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Author Topic: Can anyone help GRUMAGE fix his scope PLEASE  (Read 13174 times)
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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

R1007 and R1008 are startup resistors that provide a trickle of current to kick the oscillator into operation by biasing V1013 up , after which the positive pulse on top of V1009 via R1002 and C1012 keep it going .

How do you reason that it would be these resistors, when the supply starts normally and then goes into an alternate mode of oscillation.

I would suspect those resistors if the unit failed normally and was dead and would not start up due to many hours of operation causing upward drift of those values, in which case the oscillator would not start at all. I have seen this type of failure.

But the oscillator starts normally then goes into a different mode after about two minutes?

I suppose it is worth checking, those resistors probably will read higher than 464k due to age and heating and will probably need to be removed to get a proper reading.

This power supply oscillator has a bit of complexity and interactive functionality despite it's apparent simplicity, especially the functions of V1007 and V1014.

Does it hold to reason that a surge in the BNC ground path back to the mains would take out those resistors or cause them to change value?

Just curious, Peter, as you have probably seen a lot of this type oscillator if you worked in repair, and may have more experience with it than I.



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The circuit is quiet a bit different to what i am used to working with, but we have had strange things with the start resistors and almost as a matter of course we would check them anyway even if not related just in case they were causing strange faults, what you need to remember is that it could take me day's to find a hard fault with a diagram, but i could fix 20-30 TVs a day easily by following instink and past experience. I would personally change C1012 or at least have a good sniff(very fishy smell if leaking) and look at it's base for a brown patch.

Sorry i know you got this one covered i should not have jumped in.
   
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The circuit is quiet a bit different to what i am used to working with, but we have had strange things with the start resistors and almost as a matter of course we would check them anyway even if not related just in case they were causing strange faults, what you need to remember is that it could take me day's to find a hard fault with a diagram, but i could fix 20-30 TVs a day easily by following instink and past experience. I would personally change C1012 or at least have a good sniff(very fishy smell if leaking) and look at it's base for a brown patch.

Sorry i know you got this one covered i should not have jumped in.

Not at all "covered" as it is not repaired yet, and your experience is well respected, you are welcome to jump in since I have no suggestions outside of what I have outlined.

Yes it is true that instincts can do the job faster than pouring over each part in a schematic. In this case it is necessary to retrace the exact path of the current and take a good guess as to which part may have been most likely affected.

This kind of fault, the unusual discharge of a highly charged capacitor into the ground creates a current spike that can propagate anywhere and destroy a most unlikely part. That's why I asked Grum to try and retrace the exact path of the surge through the probe ground, BNC, circuit ground and back around the loop such that we could take a guess as to the most likely to be damaged part.

This is not at all like a normal age related failure mode, and as such it is very difficult to find.

The nose is definitely a help here, the smell of a hot part can lead to the solution, also an IR thermometer helps, but you need to know if the part is being heated beyond it's rating.

Please do comment Peter as we can use your help, instincts, and experience.


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I just spent a few minutes trying to to catch up on this subject.

Going by my understanding of the symptoms, I would look into the PS voltage control feedback circuitry.

This suggestion is due to the thought that a capacitive discharge through the shield/ground-plane is the culprit. I would suspect the PS feedback opto (H1046) and/or almost everything connected to it.

A repeating squeal from an non-isolated PSU or practically any oscillation of rail voltage would make me go after voltage control and anything with designed in isolation between circuits.

This may be completely off track because it would all depend upon the part layout and ch1 shield connection relative to the rest of the ground-plane and path to real Earth ground.


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yes indeed WW i would definitely be checking the voltage across R1049 to see if the current is creeping up across the opto LED especially as the other half of the 358 op amp is used for line triggering and is connected directly across the mains via caps.

I think it is logical to believe something is temperature sensitive due to the varying times it takes to shutdown, i would get the machine cold and see if it takes longer to shutdown (freezer spray the 358 and opto) and hair dryer the 358 & opto.
   
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From WW:
Quote
This may be completely off track because it would all depend upon the part layout and ch1 shield connection relative to the rest of the ground-plane and path to real Earth ground.

Agreed it is important to identify the complete current surge path.

Notice the positive 12 volts is used for reference and feedback via the opto isolator. This keeps all the secondary windings nominally regulated. Since the  +12 and -12 are regulating to within 100 millivolts on startup and for two minutes thereafter, I kinda ruled this out, but it is always a possibility.

