suddenly unable power up Tektronix 2235 (analog) oscilloscope

[oldway]

Ok, very goed, you have to check what's the max voltage of the 42V rail (on C940) and of the +8.6V  rail(on C960)

Don't let it switched on for a long time in such condition ....Try to measure these max voltages as quick as possible...report the results.

[oldway]

If I had to repair this scope, I would feed the 42V rail by an external power supply, I  would begin with 38V current limited at 2A, it is far safer to work and to protect the electronic circuits of the scope...

But I think it is not possible because you don't have such a power supply.
I have a HP 6296A  60V 3A for this use.

[mmFooD]

C960 - max 1.8 v and min 2V and C940 is showing min - 1.3V and max .Ol

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[oldway]

C960 - max 1.8 v and min 2V and C940 is showing min - 1.3V and max .Ol

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Not possible, there is something wrong with your measurements.
I was waiting for something like 42V or more, but lower than 51V because crowbar protection did not atuated.

[oldway]

Also check Q944 for short circuit collector-emitter.

[oldway]

I explain my reasoning .... ds856, ds858 and ds870 are spark gaps designed to protect the CRT and all circuits against excessive supply voltages from the inverter.

Testing with the flashing spark gaps should be avoided as it is risky for the entire oscilloscope.

But the lack of technical ressources of the TS does not leave the choice of the method of work.
In fact, it is a mistake to try to repair an oscilloscope under such conditions.

Nevertheless, I hope that the TS is aware of the risk that he takes to permanently damage the oscilloscope by its attempt to repair.

So I'm trying to confirm:
- that (for a reason to check), the inverter supplies a voltage too high to the secondary.
- that 42V rail regulation is not responsible for this problem.

[David Hess]

Tektronix *never* mentioned ESR or ripple current in their parts lists whether it was important or not.  The only way to know is to recognize the manufacturer's part number.

For me, there is a good reason why Tektronix never mentioned low ESR in the part list: ranges of special low ESR electrolytic capacitors is something relatively recent :  the market demand for low ESR, high capacitance capacitors has been driven by industry trends toward reduced circuit voltages operated at increased frequency with higher current demands than in the past....As far I remember, in 1981, I was working in a big power electronic manufacturer and when had a problem related with ESR and there was no low ESR capacitors on the market.

It is more or less ten years later that the low ESR capacitors appeared on the market.

Tektronix never listed ESR or ripple current rating even when these were specified as with the Sprague 30D series.

You know that Tektronix has always been very accurate and detailed in their technical specifications....It is not possible that they simply would have forgotten or neglected an information as important as "low ESR" if needed...

They did not mention it in the parts list.  Their Parts Catalog for capacitors (secitons 9 to 14 of catalog 5 of 6) likely listed it but I do not have it and I do not know anybody who does.

The explanation is simpler: low-esr capacitors did not exist at the time this oscilloscope was developed and manufactured.

If someone has access to the datasheets for the Sprague 17D and Nichicon ULB series, then we could know.  Often the parts existed as with the Sprague 30D series but were not labeled that way.

Some idea can be gained however.  I have some capacitors that I pulled from 22xx series power supplies here including a Nippon Chemi-con 1000uF 50V SM series used for C940.  The SM series was replaced by the SME series which has a max ESR of 0.116 ohms and a ripple current rating of 1530 milliamps.  I even have my metric calipers sitting here and can confirm that the diameter is the same and that the SME is slightly shorter which is to be expected.  The PW series capacitor that I recommended is comparable or superior in every way although I would try to find a higher voltage part which is taller with the same diameter.

When one of my 22xx power supplies needs to be rebuilt, I plan on making measurements of the ripple current for the various capacitors.

[oldway]

Tektronix never listed ESR or ripple current rating even when these were specified as with the Sprague 30D series.

Indeed, they do better than this, they gave you the waveform of the ripple current. 

On test point 39, you have the waveform of ripple current of the C941: that's a triangular waveform....

Amplitude measured on a 0.2R shunt resistor is 50 mV peak (150-100mV)

Ripple of C941 is a triangular current of 250mA peak !!!! No need of low ESR capacitor.....

