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Author Topic: Can anyone help GRUMAGE fix his scope PLEASE  (Read 16843 times)

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He will post information here, hopefully someone can help him fix his scope  O0
   

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He will post information here, hopefully someone can help him fix his scope  O0

Thank you Peter.

I have managed to "Abuse", accidentally BTW, my Fluke PM 3092. I managed to discharge a 50 micro farad capacitor with about 350 Vdc onto the earth clip!! This took out the 5 A fuse and upon replacement the scope comes on for a few minutes then the trace disappears coinciding with a high pitched whistle from the PSU transformer.

I now have the scope bared and I have also noticed that upon applying the probe to the signal ref pin (on the front) Ch1 shows a very deformed square wave whilst the other three channels appear to be ok.

I am open to any help!! Please ask whatever, I can even make a You tube if that is any help?

Cheers Grum.


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If it has a dc-dc psu, then thats the squeal.  Being that it happens after time on, most likely it is a capacitor that is heating and then internally shorting, thus the squeal of the psu being heavily loaded. It seems like a heat up problem. You can try using what is called freeze spray or a can of Blast Away duster turned upside down and cool caps in the circuit one at a time to see if the issue goes away temporarily. If you find a cap that reacts to cooling, then thats your bugger. ;)  It could be a semiconductor, but I would go after the caps first.  ;)

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Dear Mags.

Thanks for that I will give it a try!!

Cheers Grum.



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Quote:

Quote
I have managed to "Abuse", accidentally BTW, my Fluke PM 3092. I managed to discharge a 50 micro farad capacitor with about 350 Vdc onto the earth clip!! This took out the 5 A fuse and upon replacement the scope comes on for a few minutes then the trace disappears coinciding with a high pitched whistle from the PSU transformer.

I now have the scope bared and I have also noticed that upon applying the probe to the signal ref pin (on the front) Ch1 shows a very deformed square wave whilst the other three channels appear to be ok.

A more precise explanation of what you did to cause the failure would help e.g

Quote
I managed to discharge a 50 micro farad capacitor with about 350 Vdc onto the earth clip!!

Two points are needed to form a discharge circuit. We have one, the ground clip. What was the other connection to the capacitor?

Quote
I now have the scope bared and I have also noticed that upon applying the probe to the signal ref pin (on the front) Ch1 shows a very deformed square wave whilst the other three channels appear to be ok.
[/quote]

This is cryptic. Are you measuring the reference pin with a different scope? Are three channels of the scope operating properly before fadeout? What does "ok" mean. (no deformed square wave? no wave display at all?)

If three channels are operating  "ok" as you said, part of the the problem is in the bad channels vertical amp.

Please try to re-explain step by step what you did and what you now have. Identify if other pieces of equipment are being used to test the faulty scope.

You can learn a lot in the few minutes the scope does work before it fades out. The fadeout is most likely a power supply problem, without a schematic it may be difficult to repair except by component swap i.e. the switchmode controller chip, drive fet, transistor or shorted turn in PS transformer.

Procure a service manual with schematics as a first step. It will be handy to have for this and future reference.









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Dear ION.

Thank you for your response, Chet phoned me and said you would be in touch.

I PM'd you before I noticed your post!!

Please let me know if you need more info. BTW I now have a PDF of the service manual is there a way of attaching single pages?? The whole document is 120 MB!!

Cheers Grum.


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Please let me know if you need more info. BTW I now have a PDF of the service manual is there a way of attaching single pages?? The whole document is 120 MB!!

Cheers Grum.
[/quote]

copy and paste to another file the pages you want, and then send or post here O0

regards

Mike


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Thanks Mike!!

Attached schematic of voltage control.

C1012 was warmer than all the others for the same period of time.


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The converter is a modified blocking oscillator design as TEK frequently uses with all the frills for current sensing and limiting, hence the squeal.

While it is possible there is a bad part in the power supply, another view is that it may be in CH1 loading the supply excessively.

Best to try and determine what component has failed and is causing such a load on the converter that it has altered it's duty cycle to provide strong harmonics in the audio range. This component may not be in the power supply itself, but the supply is reacting to the excessive loading by operating closer to 50% duty cycle and eventually shutting down.

