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Where did you find a schematic? That is better than the one I have. I have been using the 2236 schematic for the 2235.
There you are, from the Tek itself..
http://www.tek.com/manual/2235-service-manual
That is the one I have. The schematic mmFooD posted is complete and not split across pages.
I tried merging the pages last year and they do not overlap so strips of the schematics are missing.
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There is an even better image here:
Click on the image (48KB) to see a larger version (566KB)
The third post tells how the person put the schematic together.
https://www.electronicspoint.com/threads/tek-2235-good-focus-but-high-intensity.276029/ -
Replacing all the capacitors, systematically, without even to look for the origin of the defect, this is, as I wrote, a complete mistake.You are going to make a fundamental mistake
... I have 40 years of experience in electronic project and repair and I can guarantee it for you.
Your oscilloscope has crashed suddenly and completely, it is not a "wear" of a capacitor that caused this.
If there is something wrong with a capacitor, it is short circuited, not to low capacitance, nor too high ESR....
Without considering that the aluminum electrolytic capacitors are old enough in this model to be worn out, I disagree because:
Normally when pressing the power button the CRT makes that same, kind of static noise that old TV's does, for a second or two. Now my scope makes this noise but repeatedly for only a tenth of a second or so and the power led is also flashing in the same manner.
It is very unlikely that it would be an electrolytic capacitor that make that kind of noise because once the thin Al²O3 insulation is pierced and destroyed, the space between the electrolyte and the aluminum becomes almost zero and it would be impossible that the power supply can restart to such a hig level that a led can be flashing.
But it is true that if there is a short circuited diode, it is also impossible the led beeing flashing....inverter should try to start but led would not be flashing.
On the other hand, it can be a high voltage arc ... Then it's probably visible.
But in any case, you have to do an investigation, not just replace electrolytic capacitors for nothing.
Helping repairing something is always very difficult : for exemple, what is the static noise of an old tv ? CRT colour TV ? tubes ? Transistors ? the noise of charging the 25KV's tube capacitor ?.... -
David knows of what he speaks.
The PSU in the 22xx series is well known for going into "tick mode" when the electrolytics dry out and go high ESR
C925 and C943 are common culprits but it can be pretty much any of them. -
David knows of what he speaks.
So, it is better to recommend simply to replace these 2 capacitors, they are small capacitors of low voltage and cheap (10 and 100?F)
The PSU in the 22xx series is well known for going into "tick mode" when the electrolytics dry out and go high ESR
C925 and C943 are common culprits but it can be pretty much any of them.
The principle of recap systematically audio amplifiers and receivers is used by scam artists to earn a lot of money of audiophiles.
It is better not to repeat this formula unnecessarily
My Marantz 2285B from 1978 that I use everyday has only one capacitor replaced.
And it still has the same performance as when it was new .... worn capacitors? They are 40 years old of good and loyal service and they will probably hold another 10 or 20 years .... It will always be time to think about recap when there will be repeated capacitor breakdowns. -
Replacing all the capacitors, systematically, without even to look for the origin of the defect, this is, as I wrote, a complete mistake.
Of course I would look for the cause of the problem including by a close detailed inspection but once I have to pull such an old piece of equipment apart to gain access, I am going to replace the aluminum electrolytic capacitors and certain other parts if they could have caused the problem. This goes double when the aluminum electrolytic capacitors are known to be a problem 20 years later as with the 22xx series of oscilloscopes. (1) (2) The capacitors get tested for capacitance, dissipation, and maybe leakage *after* being pulled to verify if they could have caused the problem.
Changing the X and Y class capacitors is a preventative measure. The ones Tektronix used fail with age. I even have some in front of me on 22xx main boards which are split open.
And if I have to order capacitors and other parts, then I would rather order them all at once than replace things individually with separate orders.
Another thing I would do it replace that connector on the TO-220 power transistor. I use those connectors on transistors sometimes when prototyping but in a finished design, I would solder the wires directly to the transistor leads. I thought there was a service bulletin from Tektronix about doing this but if so, I have lost track of it.QuoteIt is very unlikely that it would be an electrolytic capacitor that make that kind of noise because once the thin Al²O3 insulation is pierced and destroyed, the space between the electrolyte and the aluminum becomes almost zero and it would be impossible that the power supply can restart to such a hig level that a led can be flashing.
