Tektronix 2215 Possible CRT Damage....?

[Skai182]

Hi All,

About a year ago I bought a second hand Tektronix 2215. After I powered it up it worked perfectly, and it has ever since, untill yesterday... I was testing some preamplifier and I had the scope on for already about an hour and a half. I went to the bathroom and when I came back my friend had turned the scope off saying it was making a funny sound.
Now, from the beginning i.e. when I first got the scope, it always made this high frequency "startup" sound upon power up. That sound was always very clean and sounded like nothing was wrong with. After the sound occured, the scope nicely powered up and the trace appeared. Now though, after yesterday's incident, when I power up de scope, that sound is really weird and it sounds like the scope is gonna blow! The sound is really high pitched and sounds very crooked. I don't really know how to describe it exactly  but what I said above is the best I can do. Also the trace doesn't appear after power up.

I really have no clue what is going on here. Especially since I've used it for about a year already this happened so suddenly.

I've opened the scope up and did a visual inspection, I didn't see anything blown or weird. I've check the service manual and the internet to find something about the sound or anything particularly related. That's basically as much as I can do since I do not yet have the knowledge what is precisely going on in one of these oscilloscopes.

I sincerely hope anyone can help me out even I slight bit.

Thank you in advance,

Kai

[Andy Watson]

It is most unlikely to be the CRT. All the power supplies in this instrument are derived from one switching power supply - the source of the noises that you can hear. It is most likely that either a fault has occurred on one of the power supply rails (e.g. a shorted capacitor) which is bringing the remaining supplies down, or the switching power supply is faulty.

Have you obtained the service manual?

[alsetalokin4017]

Just guessing based on your description this sounds like a fault in the HV power supply. Do you see anything at all when you press "Beam Find"? You probably don't want to turn it on much, to avoid further damage, but a brief test to see if you get anything with Beam Find is probably OK.

These old scopes have a lot of tantalum capacitors that can go bad, and also regular electrolytic caps. Another very careful inspection with a magnifying glass may reveal a faulty tantalum capacitor, or maybe not. It might be worthwhile to go over it again visually, with a magnifying glass and strong light.

[Skai182]

Thanks for the replies,

When I did the first visual check I tried to get under the highly capsuled HV powersupply but failed due to the fact that I was scared to damage anything. Man they are really tucked away good in this scope. Anyway I feel you guys are right, and so I will have another go at it! Thanks again.

Any more tips, suggestions etc. are always welcome. I will keep the topic up to date.

[richnormand]

The noise you hear is most likely from the inverter power supply. frequency is normally above the hearing range but with aloose winding you could have sub-harmonics or under heavy load the frequency could go down. All depends on the PSU design.
Have a look at:

http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=Tektronix/Tektronix_-_2215A

It might be worthwhile to curl up and read it cover to cover.

Lookup the service manual and locate unit board 7. As you can see there is a single PSU to supply everything.

The clean high pitch sound before was most likely the inverter oscillator and if the main transformer is a bit old some coils may be a bit loose and will whine. The garbled sound is most likely that sound but with the supply under a bad load. The over current/under voltage protection circuit will chop the inverter on and off in a variable duty cycle and generate that weird noise. In a full short you would hear the supply emit a sharp "click" every half second or so as it tries to start and then collapses.

Other possibility is a carbon arc on the CRT high voltage power supply. Try to operate in the dark and see if that is the case. I use an extension with a switch to power the unit dow at a safe distance insted of poking my fingers in there in the dark... That certainty would present a wierd load to the main PSU!

I would first look for older bulged out or dry electrolytics capacitors. They can really upset the PSU. The service manual has a flow chart for trouble shooting that you may want to follow. I have not fully read it but, depending on how the protection circuit works, it might be allowable to remove (unplug) each load off the PSU one by one and see if ithe PSU is happy. That would place the problem outside the PSU.

There is also a full set of voltages and waveforms in the PSU to help the trouble shooting but you need to be familiar with this type of testing with potentially lethal voltages inside. If you look the the schematics my first suspect is an excessive load on one of the PSU output lines (shorted diode, cap etc..), the pre-regulator (too low voltage to invertor, wrong feedback signal), the inverter itself (switching trans or shorted winding in the transfo for example)..

Best of luck with it.

[oldway]

- Check -8.6V, +8.6V, +30V and +100V rails for short circuits.

