Tektronix 2465B 50 Ohm Overload

[Sublimis]

I was curious if any one could give me some insight on trying to find the root cause of this error. I had my scope turned on no probes about 5 min the scope threw this error. Now when I put a probe on and put it on the probe cal on the scope, doesn't matter the channel and switch it to 50 ohm DC it throws the error after about 5 seconds. I have been talking to a person here and there and has been helpful in some ways but I'm new to this and not sure where to look for some of the items that he stated. He told me to look for cracks in the 50Ohm and thermistor are these in the attenuator? Totally new to this and I appreciate any help.
 

[siggi]

Have you tried running the self-tests http://www.tek.com/support/faqs/how-do-you-run-self-tests-2465, and do they run to completion without failure?
Does the scope look to be reasonably in cal? Note that the frequency of the compensation output on the 2465B changes with the time base, so you can do a quick check of the timebase calibration by hooking a 10X probe to the calibrator, then running up and down the timebase settings. You should have the same number of cycles on screen from way low until the calibrator is at 5MHz or so.

A common problem with these scopes is leaking SMD capacitors on the A5 control board. If the scope is new to you, and you don't know whether these have been changed, that'd be my first port of call. It's quick work to change them and should IMHO be done preventatively as this is a very common problem. The A5 board is quite easy to get to - pop the case and it'll staring at you from the right-hand side of the scope. A couple of connectors and a couple of screws, and it's out.

The gunk from the caps tends to mess with components that derive the reference current for the DAC on the A5 board. Since the 2465B is pretty much all drive-by-wire, this will cause all kinds of fallout, including trouble in reading back from the thermistors in the attenuators.

Incidentally, the quickest and easiest way to diagnose DAC reference trouble is to measure the 1.36V and the -1.24V reference voltages on the A5 board. Some versions of the board break them out to test points, but on other (later?) versions you have to hunt them down to component legs or the like.
If those voltages are grossly off (say more than 5% or so), you have a smoking gun.

[Sublimis]

Thank you for the reply. Yes it passes all tests and seems to be decent cal. This is a lower serial scope and I didn't see any surface mount except in the attenuators. I would like to replace all the caps, does anyone have a cap list for easy ordering? I'll do more detail testing this week.

[siggi]

Nice, your A5 looks very much like what I remember of my 2467's, including the discrete keeper battery. That's another thing to mind - if you're not set to easily recalibrate, mind that the battery is still good. May want to change it preemptively as well, as it powers the NVRAM that stores the calibration.

As for diagnosis, since you don't have the SMD caps, go look at power supplies and ripple at J119. Also, I'm pretty sure your A5 board breaks the 1.36V and -1.24V references out to test points. If so, measure them, It's only a moment's work.

For the cap list, I'm pretty sure one was kicking about on the Yahoo TekScopes forum. You may want to enlist...


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[tggzzz]

does anyone have a cap list for easy ordering?

For the mains metal film caps in http://www.condoraudio.com/wp-content/uploads/Projects/Tektronix-2465B-Oscilloscope-Restoration-Repair.pdf
http://uk.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?partNumber=1673307
http://uk.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?partNumber=2525311
http://uk.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?partNumber=2525309
http://uk.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?partNumber=2455939

It is difficult to see, but it looks like your 0.068uF caps are waiting to emit the magic smoke. (On my board they toasted a 68R resistor and the PCB's prepreg)

[Alex Eisenhut]

Is this the series of scopes that must be run with the cover on, otherwise the air isn't flowing properly over those custom chips and they overheat?

[siggi]

Is this the series of scopes that must be run with the cover on, otherwise the air isn't flowing properly over those custom chips and they overheat?

No, these scopes aren't likely to roast themselves when run out of the case. You're probably thinking of the 2430/2440 et al, the ones with CCD acquisition hybrids, a fan that sounds like a 747 at takeoff, and an explicit warning to use a bench fan in the service manual.

That being said, I like to have a bench fan on mine. U800 in particular gets quite warm without airflow, and it sounds like thermal cycling is what does those in, ultimately.


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[BravoV]

I think its safer to put the cover on as it has specially placed vent holes (intake) to blow directly at hot chips, such as the unobtainium U800. Without the cover, its better to have big fan blow throughout the board rather than leave the ambient air to do the cooling.


Scope's bottom view, the U800 is located at the left top one. These 4 groups are the only fresh air intakes for the whole scope. Tek designed and made it for a reason.




Close up view on the vent holes, you can see the U800 is located straight below.




Hot spots thru IR cam :