Usually if the feedback fails voltages will soar a bit until shutdown, this would be a good clue if it could be noted.

The cold test is a good idea. If it is really cold outdoors, let the scope sit outside for a bit then see if the run time extends.

Cold by outdoors or spray is a good idea, but should be used carefully since the sprayed area will acquire condensation which could then cause tracking and migration of metals in DC circuits.

My best guess at this time, assuming there is not a part shorting beyond the power supply:

If the bias FET V1016 were leaky, it would begin over biasing the drive transistor V1019 preventing full cut off, heating it and causing core saturation,  squealing and a sudden drop in output. Scoping the secondary winding waveform would be the best way to identify this, as scoping the primary is not recommended without isolation transformers. A wafer puncture wound of the FET could cause this. Puncture wounds grow more leaky with time and temperature. The excesss current would keep the SCS biased close to turn on at the edge of what should have been a linear ramp across the transformer current sense resistors, but is now just a pedestal close to SCS firing threshold.

The current sense resistors R1023, R1024 which should show a linear ramp, then cutoff of current when the SCS V1014 fires at the prescribed current of around 2.5 amps, are now seeing a slowly rising bias as the steady state conduction builds in the output transistor V1019. When this current builds high enough, the SCS V1014 is not reset or is only reset at the very edge of the remaining ramp, which now rides on top of the current pedestal.


As I said priorly, I have designed these type of circuits using an SCR turnoff (instead of SCS) and a resistor bias in place of the FET. The use of an FET  in the location shown is definitely the "Achilles Heel" of this circuit design. They are using an SCS so that they can surely reset for the next cycle, which is often difficult with just an SCR once the holding current is exceeded.

Question to Grumage: does the squealing continue or does it stop completely after the two minute warmup period?

If the PS unit could be removed and fired up with dummy loads as outlined in the manual, that would definitely isolate the problem, but that is a real pain to do.
« Last Edit: 2013-11-30, 15:08:50 by ION »


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Normally, it isn't difficult to temporarily replace the power supply with feeds from a bench supply. This is a great way to isolate problems by removing whole blocks of the block diagram.

Worst case, you'll have to lift a couple of leads and connect your bench supply at those points. One advantage is you can then see exact current draw and limit it to prevent further damage.

Don't discount the opto as these and most types of op-amps (especially 358, 741, 07x, 08x, etc.) and electrolytics are the first things we replace when a device hits one of our service benches.

The electrolytics die with simple age (their shelf-life is actually shorter than their in-use life). Anything with GaAs and many MOSFet junctions of all types suffer from simple age problems (normally they last a long time but their susceptibility to failure from transients increases drastically over time).

I've been fighting my garage door opener since the lightning strike several months ago. The signal sensitivity dropped and continued to get worse (remote controls). Yesterday I PM'd the damned thing by replacing all of the op-amps, electrolytics and signal transistors.

The electronics work better now than they ever did - unfortunately, the gearbox cracked during my range testing  :'(


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WW

A good 5 minute epoxy will make your day for the Gear box repair,a quick solvent clean up on the outside get a bit of fiberglass matt to hold the epoxy and add some extra strength.

whallah.......

Chet
   

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Great suggestion, ramset.

I wish the same would work for the missing teeth on the nylon gear set  :(

--------------

I just checked our repair line-card for Grummage's scope. It was black-listed several years ago because of the non-isolated supply and other design issues. Normally, I can grab a list of common failures with corrections and suggested parts. This is what happens when your employer touts reliability of repairs and only sells to industrial and government customers.  :-[



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Dear All.

Although on line I have been underneath my Son's car most of the day. I have just got back to my desk.

I really DO appreciate your help.  Now to answer your question ION, There is no squeal from the PS until the component?? fails. It is then that the squeal appears and the voltage drops off.
I am such a novice with thing's electronic and I am REALLY daunted about continuing!!

Dear WW.

As regards your Nylon gears teeth is there any "meat" underneath where the teeth were?? I could suggest drilling, tapping and screwing Nylon studs into the holes then filing the tooth profile. This has been something I have done on more than one occasion when a gear has stripped and a replacement was not forthcoming!!

Cheers Grum.


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 There is no squeal from the PS until the component?? fails. It is then that the squeal appears and the voltage drops off.