Low capacitance and/or high ESR from a worn out aluminum electrolytic capacitor can cause all kind of strange behavior in switching power supplies including hissing and buzzing.My Marantz 2285B from 1978 that I use everyday has only one capacitor replaced.
And it still has the same performance as when it was new .... worn capacitors? They are 40 years old of good and loyal service and they will probably hold another 10 or 20 years .... It will always be time to think about recap when there will be repeated capacitor breakdowns.
The common case that I run across is that once one of the aluminum capacitors has failed do to less derating than the others, the others have exceeded their rated operating life by several times and are displaying low, but not zero, capacitance and high, sometimes 10 times high, dissipation. This is not such a big deal with bulk decoupling capacitors which may only marginally affect circuit performance but input and output capacitors are more important and especially so in switching power supplies where ripple currents are higher.
(1) There is nothing wrong with the 22xx design in regard with the capacitor operating life. That is just the way it goes with aluminum electrolytic capacitors; they have known wear out mechanisms so do not last forever and degrade even when not used.
(2) There also seems to be a failure mode where old 22xx switching power supplies self destruct and I have not ruled out bad capacitors causing this.
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I notice that we have a completely different conception of what is repairing an electronic equipment ... Your conception is rather what is called "reconditioning" and not repairing.
I do not like preventive electronic maintenance because I believe that in electronics, everything is measurable and one can find a defect in a component by measures, even sometimes simple measurements as ripple.
In my experience, the more you want to do (and in particular replace components unnecessarily), the more you create problems.
The systematic replacement of components is often a solution for those who do not understand what they are doing (not your case, of course) or beginners .... I am totally opposed to it.
It is sometimes also a solution in the factory because the time is expensive and the repaired SMPS must pass by the same tests as a new power supplies .... one can not afford to see the power supply refused to the tests, then, one changes all the capacitors of output filters to be sure ...That's what I was doing when I was working by Cherokee Europe. In factory, the capacitors are much cheaper than the labor.
But for a Tek 2235 oscilloscope that is old-fashioned, which is worth 100 bucks and is likely to be in the trashbin in a few years (nobody want an analog oscilloscope anymore), for me, what must be done is to look for the faulty component, replace it , and check the power supplies, ripple, correct functional operation, and make an approximate calibration and it is good for some more years of use.
And do not pull such an old piece of equipment apart If not absolutely necessary...!!!! (sometimes, no way, you have to pull it apart...See picture of repair of an Hameg HM 605 with bad solder in output horizontal amplifier).
Ten or fifteen years ago, I was repairing a lot of analog oscilloscopes of different brands, but now, there is no more analog oscilloscopes to be repair....People prefer to buy a new DSO than to spend time or money repairing old analog scopes.
I still have some success with vintage audio and I am applying my conception of repair without any problem....all my clients are very satisfied and I had no return of any problem after my repairs.
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I notice that we have a completely different conception of what is repairing an electronic equipment ... Your conception is rather what is called "reconditioning" and not repairing.
I would reserve reconditioning or refurbishing to cases where I do so comprehensively. For a power supply repair, I tend to refurbish anything I have to gain access to so I only have to do so once. Further I know old aluminum electrolytic capacitors have reliability issues and they are easy to replace.QuoteI do not like preventive electronic maintenance because I believe that in electronics, everything is measurable and one can find a defect in a component by measures, even sometimes simple measurements as ripple.
The systematic replacement of components is often a solution for those who do not understand what they are doing (not your case, of course) or beginners .... I am totally opposed to it.
Sometimes it is less time consuming and safer to replace multiple suspected parts and if I have to pull a part to test it, I am not likely to reinstall it. There is also a risk tradeoff to consider.
For example if I suspect a high voltage DC restorer problem, then I am likely to replace the diodes and capacitors without testing them in circuit because testing is both hazardous and very difficult. Once I pull those parts making testing slightly easier, I am hardly going to reinstall them so they are all going to be replaced.
