suddenly unable power up Tektronix 2235 (analog) oscilloscope

[David Hess]

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.

The neon bulbs usually flash on startup and shutdown and this is normal.  They are being used as low capacitance low leakage surge suppressors to prevent excessive voltages on the CRT elements.  They are inexpensive alternatives to gas discharge tubes.

[mmFooD]

Turned it on again: No flashing -53.6V on TP940 -8.5V on TP961 and it showed a trace on the display  . But it is not stable

[mmFooD]

This is at 2mV/div (1x) vertically and 10us/div horizontally

Skickat från min ONEPLUS A3003 via Tapatalk

[oldway]

Turned it on again: No flashing -53.6V on TP940 -8.5V on TP961 and it showed a trace on the display  . But it is not stable

Measure ripple on C940 with your Fluke on AC range. (directly between the + and - connections of C940)

And you can change the tittle of your topic:

suddenly unable to start Tektronix 2235 (analog) oscilloscope

[mmFooD]

on range 40 it is recording an avarage of about 1.8V and max 7.6V. Min is 0.12V

[oldway]

That's far too much, replace it.

[David Hess]

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).

Like I said, the transformer provides isolation so measurements between the primary and secondary side have little meaning.  The input voltage which matters is between TP950 and TP940 and should be +42.8 volts

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 

If the +42.8 volts at TP940 is high, then SCR crowbar Q935 fires shorting out the preregulator and protecting the inverter.  This resets the preregulator which restarts after a short delay.

If C940 or maybe C906 is worn out, then the preregulator will have problems regulating the output voltage.

This is at 2mV/div (1x) vertically and 10us/div horizontally

This is switching noise from the inverter which operates at about 20 kHz.  It implies that the input voltage is just low enough to prevent regulation and the output capacitors are bad.

[oldway]

Replace first C940 and everything will be ok....

Have you some capacitor 50 or 63V 470 or 1000µF ?
You can even solder the capacitor in parallel to make a quick test

[mmFooD]

David Hess: I did a measurement between TP950 and TP940 - max 75.2V. Now that I see the TP950 on the circuit it seems obvious that that is the ref point to measure against. Thanks!


I also measured ripple over C906 - max 2.8V. Should I replace that one as well?


Off topic:
Can anyone give me a hint as to what the purpose of the regulator vs the "regular" regulator and what part linear, LDO, and switch mode regulators have to play in the matter.

I am currently trying to regulate a 12V PLC voltage to 5V to feed a MCU and some other stuff. Preferably it should be able to handle at least 500mV. First I tried a single LM7805 but it gets very hot even with the biggest heat sink I could make room for and since it will be placed in a poorly ventilated housing that is not a good solution.

So I tried the MC34063 switch mode regulator. But the calculations where made on an input voltage of 12V and I am using an arduino uno with a 12V input as PSU and it seems to have a voltage drop of 0.7V which I think messes up the operation of the MC34063. I get a output voltage of about 5.2V and the MCU and 7segment LED display is flickering and not working proberly.

Should I use a linear and switch mode regulator in pairs, with one as a preregulator for the other?

I get that this might be a little to OT. But if anyone know some good literature online or on paper on this topic I'd appreciate it a lot if you could send me a link or other info in a PM.

[mmFooD]

I have a 1000uF and several 470uF but they are a lot smaller. Only 35 V.


Another kind of OT question. Is it a good idea to have >50-63V caps as stockpile caps. The ones I have atm is 35V and 16V.

[oldway]

I also measured ripple over C906 - max 2.8V. Should I replace that one as well?

Not necessary.

I have a 1000uF and several 470uF but they are a lot smaller. Only 35 V.

For test, if you have 8 x 470µF 35V, you can make a 940µF 70V by association in serie/parallel....Don't forget 2 resistors 10K 1/2W as voltage divider.

Another kind of OT question. Is it a good idea to have >50-63V caps as stockpile caps. The ones I have atm is 35V and 16V.

You should know better than us what you need as components...

Question off topic: open a new topic.

[David Hess]

I also measured ripple over C906 - max 2.8V. Should I replace that one as well?

I would replace them all but as for the ripple across C906, I am not real clear what the designers were intending.  The ratio of capacitance between the input, C906, and output, C940, capacitors is oddly high for an offline switching power supply and later models of this power supply increases the value of C906 to 270 microfarads which seems more reasonable although they had to add a PTC for inrush current limiting as well which may explain the low value of C906 in the 2235.  C940 actually seems to have too much capacitance unless that was needed to increase the ripple current rating or lower the ESR.  (1)

The ripple across C906 should actually be higher at roughly 26 volts peak-to-peak when operating on 60Hz 120VAC power.