Im still leaning toward a bad cap.  More than often, a semiconductor issue tends to be an issue 100% or the time. But a cap that has been damaged somewhat, will show itself once heated up. In these cases the temp difference may not be easily noticeable but enough to induce a short in the cap once heated.

I could be wrong 'here'. Say for instance, you turn the unit on, then a time period goes by and then squeal. Then turn it off and then back on right away, is there still a time period before squeal?  I say because the cap should have already heated up and the no squeal time period should be shorter after re turn on, being that the cap already had a chance to heat up. ;)

But if the time period is the same after on, off, then on again, then I might say that it happens after a portion of circuitry is initialized, possibly a normal function of the circuitry.

Also you mentioned shielding covering some circuitry and is soldered down?  First thing I would do is unsolder and lift the shield and look for damage on the board, if you havnt seen any on other readily visible parts already. The amount of energy from the cap could have blown traces on the board, possibly even shorting to the soldered shield. Even then, there may be bad components.

mags
   

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Dear Mags.

Thanks for the reply.  Strangely there is a longer period of time before the next squeal occurs !!  I have also removed the soldered cover ( it was where ION said the little chip was ) and no sign of damage there either !!

I am truly lost at the moment !!

Cheers Grum.



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Dear Mags.

Thanks for the reply.  Strangely there is a longer period of time before the next squeal occurs !!  I have also removed the soldered cover ( it was where ION said the little chip was ) and no sign of damage there either !!

I am truly lost at the moment !!

Cheers Grum.



Maybe best to contact the company to see what they will do for you in way of repair.

I have a Novation synth keyboard than went out on me. The 5v supply was being dragged down.  My Wavetek meter measures down to .01ohm, 10 years old, so meters today should be able to do it. But it can help track down shorted parts. I tracked the short to a tiny surface mount cap next to an SM processor.  Once the cap was removed, the short was still there, it was the IC.

Cant get it. I called Novation and they referred me to a local repair station.  $350 unit fixed for less than $100.

Mags
   
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Yes, leakage building in electrolytic caps is common after warmup and can be excessive if they are failing.

The section in the manual regarding the power supply outlines a method of removing each smoothing inductor and measuring the current in each section of the isolated secondary of the power supply, Page 8-9.  This is messy and can destroy the board if not done very carefully. I don't recommend it. Better to just remove the board and see if the supply runs without going into squeal mode. This should vindicate all components on the PS board, so the progressive load must be on the main board. Running the supply removed and with dummy loads is a better method.

Logic dictates that the malfunction must be isolated to either:

1) a faulty component (temperature dependent) on the mains side of the converter, primary side of T1001

2) a faulty component on the isolated secondary of T1001, causing one of the supplies progressively drawing more current.

At lighter loads, the operating frequency will go up. Typically it is between 20 to 50 kHz.

A way to measure the rectifier diode temperatures of each supply rectifier  would also give some clues.

Due to the capacitors from earth ground to either side of the line, the transient could have easily ripped into the power supply degrading the FET or drive transistor as stated in my earlier post.

Grum: even though there was no visible damage to the AD548, it could be failing, after all Ch1 low frequency buffer is not working.


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Dear ION.

So, and please pardon me for being "Simple" !! can I just slide out the PS board from the large ribbon socket and power up just the PS and HT circuit ( As the HT circuit is on the same board ) ?? Would an open circuit be ok??

My own thought is that if the fault is on the PS it will just start to squeal as before??

Cheers Grum.


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Dear ION.

So, and please pardon me for being "Simple" !! can I just slide out the PS board from the large ribbon socket and power up just the PS and HT circuit ( As the HT circuit is on the same board ) ?? Would an open circuit be ok??

My own thought is that if the fault is on the PS it will just start to squeal as before??

Cheers Grum.

Yes, this is correct logic, however it is sometimes dangerous to run a supply with no load. Also the CRT HV section may go too high and break over since it is meant to operate with a load.

Add to this that you will need a ground to the power supply if you keep the HV and filaments connected to the crt.

This is why loads are recommended and specified in the manual. No easy ways here. Try it at your own risk but it is not my recommendation.