The 22xx switching power supplies count here because I know they have a mysterious destructive failure mode which seems to occur when the capacitors are worn out. The only reason it may be mysterious to me is simply because I have never gotten my hands on one which has failed destructively. All of mine work perfectly.QuoteIn my experience, the more you want to do (and in particular replace components unnecessarily), the more you create problems.
I agree and this is especially a problem when lack of experience is a factor in which case I council very limited tests and repairs.QuoteBut for a Tek 2235 oscilloscope that is old-fashioned, which is worth 100 bucks and is likely to be in the trashbin in a few years (nobody want an analog oscilloscope anymore), for me, what must be done is to look for the faulty component, replace it , and check the power supplies, ripple, correct functional operation, and make an approximate calibration and it is good for some more years of use.
I would not use a 2235 but only because I have better old oscilloscopes some of which have useful capabilities not duplicated in many modern DSOs unless you want to spend a lot of money.QuoteI still have some success with vintage audio and I am applying my conception of repair without any problem....all my clients are very satisfied and I had no return of any problem after my repairs.
Most audio clients would not notice subtle problems.
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This is the rate of failure of electrolytic capacitors....
If you replace a capacitor by a new one, you must be sure you are in the wear-out failure segment...And to be sure of that, you must have first several failures of the same brand of capacitor in the same equipment...
If you replace them too soon (during the random failure segments), you will have more failures than if you don't replace them.
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And rate of failure of a new electrolytic capacitor may even be higher than of the old one.
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I found the schematic by doing a web search for "tektronix 2235 schematic" not sure what site.
So I finally removed the PSU cover and could have at least a visual inspection of all components. I can't see any signs of any leaking caps. But I can't be sure they aren't defective since I don't have the proper equipment.
I did however try and test the diodes around the T948 and it seems as if most of them are shorted. Could this be correct? If so how could that have happened? I tried two multimeters, one fluke 83 and a cheaper uni-t UT132C. They pretty much showed the same results but the uni-t one showed ".Ol" for values over 1.5 V.Voltage drop (mV) # fwd rev CR960 50 50 CR962 50 50 CR961 0 0 CR963 0 0 CR956 337 407 CR957 337 403 CR954 439 2527 CR955 439 2526 CR879 492 Ol CR907 493 999 CR970 99 157 CR967 99 157
I tested even more diodes around the board(s) and found several more that were shorted. It all seems very weird to me.
I will try and read through the linked thread and see if I learn something useful.
Thanks for all the help so far.
EDIT:
If I replace the caps. What should I be looking for in terms of specs and what should I avoid. For instance should they be Japaneese named brands only? Low ESR? -
This is the rate of failure of electrolytic capacitors....
If you replace a capacitor by a new one, you must be sure you are in the wear-out failure segment...And to be sure of that, you must have first several failures of the same brand of capacitor in the same equipment...
If you replace them too soon (during the random failure segments), you will have more failures than if you don't replace them.
Not having the aluminum electrolytic capacitors in the wear out portion of their reliability curve is hardly going to be a problem in a piece of test equipment which at a minimum is 28 years old. Input capacitors in linear power supplies last the longest because they end up massively derated but even they can fail at that point though corrosion of the lead connections as water or oxygen leak through the rubber seal; eventually they dry out also.
I leave 60 Hz input capacitors alone until one actually fail but the ones in my 7603 are finally going bad do to low capacitance so it is time to rebuild its power supply. When I had one of the big input can capacitors fail in my PS503A power supplies, I bought 4 replacements and changed them all in 2 different power supplies. I changed the other capacitors as well and they tested as worn out with a dissipation and ESR 10 times greater than their operating life specification although their capacitance was still within tolerance. There is a Sprague 30D series aluminum electrolytic capacitor in the z-axis amplifier of the 7904 which in my case was bad *despite* testing good for capacitance and dissipation at 1000Hz; its high frequency performance had decreased.
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So I finally removed the PSU cover and could have at least a visual inspection of all components. I can't see any signs of any leaking caps. But I can't be sure they aren't defective since I don't have the proper equipment.
