Tektronix Oscilloscope problem

[sido]

Hi all, must be the time of Oscilloscope failures. Just read Tinkerers thread as he is having faults with his. Mine seam more serious though.

Took out my Tek 2215A 60Mhz Oscilloscope from storage to use on projects here. Was working a year ago since being in storage. Well.......

Powered it up today, unit ran perfect for about 5 minutes, then i noticed the square wave signal on the CRT i was viewing began to shrink slowly towards the center on the screen, then followed a burning bad smell beginning to emerge from the case. (similar to a bad cap).

I quickly switched unit off from the power switch on the unit.

Note: The unit's fuse did not blow while this happened at all.

Interesting thing happened though......Upon switching unit off, 3 seconds later my house safety switch tripped and i lost AC 240 Volt to my power points.....HOW Can that be?

Any way, i removed the covers to inspect the unit and this is what i have discovered so far. I suspect the power supply has given up, considering the unit is 28 years old and looks like it has not been refurbished since being built from factory. Original factory caps noted.

The Pics's below show the suspected faulty visible part. Its a 0.01 mf Capacitor which is connected to ground. Not sure what other damaged parts could be there.

I have the Full Service manual for this scope, but not familiar with SMPS, apart from the obvious warnings of high voltage's floating around this area and the anode lead that goes to the CRT from the flyback transformer. I have discharged that lead to ground but have not done the SMPS caps so far.

I have signed up with the Tektronix forum, but i am waiting for moderator approval for now.

In the meantime any opinions or suggestions are welcome as to how to go about fixing this problem, Thanks

Cheers Sido

[WM6]

In old devices mostly electrolyte C are problem.

Here we have high voltage which can be dangerous too to (foliar) C which lost their dielectric properties.

If your HV C is heating, then it is short-circuited inside. If all other part around are OK (take a look at bottom side of PCB too), you can try to broke visible C pin and put in series with broken capacitor new C with the same value as on picture:



If yo do not have such HV capacitor in your stock, you can try even without this new C, only with broken pin of existing C (seems smoothing C for HV) without some fatal outcome. If burning bad smell appear again there must additional error somewhere.

[sido]

Hi WM6, thanks for the reply.

I was thinking of breaking the connection of the Capacitor after seeing the schematic. You are possibly right that it is a smoothing capacitor that may have shorted.

There is another 2 identical Capacitor's that a connected in-line and also go to ground.

See schematic below.

I dont have an identical or equivalent at hand now, but will see if i can source one out of a old CRT computer screen in the shed.

There is no other visible burnt marks on other components or the under side of the PCB that i can see.

I think if i break the connection it might give me a quick test. What do you think? What possible damage can be done? Maybe the other identical capacitor might burn? If so then i might have some other issues other than that bad looking capacitor. (Waiting for all replies before attempting the above)

Oh BTW i have a portable safety switch that i will plug the mains power cable too for extra safety.

Cheers Sido

[WM6]

I think if i break the connection it might give me a quick test. What do you think? What possible damage can be done? Maybe the other identical capacitor might burn?

Yes, you can break the upper pin of damaged capacitor for quick test without tragic outcome.

Other capacitor might burn if it is in same bad condition as first, otherwise not.

But if you wish, you can replace bad C for test with more lover voltage capacitor in series too. Maybe you have 7 capacitor of 68nF suitable for 500V=. If you put those 7 capacitors (or at least 6) in series, you can get approximate C replacement for test. Be careful to not to make some new shortage with those chain of capacitors.

[sido]

Will do so WM6....But it will have to be tomorrow as now its dark and cold here and i dont want the house power going off if problem continues. I have a isolation transformer which i could use but it is in storage. Thanks for the advise and will keep you posted.

Cheers Sido

[WM6]

Interesting thing happened though......Upon switching unit off, 3 seconds later my house safety switch tripped and i lost AC 240 Volt to my power points.....HOW Can that be?