[mmFooD]

Well there is definitely no  short in Q944. And as to the C940 and C960 I tried setting the fluke 83 to manual range 400V and using the min/max button. I record similar values as before but a little higher this time. About 16V on one of them but no 45V.


C960 2V max ~ 0V min
C940 16V max -2V min


The fluke specs says that it is sampling at 100ms intervals from what I can tell. I am measuring from the back of the PCB over the footprint of the capacitor (black probe to negative and red to positive). Maybe I am doing something wrong?

[mmFooD]

This is where i did the measurements

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[David Hess]

Well there is definitely no  short in Q944. And as to the C940 and C960 I tried setting the fluke 83 to manual range 400V and using the min/max button. I record similar values as before but a little higher this time. About 16V on one of them but no 45V.

C960 2V max ~ 0V min
C940 16V max -2V min

The fluke specs says that it is sampling at 100ms intervals from what I can tell. I am measuring from the back of the PCB over the footprint of the capacitor (black probe to negative and red to positive). Maybe I am doing something wrong?

Sampling for how long?  In my experience multimeter min/max modes are only useful for steady inputs.  I am sure the peak voltages are much higher than that to be illuminating the power LED.

C940 is referenced to the isolated ground on the primary (off-line) side and not chassis ground.  This point is test point TP950 in the lower left corner of the schematic.

If you want something to measure, pull C940 and check it for capacitance and leakage and if you have the test equipment, dissipation and ESR.  Messing around on the off-line side of a malfunctioning switching power supply with just a multimeter would not be my first choice.

Or if you have suitable capacitors, tack solder them in parallel with C940, C906, and maybe C925 on the back of the board. 

[oldway]

You need a power supply capable of supplying 45V 2A and another oscilloscope.

The solution is to feed the 42v rail (C940) by an external power supply set at about 38V and adjusted it to have the -8.6V at the test point ... It is then possible to check whether the potentiometer R938 acts or not

Without technical means, and in addition, at a distance, what should be very simple becomes very complicated.

[TerraHertz]

Are you naming who answered as "bastards" ?
I did not like your answer at all....

Oh, sorry for the misunderstanding. And I did not come back to this thread till now, to notice the problem.
As Dave said, it's an Aussie joke. I'll explain for all non-Oz-English speakers:

The OP used slightly unusual English in the title. A native English speaker would have said 'unable to power up Tek 2235..." The phrase "unable to start" tends to imply something like an internal combustion engine won't start.
Now, if you'd watched the video you'd see it's a parody, a fake product advertisement. It plays on the very typical Aussie sketch of some guy trying to start a reluctant lawnmower or car engine, and ending up yelling "Start, you bastard!" at it in frustration. Actually this is an abbreviation, he'd be more likely to phrase it "START, you F*CKING BASTARD!"
Anyway, the product is supposed to be able to make any engine start easily. (We all wish!) Bearing in mind that in Oz  the word 'bastard' is considered mild and commonly used. Can even be a term of friendship. Depends on the context and tone of voice.

And so, the joke - an image of OP using this product to get his scope 'started'. Funny because it's absurd.

I think if you are easily offended, better not take a holiday in Australia. If the drop bears and bunyips don't get you, a heart attack will.

Meanwhile I have a faulty Tek 2232 that I really must get around to fixing. Been too busy.

[oldway]

I would never understand that your post had something to do with the tittle of the topic without your explanation....
In Europe, I never heard about this product, we use "start pilot" to start engines by cold wheater.

I think if you are easily offended, better not take a holiday in Australia. If the drop bears and bunyips don't get you, a heart attack will.

I am only waiting to win at the lottery, but no chance this would happen, I don't bet 

Heart attack ? I allready had one, why not another one ? 

[oldway]

@mmFooD:

You wrote:

Turned it on and these are the buggars making the noise I think. And they are flashing.
And they are referenced as DS856, DS858, DS860 on the PCB.
The power led is flashing just the first two burst in sync with those three components but then it fades out completely.

1) DS860 ? I did not find it on schematics....Is it not DS870 ?

2) Are you 100% sure that all the 3 spark gaps are flashing ? Not only two of them ?