The designer properly derated them so they do not generally show external signs of leakage.QuoteI did however try and test the diodes around the T948 and it seems as if most of them are shorted. Could this be correct? If so how could that have happened? I tried two multimeters, one fluke 83 and a cheaper uni-t UT132C. They pretty much showed the same results but the uni-t one showed ".Ol" for values over 1.5 V.
The diodes are almost certainly going to test as a short or close to it in-circuit because of the load from the rest of the circuits through the transformer windings. I do not see any measurement in your list indicating a problem.QuoteIf I replace the caps. What should I be looking for in terms of specs and what should I avoid. For instance should they be Japaneese named brands only? Low ESR?
Look for low ESR, high frequency, and high ripple current capacitors intended for switching power supply applications. I like the Nichicon PW series for general use but there are many options.
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Why don't you power on your oscilloscope with the power supply cover removed (beeing carefull !!!) to listen where is the noise coming from and to look if there is no arcing ?
I use to test 2235 feeding C940 with an external power supply 42V current limited to 2A.
If the scope is functioning correctly, then you know the problem is coming from pre-regulator. -
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Not having the aluminum electrolytic capacitors in the wear out portion of their reliability curve is hardly going to be a problem in a piece of test equipment which at a minimum is 28 years old. Input capacitors in linear power supplies last the longest because they end up massively derated but even they can fail at that point though corrosion of the lead connections as water or oxygen leak through the rubber seal; eventually they dry out also.
there are a lot of parameters influencing the lifetime of the capacitor.
Lifetime is given in hours of working...this can vary a lot....
How many years the oscilloscope old is, that does not matter.
What matter is how many hours did it worked and at what temperature...Every 10°, lifetime is reduced by 2....
As I said, my Marantz from 1978 is still working with all is electrolytics, only one has been replaced...And this receiver is 39 years old....
I have also a SANSUI AU-5500 from 1975 working very well with all his originals electrolytics...42 years old !!!
Others Sansui's AU-7700, AU-999, AU- 9500 from the same time are also still working with their originals electrolytics ....It is prooven that the fact the oscilloscope is 28 years old does not mean that the capacitors are in the wear-out failure segment.
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Quote
Look for low ESR, high frequency, and high ripple current capacitors intended for switching power supply applications. I like the Nichicon PW series for general use but there are many options.
Not at all, if you replace capacitors, buy common capacitors of good brand, no need to be low ESR.
But buy those capacitors from reliable electronic parts suppliers NOT FROM EBAY...!!!!
Why it is not neccessary and even wrong to replace the capacitors by special low ESR capacitors ?
Some people suffers what I call "ESR madness".....It seems that ESR is the name of some godess and that they are repeating again and again their prays to the adoration of this godess...
1) Look at the Service manual of 2235: you have the list of replaceable electrical parts of A1 with all the specifications of the components....none specification of low ESR capacitors...!!!!
One of the rules in repairing is to respect the technical specifications of the manufacturer when you replace a component....Not better, not worse component, only the same as specified.
Could replace a component by a better one cause any harm ?
Yes, it can: I have here a Marantz Model 170dc to repair...It has beeing repair before and the "clever" guy has replaced some transistors by new ones, other types than what the manufacturer had recomended...
Those more moderne transistors has greater gain....What happen ? the power stage where he replaced the transistors is oscillating at about 2 Mhz...
2) What are the capacitors with high ripple current ? (NB: if there only low ripple current, for sure, there is no need of a low ESR capacitor)
- C906: has both ripple current of (120) 100Hz of rectified main's voltage and ripple from buck pre-regulator but ripple voltage on capacitor does not seems critical....there no specification of max ripple, nor test point on this capacitor.
- C925: has no ripple current but is very critical for leakage....should be replaced but low ESR is not needed.
- C940: Has both ripple current from the pre-regulator and from the square wave inverter T948. The square wave inverter has very low ripple current, its nearby dc current. Ripple current is limited by the inductance of T906. No high ripple current, no need to be low ESR.
- C944: very low ripple current because drive current of Q946 and Q947 is square wave. No need to be low ESR.
- C943: very low ripple current because collector current of Q946 and Q947 is square wave. No need to be low ESR.
- C942: no ripple current, no need to be low ESR.