Off topic:
Can anyone give me a hint as to what the purpose of the regulator vs the "regular" regulator and what part linear, LDO, and switch mode regulators have to play in the matter.

I do not understand the question.

Should I use a linear and switch mode regulator in pairs, with one as a preregulator for the other?

That can be done but it is usually not required.  Linear regulators are poor at removing noise from switching regulators when better filtering is usually called for.  This is why the 2235 uses PI output filters after the transformer and no linear regulators; there are some subtle design features which produce a low noise output.

I get that this might be a little to OT. But if anyone know some good literature online or on paper on this topic I'd appreciate it a lot if you could send me a link or other info in a PM.

Start by reading through the free Linear Technology application notes which discuss switching and linear regulators.  Several discuss the trade offs between switching and linear regulators.

(1) This is why I do not care about the measured capacitance of C940 or its ripple current rating.  That capacitor is worn out because the simplest explanation is that Tektronix selected its value to meet ESR requirements and there is no way it is still within its ESR specifications after 27+ years.  The same goes for almost all of the other capacitors; they are worn out.

I have a 1000uF and several 470uF but they are a lot smaller. Only 35 V.

A pair of the 470uF capacitors in series should be good enough to verify if there is a problem.

Another kind of OT question. Is it a good idea to have >50-63V caps as stockpile caps. The ones I have atm is 35V and 16V.

If you regularly work with 50 volt power supplies, which used to be common in analog circuits, then you should have them.  Some economy in parts selection can be had by stocking higher voltage capacitors for use in lower voltage applications; in most cases you do not need capacitors with every available voltage rating if you have higher voltage ones.

[mmFooD]

I do not understand the question.

Sorry it is supposed to say "the purpose of preregulator vs 'normal' regulator". I guess the preregulator is regulating the voltage to a more suitable one for the 'real' regulator. But when are they used and what type (linear, LDO, and switch mode) are used as one or the other.

That can be done but it is usually not required.  Linear regulators are poor at removing noise from switching regulators when better filtering is usually called for.  This is why the 2235 uses PI output filters after the transformer and no linear regulators; there are some subtle design features which produce a low noise output.

Ok, I looked up PI output filter and I think that is the optional filter mentioned in the MC34063 datasheet so it would be sufficient I think.

Start by reading through the free Linear Technology application notes which discuss switching and linear regulators.  Several discuss the trade offs between switching and linear regulators.

Will do!

A pair of the 470uF capacitors in series should be good enough to verify if there is a problem.

ok, I might try that then.

[David Hess]

Sorry it is supposed to say "the purpose of preregulator vs 'normal' regulator". I guess the preregulator is regulating the voltage to a more suitable one for the 'real' regulator. But when are they used and what type (linear, LDO, and switch mode) are used as one or the other.

There are a bunch of examples in Linear Technology application note 2 including some that use SCRs for preregulation that are very similar to how the original 22xx power supply in the 2213 and 2215 worked.

Ok, I looked up PI output filter and I think that is the optional filter mentioned in the MC34063 datasheet so it would be sufficient I think.

Linear Technology application note 70 discusses noise in switching regulators.  There are all kinds of subtle issues.

[mmFooD]

Ok, so i finally replaced C940. But it didn't fix it. I did a ripple measurement on the new one and it was high as well. I didn't have the fluke with the min/max function. But I measured peaks of 12V. Did I botch the soldering?

Skickat från min ONEPLUS A3003 via Tapatalk

[David Hess]

Ok, so i finally replaced C940. But it didn't fix it. I did a ripple measurement on the new one and it was high as well. I didn't have the fluke with the min/max function. But I measured peaks of 12V. Did I botch the soldering?

So what parts have been replaced at this point?

Is the oscilloscope still blinking the power light?

[oldway]

Ok, so i finally replaced C940. But it didn't fix it. I did a ripple measurement on the new one and it was high as well. I didn't have the fluke with the min/max function. But I measured peaks of 12V. Did I botch the soldering?

Skickat från min ONEPLUS A3003 via Tapatalk

Again a T906 failure ? 

[mmFooD]

Nope, no flashing on the power led. I've only replaced C940 at this point.

Skickat från min ONEPLUS A3003 via Tapatalk

[mmFooD]

Ok, so i finally replaced C940. But it didn't fix it. I did a ripple measurement on the new one and it was high as well. I didn't have the fluke with the min/max function. But I measured peaks of 12V. Did I botch the soldering?

Skickat från min ONEPLUS A3003 via Tapatalk

Again a T906 failure ? 

You think the problem is with the transformer?