The schematic indicates around a 25% duty cycle for the converter. This will be difficult to probe so don't even try it with another scope unless you know how to get around the ground loop problems you will have or more damage will ensue. Probe only the secondary isolated section.

After I download the manual I will see if Ch1 can be isolated or removed to see if the scope operates normally until the faulty component(s) can be found.
« Last Edit: 2013-11-28, 14:05:00 by ION »


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Ok Got the complete manual. Note of caution attached.

More to come once I give it a look.

Question: Was the probe tip of CH1 hooked to anything when the capacitor short occurred? If not, then the transient induced in the probe shield may have coupled into the inner conductor and taken out the input buffer amplifier. Since you get a distorted waveform on CH1, the buffer may not be operating properly and you are only getting pulse edges without the low frequency information.

See if the Ch1  Low frequency impedance converter IC type AD548 (N1001) is running much hotter than equivalent parts in CH2,3,4 (input attenuator board)

The fact that the signals are good on CH2,3,4 but distorted on Ch1 points to a defective part such as the AD548 in the attenuator section. This part gets hit first and hardest with a large transient on that channel and the shorted part may be loading the supply.

A careful check of startup power supply voltages especially +/-12V would be helpful.

« Last Edit: 2013-11-27, 22:43:07 by ION »


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Ok Got the complete manual. Note of caution attached.

More to come once I give it a look.

Question: Was the probe tip of CH1 hooked to anything when the capacitor short occurred? If not, then the transient induced in the probe shield may have coupled into the inner conductor and taken out the input buffer amplifier. Since you get a distorted waveform on CH1, the buffer may not be operating properly and you are only getting pulse edges without the low frequency information.

See if the Ch1  Low frequency impedance converter IC type AD548 (N1001) is running much hotter than equivalent parts in CH2,3,4 (input attenuator board)

The fact that the signals are good on CH2,3,4 but distorted on Ch1 points to a defective part such as the AD548 in the attenuator section. This part gets hit first and hardest with a large transient on that channel and the shorted part may be loading the supply.

A careful check of startup power supply voltages especially +/-12V would be helpful.



Dear ION.

As far as I recall the probe tip was not connected when the accident occurred.

I have run up the scope with the main board's underside exposed and cannot detect any difference in temperature using the back of my finger as a guide.  As it is getting late here I will have a look at the +/- 12 v area tomorrow. Can you suggest where I might find that test point??

Cheers Grum.


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Dear ION.

As far as I recall the probe tip was not connected when the accident occurred.

I have run up the scope with the main board's underside exposed and cannot detect any difference in temperature using the back of my finger as a guide.  As it is getting late here I will have a look at the +/- 12 v area tomorrow. Can you suggest where I might find that test point??

Cheers Grum.

Grum:
The part I am talking about is on Ch1 of the vertical amplifier /Attenuator which is on a separate board where the probes plug into. Each of the channels has some power supply decoupling to prevent crosstalk.

Voltages can be measured on the large connector at the output of the power supply board. If you intend to look at the duty cycle, and frequency, do it on one of the isolated secondary windings for safety.

It will be a good idea to get really familiar with the power supply switcher circuit and how it operates. Read section 5.6-1 You will learn it has current sensing and limiting built in, so under heavy load from a shorted IC it will work somewhat, but will quit after some time.

Since Ch1 is not working during start up, we can have good reason to look for a shorted part there. The signal unit A1 board is not part of the main board, but is the board where the input channels plug into. Can you post a picture of ch1 hooked to the ref pin so we can determine the nature of the distorted signal? Also show a sample of Ch2 if possible.

The use of a BRY62 silicon controlled switch for current sensing / switch turnoff is interesting. I have used ordinary SCR's as current sense / turnoff for this type of switcher, but the use of the SCS is a new twist. Always learn something new from those TEK engineers. This is a well thought out flyback converter, far more elegant than a simple JT or blocking oscillator.

Attached is a clearer copy of the power supply printed to pdf from the original document. notice that it is only 225 kb compared to 348 for the .png, but when zoomed you can read the component values clearly.
« Last Edit: 2013-11-28, 14:53:24 by ION »


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Dear ION.

Please find attached photos as requested.

I have shown both the front and rear of the main board. As you can see the input sockets are directly on this , not a separate board.

Cheers Grum.


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I nominate myself for the TWIT of the year 2013!!