- C968 : very low ripple current because square wave inverter and load not varying very much, not necessary to be low ESR.
- C970 : very low ripple current because square wave inverter and load not varying very much, not necessary to be low ESR.
- C960 : very low ripple current because square wave inverter and load not varying very much, not necessary to be low ESR.
- C961 : very low ripple current because square wave inverter and load not varying very much, not necessary to be low ESR.
- C956 : very low ripple current because square wave inverter and load not varying very much, not necessary to be low ESR.
- C954 : very low ripple current because square wave inverter and load not varying very much, not necessary to be low ESR.
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1) Look at the Service manual of 2235: you have the list of replaceable electrical parts of A1 with all the specifications of the components....none specification of low ESR capacitors...!!!!
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. They also did not distinguish between tantalum and aluminum electrolytic capacitors.QuoteOne of the rules in repairing is to respect the technical specifications of the manufacturer when you replace a component....Not better, not worse component, only the same as specified.
Unless you find the specifications based on the manufacturer's part number or maybe the entry in the Tektronix Common Parts Design Catalog which I do not have, then you will never know. The best you can do is to reverse engineer the design and sometimes this will reveal shortcomings where better parts should have been used.QuoteOne of the rules in repairing is to respect the technical specifications of the manufacturer when you replace a component....Not better, not worse component, only the same as specified.
This gets really tricky with capacitors in old switching power supplies because modern capacitors with the same voltage rating and capacitance will often have a *worse* ripple current rating. This is simply because they are smaller. One way to make up for this is to voltage derate the capacitor forcing it to be physically larger. I would actually voltage derate C906 and C940 for this reason.
I did not want to complicate matters by discussing the requirements of every capacitor in the circuit. I used the Nichicon PW series as a low cost example which met the requirements for all of the capacitors.
The capacitors on the inverter do not require high ripple current but low ESR is a good idea for them anyway to do filtering requirements.
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So there is nothing wrong with the diodes?
I am a bit curious as to what I would find if I tried to start the scope. Is there some easy way of discharging the caps after pulling the cord again? Or how long should I wait before it is safe to poke around?
Is nichicon the same as nippon chemi-con? That is the closest brand name I Could find at my local retailer.
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So there is nothing wrong with the diodes?
As far as I can tell from your measurements, there is nothing wrong with the diodes.QuoteI am a bit curious as to what I would find if I tried to start the scope. Is there some easy way of discharging the caps after pulling the cord again? Or how long should I wait before it is safe to poke around?
In this design, the capacitors will discharge very quickly once power is removed so manually discharging them is not necessary.QuoteIs nichicon the same as nippon chemi-con? That is the closest brand name I Could find at my local retailer.
Nichicon and Nippon Chemi-Con are different companies. They both make suitable capacitors.
I have been working on a spreadsheet showing recommended replacement parts for the 22xx series power supplies however I have not gotten up to the capacitors except to note which models used which values.
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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.
My thinking is that it might be caused by too high ripple in the PSU and thus faulty caps in there somewhere? Am I totally confused? -
Quote
So there is nothing wrong with the diodes?
It seems all right.QuoteI am a bit curious as to what I would find if I tried to start the scope. Is there some easy way of discharging the caps after pulling the cord again? Or how long should I wait before it is safe to poke around?
The only dangerous capacitor is C906....It will discharge slowly, you can measure the voltage and disharge the capacitor with a 1K 5W resistor if necessary.
There may be some high voltage on the PDA connection of CRT, but no real danger of this side.
Power on this scope, and report what you see and listen.....Ok, done, we have a solution now !
QuoteTektronix *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 whe 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.
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...
The explanation is simpler: low-esr capacitors did not exist at the time this oscilloscope was developed and manufactured.
Later, Tektronix has used OEM SMPS and did no more give detailed schematics and part list of their power supplies. -
I did just that, in my post below . Not sure what the problem is though. Do those components behavior mean anything to you?
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I did just that, in my post below
Yes, it does....they are protecting the power supply and other components if the power is not regulated anymore and voltages goes too high.... . Not sure what the problem is though. Do those components behavior mean anything to you? -
Have you a multimeter with min/max recording ?
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Yes! Seems like the fluke does
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