[mmFooD]

Quote from: oldway on Today at 11:44:37

@mmFooD:

You wrote:


Quote

Turned it on and these are the buggars making the noise I think. And they are flashing.
And they are referenced as DS856, DS858, DS860 on the PCB.
The power led is flashing just the first two burst in sync with those three components but then it fades out completely.

 
1) DS860 ? I did not find it on schematics....Is it not DS870 ?

2) Are you 100% sure that all the 3 spark gaps are flashing ? Not only two of them ?

You are correct. I misread the R860 and the DS870. And, of course, only two spark gaps are flashing. As shown in the pictures. DS870 is the one not flashing.

[oldway]

I am little confused...They are neon bulbs, not spark gaps....they are limiting voltage at about 60V each....

And you said they make noise ? How is it possible ? I have never heard a neon bulb making noise....Are you sure that the noise come from those neon bulbs ?

I think they are glowing because something is arcing.

[mmFooD]

Yes, even though I read and listen to english everyday without trouble it is a different matter of expressing one self. It is a great lesson in how it is not enough to just read up on a subject without practicing what you learn.

[mmFooD]

I have no idea what they are. You called them spark gaps so I figured that's what they are.


I can't say for sure that they are the ones making the noise. It is difficult to tell but it is definately coming from that area, and the noise is occuring in sync with the flashes. I'll se if I can pin point the source of the noise.

[oldway]

As I said, we should check the max output voltage of the inverter to know what is happening, we tried with the min/max function of your Fluke 83 and it did not worked.

You need another oscilloscope, using it to measure the max voltage of 8.6V (or -8.6V) supply to see if inverter outputs are going overvoltage or not.
Have you another oscilloscope ?

[mmFooD]

you are also correct in that I don't have a bench power supply . Allthough I really want to build one with a max output of min 12V (maybe it should be able to output around 50V with this experience) and 1A. More if not to expensive so if your know of any good DIY projects that I could take notes from or where I can read up on PSU theory and -configurations that would also be greatly appreciated.


With the lack of proper equipment it seems to me that replacing the caps in the PSU area might be a good thing. If only to check that one of the list of possible errors. And then do some more trial and error replacements and see if any of it helps. But at the moment buying a DSO is not within my budget. Analog osciloscopes are kind of hard to find in this part of the world, at least for around the $60 that I payed for it. So I have to make this work.

[oldway]

There is something arcing, we must know what is arcing and why....Too high voltage or isolation failure ?

We must answer this question first.

Wear-out capacitors generaly results in a too low voltage supply, not a too high voltage....so replacing the capacitors will not solve the issue.

You do not have a friend who can lend you an oscilloscope?

[mmFooD]

No, I can't think of anyone I know that would have a scope. And I read up on he fluke specs again and found that it only measures capacitance up to 5 uF so no help there either. I'll do some more voltage probing to see if I can find the 40V somewhere

[oldway]

Repairing an analog scope is a very specialized work and it need very specialized equipment....

Such a failure as your oscilloscope, I repair it in less than one hour for sure.....

But without another oscilloscope, HV probe, an isolation transformer and a power supply, how do you want to repair an analog oscilloscope with a minimum of safety?

Do not worry with pre-regulator and 42V rail anymore.....Look at the 8.6V (or -8.6V) rails.....have you an analog multimeter ?

 

[mmFooD]

I think we have some success here finally. I tried setting the range to 4000 manually on the fluke and then used the min/max mode to measure over TP900 and TP940. I recorded a maximum spike with a value of 72V. (-72V).


I then tried measuring over TP900 and one lead of W961 marked with "-8.6V"  and around the time of this measurment the flashing stopped. Suddenly I recorded -8.55V at this point and around 48V on TP940. But still nothing on the display. But then it started flashing again, i think after I turned from the side and placed it bottom down it started flashing again. And this time a trace was visible in sync with the flashes and the voltage values on TP940 and the -8.6V where sketchy again. (a couple of volts on the -8.6V point and 74.4V max spike on TP940).


I guess new is better when it comes to troubleshooting but the question remains: what is going on