They are all upside down!!  But why ? They were all ok viewed on my PC!!

I shall rotate and repost!!.

Cheers TWIT.

Just tried with rotated images. Still the same!!


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As I suspected the bad channel is only passing high frequency information. This points to the AD548 low frequency impedance converter IC as I stated earlier:

Quote
See if the Ch1  Low frequency impedance converter IC type AD548 (N1001) is running much hotter than equivalent parts in CH2,3,4 (input attenuator board)

The fact that the signals are good on CH2,3,4 but distorted on Ch1 points to a defective part such as the AD548 in the attenuator section. This part gets hit first and hardest with a large transient on that channel and the shorted part may be loading the supply.

You are very lucky, a lot worse could have happened to this scope, like destruction of the uP IC or one of its peripherals. The fact that the functions seem to work is a sigh of relief.

The trick now is to do the repair without causing further damage, as this is very easy to do when probing around. You must be very patient, work slowly and very carefully especially if you are not familiar with repairing this type of equipment. One slip of a probe could short something causing chain reaction destruction of a number of chips rendering the scope a useless boat anchor.
I have done this in my early days and have learned the lesson.

If it's not the IC AD548 loading the supply, then it could be a shorted decoupling Zener or other component in that section . Study the attenuator front end and you will see how the power is routed.

Edit: It is  on the back side of the board(small component side) in the vicinity of ch1. see attachment
it is a part of the main board, not a separate board as I first thought.

I have some thoughts on how we can disconnect power to that section to see if the short is there.

If you have a kill a watt meter you will probably find the input current to the scope is higher than its rating by a bit because of the extra power supply load. no need to re-post the picture, I get the idea.
« Last Edit: 2013-11-28, 18:11:52 by ION »


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Dear ION.

Strangely my Wattmeter reads 20 W under the rated Scope usage (BTW also re checked same using 40/60 W Incan bulbs)

I managed to get the Steel sheet cover removed that was soldered and again there is no difference in op temp between the 4 little chips.

I look forward to receiving your next instructions!! Is it holiday time for you??

Cheers Grum.


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Hi Grum

Yes it is the holiday time here.

 I'm surprised all 4 little chips read the same temp, so the problem could be the power source to the chip.

I will probably not be back on this until tomorrow, meanwhile, patience, we will slowly home in on the problem.

If you are in a rush then I'm not the one to help. I find it best to use simple logic and process of elimination to find the faulty part and to proceed cautiously.

Great that the power supply can start up and run for a bit. This narrows the possibilities a great deal.

We need to check the power supply voltages on the secondary and see which one is out of spec. or we can take a closer look at the power that runs the Ch1 AD548 to see if it is up to snuff. I will take a look at the schematic as time permits.

For future reference, it is a good idea to get an average power drain and note it on the back of the unit when it is running on spec and with the intensity set to a medium level. Lately I've been doing this as part of documentation for all my equipment.

Odd that you are drawing less but it may be because the supply is in current limit fold back mode.



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Dear ION.

My heartfelt thanks to you taking time out during the holiday period!! Please do not worry I am a very patient type "slowly and surely wins the race" !!  O0

Take your time and enjoy the day.

I'm off to bed soon!! Cheers Grum.


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Dear ION.

My heartfelt thanks to you taking time out during the holiday period!! Please do not worry I am a very patient type "slowly and surely wins the race" !!  O0

Take your time and enjoy the day.

I'm off to bed soon!! Cheers Grum.

Dear Grumage:

After giving it more thought it is always possible that there is more than one failed part in the scope. We know for sure that low frequency information is not getting through Ch1  and this points to a failed AD548 but it could also be a part surrounding that chip such as a failed Zener or capacitor. Only high frequency info is getting through and this is capacitively bypassed around the chip.

We know that something is loading the power supply causing a current limit / shutdown mode, but this could also be due to a failed component in the power supply causing it to drive too hard and shutdown.

The two symptoms together could be caused by one part failed in each section mentioned or two separately failed parts in the two separate sections (more unlikely).

If I understand correctly, the capacitor discharge went through the probe ground and somehow completed a circuit back to the capacitor. This surge would have propagated through the BNC ground connector back to earth ground so we can assume that the other end of the capacitor was somehow grounded. It would also have induced a voltage into the center conductor of the probe wire, possibly taking out the buffer chip.

