaired one like this, but I can't remember which model - one of the TDS700 series. Turned out to be the power supply. Rather than making an attempt to fix it, I just replaced the supply.

Good luck.

Jay
Jay

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Online nctnico

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #5 on: February 06, 2015, 03:40:16 pm »
There is a schematic of the PSU in the 'TDS520B component level service manual'. The problem is that there seem to be many versions of this PSU with slightly different PCB layouts. You can test the PSU outside the oscilloscope if you put it on isolating studs and leave the acquisition board disconnected. The PSU has a latching relay which switches between power on/power off. The problem must be somewhere between the power on buttons and the relay. Maybe it's just a matter of replacing an electrolytic cap which went bad but I'd test it first with the front panel disconnected to see if it isn't a problem with the rubber keys.
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Offline rx8pilot

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #6 on: February 06, 2015, 05:58:54 pm »
I am hoping it is as simple as cleaning the switch pads or a capacitor. The dis-assembly is the most time consuming part and risks damaging something that is not presently broken. For the moment, I cannot risk not having a scope so I have to wait until I get my new one to repair this guy. I hope the issue does not grow into something worse to where the scope cannot be used at all. right now it is a minor inconvenience to turn it off on the rear switch.

My only backup is a Rigol DS1102E.
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Offline rx8pilot

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #7 on: February 26, 2015, 06:44:17 am »
I may have found a problem that could lead to the scope turning on by itself. The ground is floating for some strange reason. I was getting all sorts of AC in the signals and could not figure out why. I measured 57 VAC from scope ground to bench ground! not good at all.

I figured maybe the last owner of this thing may have a loose ground wire in the chassis, so I have taken it apart. The AC input is (not surprising) in a fully shielded housing that is soldered to the PCB with a big piece of sheet metal soldered around it. Essentially, and all day affair to gt to it. I am, however, able to get probes on it. And there is no connection at all from the GND pin on the AC jack to the GND pin on the PCB side. Not even high-impedance, nothing.

Not sure how or why this would be. The question now is would it be just fine if I connect an earth wire to the common bench ground to 'fix' this. Also, anyone have an idea what would have caused the GND to be disconnected. I can only think it would be a manufacturing flaw of some sort.
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Offline tautech

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #8 on: February 26, 2015, 07:23:49 am »
Quite possibly the last owner preferred to "float" his scope.
I have met a few experienced users that prefer this MO.

If you are going to repair it do it properly, have an Earth via the mains lead, then you'll have it as it should be.  :-+
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Offline rx8pilot

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #9 on: February 26, 2015, 07:54:40 am »
To fix it properly will be a major effort. I was thinking that if the user wanted a floating scope they would simply lift the ground on the mains cable right? This thing does not have any evidence of modification. The ground appears to be disconnected at the AC input module that is totally and permanently sealed. Never seen one with a bad ground before and this scope has a ton of leakage (maybe the CRT??)

At the moment, I just connected a banana cable from the scope to the bench and that is enough to keep it from turning on by itself and the DC measurements are clean and accurate.
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Offline tautech

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Re: TDS754C Scope turns on by itself. Feature?
« Reply #10 on: February 26, 2015, 08:57:48 am »
If an error was made with previous use of the scope (say reference clipped to mains) the GND connection may have fused OC.
Tek had a general recall with the TDS1000 500 DSO's for a similar issue where the connection from BNC's to mains GND could be blown OC by misuse.

This is the likely explanation IMO.

If you intend to use a seperate GND.... put a large label indicating the scope has NO GND by default or one day you'll forget.  :o  :wtf:  :scared:  :phew: that's if you're lucky.
« Last Edit: February 26, 2015, 06:46:04 pm by tautech »
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