Skickat från min ONEPLUS A3003 via Tapatalk

[oldway]

Ok, so i finally replaced C940. But it didn't fix it. I did a ripple measurement on the new one and it was high as well. I didn't have the fluke with the min/max function. But I measured peaks of 12V. Did I botch the soldering?

Skickat från min ONEPLUS A3003 via Tapatalk

Again a T906 failure ? 

You think the problem is with the transformer?
Skickat från min ONEPLUS A3003 via Tapatalk

Too much ripple on a NEW C940, that's very strange....a faulty T906 could explain this.....To check it, you need an isolation transformer and another scope to check the current waveform on r907. (waveform 39)


A faulty T906 causes all kinds of phenomena:
1) Mosfet Q9070 becomes too hot
2) the operation of the inverter becomes erratic because of the ripple
3) the voltages are no longer properly stabilized
4) there is sometimes a high pitch sound that comes from T906 and which varies with the brightness setting

Another way to diagnose a faulty T906 is to feed the 42V rail by an external power supply....If all is working good with this external power supply and a new C940, you can be sure that T906 is faulty.

That's why I always use first an external power supply for testing and repair this kind of scopes

[mmFooD]

Yes, I am starting to realise that another scope and psu would be handy. I'm thinking I could buy a Rigol DS1074Z and maybe make some PSU. What do you think?

Is it better to make a big 48+ V 1-3 A PSU or a couple of 15 V 1 A PSU to put in series or parallel to get higher voltages or currents?

I no longer have access to the fluke either, so I'm thinking I need a better DMM as well to compliment my cheaper UNI-T. One that could measure at least 10mF, temperature, µA etc. The only one I've found os far with a reasonable price with good specs is the EEVblog BM235.

[David Hess]

Yes, I am starting to realise that another scope and psu would be handy. I'm thinking I could buy a Rigol DS1074Z and maybe make some PSU. What do you think?

Is it better to make a big 48+ V 1-3 A PSU or a couple of 15 V 1 A PSU to put in series or parallel to get higher voltages or currents?

I know of at least two cases now were people replaced the 22xx preregulator circuit with a 48V switching power supply brick adjusted to 42.5 volts.

I do not suggest designing and building an off-line switching power supply.

[velik_kazakov]

Maybe I wast in the row if someone is writing before for this, but have some 2236 (same power supply) repaired in last 6 months (most with problems in PS) and always remove P9070 and solder wire to Q9070. P9070 always can make some problems - I have the same trouble with one 2236 and after removing and solder the wire to Q9070 all was OK.
Tek kit for this PS components replacement is 050-2242-03. Some good information about 22xx power supply repair here. I think that have one more recommendation from Tektronix for this power supply but not sure now.
And something else - +1 install fan. It is too hot w/o fan. The board is the same as 2236 and just you need to solder the fan to the power supply. If you need, I can to make some pictures how it is installed on  2236.

[mmFooD]

Quote from: mmFooD on 2017-08-22, 09:51:46

Yes, I am starting to realise that another scope and psu would be handy. I'm thinking I could buy a Rigol DS1074Z and maybe make some PSU. What do you think?

Is it better to make a big 48+ V 1-3 A PSU or a couple of 15 V 1 A PSU to put in series or parallel to get higher voltages or currents?

I know of at least two cases now were people replaced the 22xx preregulator circuit with a 48V switching power supply brick adjusted to 42.5 volts.

I do not suggest designing and building an off-line switching power supply.

By brick, do you mean I should buy a module and then add/replace some components to get the ajd voltage?


I guess buying two such modules I could use the second one to build a  45V lab PSU unit?

[mmFooD]

Maybe I wast in the row if someone is writing before for this, but have some 2236 (same power supply) repaired in last 6 months (most with problems in PS) and always remove P9070 and solder wire to Q9070. P9070 always can make some problems - I have the same trouble with one 2236 and after removing and solder the wire to Q9070 all was OK.
Tek kit for this PS components replacement is 050-2242-03. Some good information about 22xx power supply repair here. I think that have one more recommendation from Tektronix for this power supply but not sure now.
And something else - +1 install fan. It is too hot w/o fan. The board is the same as 2236 and just you need to solder the fan to the power supply. If you need, I can to make some pictures how it is installed on  2236.

I will definateyl look into that, especially installing the fan. Where did you plug it in exactly?


I also found this in the document you linked:

If there is an audible ticking indicating that the scope is continuously starting and
shutting down there is secondary problem. The most common fault then is one or more
shorted rectifier diode(s) on the secondary windings of the transformer. Also a bad HV
Multiplier or a supply shorted in any other way would produce the power supply burst
mode.

Is that refering to CR960-3 or is it refering to CR901-4? Allthough I have already tested those and was told that the values where fine.