IC's can fail several ways, they can have an input transistor fail from the transient and still draw the correct current or they can destroy the part causing it to present a hard short or a soft short to the power supply. If a hard short, the chip will be colder than normal but some other part will absorb the current and get hot, if a soft short,the IC will be hotter than normal. It can also have a normal  temperature and be failed.

Somehow Ch1 must be isolated from the power supply to see if it is causing the load condition.

Alternately you can test the voltage +12AT1 and -12AT1 that supply power to the IC, pins 7 and 4 w/ respect to ground. These voltages are also available across C1085 and C1087 as shown on the attached schematic.

The fact that the other channels use the same +/- supply voltages and work properly points more to the failed chip as open. But something is still loading the supply. Look for any part that seems to be running hot compared to other parts in the other channels and the unit in general.

Testing the power supply can be difficult. Read the attached to see if anything is worth doing as it is a lengthy procedure.

Cheers ION
« Last Edit: 2013-11-29, 18:32:14 by ION »


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Dear ION.

Many thanks for the on going info.

My poor old eyes are having a terrible time!! I got +11.9 on C 1085 and -12.00 on C 1087. Is that 0.1 V a clue??

As stated previously I have been feeling all over the board with no sign at all of any noticeable excess heat. One thing that is noticeable is that the scope is only staying on for shorter periods than earlier!!

Cheers Grum.



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Dear ION.

Many thanks for the on going info.

My poor old eyes are having a terrible time!! I got +11.9 on C 1085 and -12.00 on C 1087. Is that 0.1 V a clue??

As stated previously I have been feeling all over the board with no sign at all of any noticeable excess heat. One thing that is noticeable is that the scope is only staying on for shorter periods than earlier!!

Cheers Grum.



Those values are within spec...but do they stay at that value even when the supply goes into squeal mode?

As I mentioned earlier, troubleshooting the power supply is difficult. It can be tested separated from the unit with appropriate dummy load resistors, but this is rather difficult. Also a procedure to check the current drain on each individual supply voltage is noted at the bottom of the pdf....also a difficult enterprise.

Fluke also sells an extender board to facilitate testing the supply, but this still (I presume) allows normal circuit loading, probably just makes it easier to probe. You could probably get some info from Flukecustomer service as I am sure they have seen this before.

Yes, squinting at all those tiny surface mount parts can be tough on old eyes (like mine also, 67)

I do not recommend the shotgun approach of random part replacement in the power supply, as the problem might not even be there and you will wind up with a damaging of a the pc board, and probably still not solve the problem.

I'm stuck for ideas right now but hope to be unstuck in a bit. Maybe someone else watching this has had similar problem and can chime in.

More to come

Cheers, ION

Edit: I have been referring to this as a TEK skope, should be Fluke / Phillips


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ION

can ch1 power supply be removed without affecting the other channels? if so then we will know if it is the supply or just ch1 !!! just my thoughts to narrow it down to which board.

regards

Mike


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Dear ION.

Message received and understood.

Voltage drops to +/- (SAME) to 0.95 in squeal mode!!

Also read Mike's post, thanks BTW. There are two Ribbon flex onto the board one small and one large. I wondered if they could be disconnected??

Cheers Grum.


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Dear ION.

Message received and understood.

Voltage drops to +/- (SAME) to 0.95 in squeal mode!!

Also read Mike's post, thanks BTW. There are two Ribbon flex onto the board one small and one large. I wondered if they could be disconnected??

Cheers Grum.

So it seems in squeal mode the power supply collapses. +/- 12 volts feeds all input channels so only one supply for all thus hard to disable. (to answer Mike's question)

Now the challenge will be to keep the power supply up (find the ps fault) and troubleshoot ch1 later as it seems to be a separate problem.

Need to find out what the cables are for before randomly disconnecting.

taking a break!


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OK looking at the diagram briefly i would cold check the 2 resistors in the primary drive R1007 & R1008 both are 464K and will be prone to go high resistance as they warm and maybe high anyway even when cold, be carefull in case there's charge in the mains smoother caps.

In fact measure the voltage across each resistor to check the voltage first you dont want to damage the meter on ohms range during resistance measuring  O0
   
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