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Grum
These people will give you a free repair quote if you can get it to them, but how far is Wigan from your location.

http://www.tercalibration.com/ContactUs.aspx
   

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Grum
These people will give you a free repair quote if you can get it to them, but how far is Wigan from your location.

http://www.tercalibration.com/ContactUs.aspx

Good find Peter

I would say about 1hr from Grum

regards

Mike


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They seem like a fair company, they even calibrate while you are there, maybe they would look at it while there, they even make tea  ;D

First though you need to get it back together without making it too obvious it's been apart, there's nothing worse than the sinking feeling you can get when you take the top off and see cowboy hoofs all over the pcb  ;D no disrespect meant LOL you would be surprised to know that it takes a real pro to put all the screws back in place.
   
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If you can get your hands on another scope, you can make a sniffer loop to determine if the oscillator is totally quitting after two minutes. It seems to do this from previous info (squeal quits, +/-12 volts fully collapses, scope totally dies, no display)

When and if you determine the oscillator is totally quitting then:

measure the voltage on the current sense resistors R1023, R1024 using an ordinary DMM when it has shut down. Exercise care, this part of the circuit is at mains potential

If there is a steady DC voltage on those resistors after the oscillator has shut down, the problem is as I have outlined in bold in the post # 30 and we can do some calculations to determine the saturation current.

quote:
Quote
My best guess at this time, assuming there is not a part shorting beyond the power supply:

If the bias FET V1016 were leaky, it would begin over biasing the drive transistor V1019 preventing full cut off, heating it and causing core saturation,  squealing and a sudden drop in output. Scoping the secondary winding waveform would be the best way to identify this, as scoping the primary is not recommended without isolation transformers. A wafer puncture wound of the FET could cause this. Puncture wounds grow more leaky with time and temperature. The excesss current would keep the SCS biased close to turn on at the edge of what should have been a linear ramp across the transformer current sense resistors, but is now just a pedestal close to SCS firing threshold.

The current sense resistors R1023, R1024 which should show a linear ramp, then cutoff of current when the SCS V1014 fires at the prescribed current of around 2.5 amps, are now seeing a slowly rising bias as the steady state conduction builds in the output transistor V1019. When this current builds high enough, the SCS V1014 is not reset or is only reset at the very edge of the remaining ramp, which now rides on top of the current pedestal.

While you don't need to know how it works to repair it with a shotgun approach, it is really quite necessary to fully understand the operation of this switcher circuit to make a proper diagnosis rather than guesswork. It's operation is well explained in the manual, and it is a good education to learn this for future reference, since you do build power switchmode circuits, besides it is a rather elegant design worthy of understanding.

If you have a IR thermometer and can (non-contact) measure the temperature of the drive transistor and FET you will probably see an excess temperature rise in either of these parts which will stay hot after the scope has quit.


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Dear ION.


"It seems to do this from previous info (squeal quits, +/-12 volts fully collapses, scope totally dies, no display)"

Sorry it is the other way round. Scope comes on with various relays clicking then settles with no noise then there is a relay click followed by the high pitch squeal and the trace collapses.


https://www.youtube.com/watch?v=BFZrJpxbnl4

It would now appear that things are getting worse as the scope falls into squeal mode almost immediately!!

Cheers Grum.


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Dear ION.


"It seems to do this from previous info (squeal quits, +/-12 volts fully collapses, scope totally dies, no display)"

Sorry it is the other way round. Scope comes on with various relays clicking then settles with no noise then there is a relay click followed by the high pitch squeal and the trace collapses.


https://www.youtube.com/watch?v=BFZrJpxbnl4

It would now appear that things are getting worse as the scope falls into squeal mode almost immediately!!

Cheers Grum.

Are you saying the squeal starts exactly when a certain relay clicks, and then some time? later the scope goes dead but is not present before that? The exact sequence of events starting at power on is very important. An exact timeline of events would be helpful, and identification of which relay causes this.


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Dear ION.

Button ON... graticule illumination...... and voltage range selection relays click. And then almost immediately after..... all go off and squeal starts.

As stated earlier the time period has now become a lot shorter.

Cheers Grum.

PS. The relay is not, IMO a cause but rather a consequence of the PS shutting down!!


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Dear ION.

Button ON... graticule illumination...... and voltage range selection relays click. And then almost immediately after..... all go off and squeal starts.

As stated earlier the time period has now become a lot shorter.

Cheers Grum.

PS. The relay is not, IMO a cause but rather a consequence of the PS shutting down!!

Does squeal continue on? for how long?


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Does squeal continue on? for how long?

Dear ION.

Yes the squeal continues until I press the power off button.

Cheers Grum.


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