Look at schematic for Switch ON/OFF position. It would be redundant to measure resistance on your scope power cable at switch ON position (not connected to main power point of course) first. Check on possible shortage at main input, you have to measure value of resistance of primary windings of power supply main transformator.

[sido]

Hi WM6, i assume you mean to have the power switch in the "ON" position without power connected and measure the resistance across active and neutral.

Well i did so and there is no short showing up either active to neutral, active to earth or neither neutral to earth.

Still wondering why the Oscilloscope fuse did not blow under the excessive load generated from the shorted capacitor??. I turned off the Oscilloscope power switch, with power cable still connected to power, but a few seconds later the house safety switch turned off.

Maybe the slow time it took the excess current draw took to build up in the scope caused excess heat to the power cable on the AC wall plug which happens to be a big run of cable to the fuse box at the front of the house. You would think that the safety switch would have activated well before i switched the unit off.

At least we can say that is a good sign, no short on input power.

I am eager to snip that Cap lead, but will have to wait till tomorrow.

Cheers Sido

[WM6]

You have filter between switch ON/OFF and cable and this can cause main house safety switch turn off. Main house safety switch work on a little different way than usual mains fuses (of course, if we are talking about same thing here called PID switch).

You can try this:
switch scope ON and then take main scope fuse out of the scope (without switch OF using scope main switch). In this case main house safety switch should survive, which mean that cause is to find between scope main switch and power cable connector on scope housing (mean in filter module).

Please, post pictures of schematic in a little better resolution.

[sido]

Sorry about the picture quality. Hope this is much clearer.

Yes the house safety switch is of PID type.

Do you mean remove fuse, then apply power with switch on? to see if the safety switch drops? If not then check this filter section with power disconnected for faulty parts?

Cheers Sido

[sido]

Or do you mean that if the safety switch trips, then i should check the filter section for faulty parts in case neutral power wire is shorting to earth?

Cheers Sido

[WM6]

Do you mean remove fuse, then apply power with switch on? to see if the safety switch drops? If not then check this filter section with power disconnected for faulty parts?

Cheers Sido

No, first apply power switch ON (and left it at ON position) and then "switch OFF" scope by scope fuse removing only.

If after about 3 sec PID switch not dropped, you need to check filter module (section between power cable connector on scope housing and scope power switch) for faults.

Of course if filter board is not potted with epoxy resin.

[WM6]

Or do you mean that if the safety switch trips, then i should check the filter section for faulty parts in case neutral power wire is shorting to earth?

Cheers Sido

It can be faulty part or too high humidity in isolating parts (due long storage) in connection with back EMS occurrence inside filter module.

After long storage it is advisable to adopt such device to working conditions in warm and dry place a couple of days.

But PID is very sensible switch and some other reason, not directly connected with possibly faulty scope parts, can occur also.

[WM6]

Slip - Correction: not PID but FID switch.

[sido]

Hi WM6,

I snipped the faulty capacitor and the unit fired up as in the picture below.

I measured all voltages as per schematic below and all are spot on. The only voltage i did not measure is the 2Kv going to the CRT as i do not have a meter capable of do so, but seeing the CRT works, it must be ok.

I tried the switch off, like you suggested from fuse. The safety switch did not trip.

I have the unit running now for 2 hour and power supply seams ok.

I Still have a problem as i cannot get the baseline to center in the middle of both channel traces on the display and it seams that the input channels are not responding to a input signal source? I can only adjust both channels one graticule division only (as marked in red in picture).

I measured the horizontal amplifier regulator 79l05 for a input of -8.6v which is present for a output of -5v as per schematic. So that is working ok.

I cannot adjust both trace's to be in the center maybe because the vertical amplifier has a problem? Not sure where else to check.

Would that not working capacitor i snipped be a problem? I doubt it.

I have had a look at the schematic, but not familiar in operation.

Cheers Sido

[sido]

Cannot adjust to here.

Cheers Sido