If everything is all right,

- disconnect ac side of HV multiplier (U990) and switch the power supply on: can you still hear that noise ?

[Skai182]

Ok so I've tested it again, when I turn it on there's a super high super annoying high frequency sound. A trace weirdly but straightly appeared on the crt. I turned it off and left it. Today I turned it on again, same sound, no trace appeared. When I hit beam test there was this vague quite large dot on the screen. I opened up the HV  PSU section. I can not find anything looking blown or weird at all.... There are some 840MDF film capacitors on the main board, should I replace those? I don't have a capacitor tester or anything just a small multimeter.

[oldway]

I asked you to disconnect ac = alternative current side of HV multiplier (U990) , not to disconnect the crt.
You must desolder and isolate the wire between Ferrite Transformer T940 and HV multiplier U990.
It is possible that the HV multiplier is bad.

I have the service manual of the 2215, the preregulator is triac controlled with a big 50/60Hz inductor mounted on the rear panel.

[Skai182]

Hi oldway,

I disconnected the preregulator/inverter board. As you suggested the problem lays within that board. When I disconnected it and powered it up, sound went away. I cannot visually see anyting gone bad on the board so hmm.. The problem I'm facing now is that I can't find the schematic for this board in the manual (yes I have the 2215 version).

[oldway]

I disconnected the preregulator/inverter board.

I never asked you to do this 

As you suggested the problem lays within that board.

I never suggested this. 
I suggested the HV multiplier is defective.

And what about the  -8.6V, +8.6V, +30V and +100V rails ?

What resistance are you measuring between those rails and commun ?

[David Hess]

The 2215 and 2215A are very different designs so you will need the proper service manual for the schematics.  Also there are two major variations in the 2215 power supply; the early one uses an SCR preregulator.

The problem might be as simple as the aluminum electrolytic capacitors in the power supply needing to be replaced.

[richnormand]

The 2215 and 2215A are very different designs so you will need the proper service manual for the schematics.  Also there are two major variations in the 2215 power supply; the early one uses an SCR preregulator.

The problem might be as simple as the aluminum electrolytic capacitors in the power supply needing to be replaced.

Sorry, I did not realise there were that much differences in the PSU between these two models of the 2215 series. Most of my Tek encounters had some differences between version A B C etc... but not all that drastic. At the end of the day most inverter PSUs have several things in common.

My first guess, as previously mentioned by oldway, is to disconnect the input power to the HV module and see if the PSU recovers. Then measure the line outputs.

I also got from ZL1CVD in this forum a very useful reference that introduces the workings and troubleshooting of these switched inverter PSUs
  http://www.repairfaq.org/sam/smpsfaq.htm

[David Hess]

Sorry, I did not realise there were that much differences in the PSU between these two models of the 2215 series. Most of my Tek encounters had some differences between version A B C etc... but not all that drastic. At the end of the day most inverter PSUs have several things in common.

The 2213 uses the same power supply so its service manual will be just as good.

Early versions of the 2213/2215 used an SCR preregulator and clever current limit circuit.  Later this was replaced with an upgrade that included a buck switching preregulator which is what the 2213A/2215A used.  There were some other changes made as well because of various problems.  The 2213A/2215A are part of the 2235 series and completely replaced the earlier 2213/2215.

My first guess, as previously mentioned by oldway, is to disconnect the input power to the HV module and see if the PSU recovers. Then measure the line outputs.

I do not really recommend this as the design does not make it easy and it is probably unnecessary.  There is no separate high voltage inverter so the only thing you can disconnect is the three terminal high voltage multiplier which produces both the cathode voltage and the PDA (post deflection acceleration voltage).

I also got from ZL1CVD in this forum a very useful reference that introduces the workings and troubleshooting of these switched inverter PSUs
  http://www.repairfaq.org/sam/smpsfaq.htm

That guide is not going to be very much help in this case.  The power supplies in the 2215 and 22xx series do not use the usual topology.  They include an SCR or buck switching preregulator followed by a two transformer saturable core inverter with variable base drive to control the output voltages.  The inverter allows good primary side voltage regulation and low output noise.  They do not make them like this anymore.

[richnormand]

Thanks David,

thanks for the info.

I have a 2215 and , fortunately, never had to open it for repairs.
I downloaded the pdf service manual for the 2215A  a while ago in case I ever need it. So now I'll hunt for the proper version.
Cheers.