Re: Tektronix 2215, pulsing/chirping noise, no trace.

tekscopegroup@...
 

I doubt that you have a problem at the output of the pre-regulator that is causing the problem and it shutting down due to an abnormal condition on the load side. As you pointed out the scope works fine with an external +43 volts being fed into the secondary power supply. So that would point towards a faulty component that is not allowing the prereg to start up. In my experience repairing this section in a 2213A some years ago was that it is absolutely necessary to check everything in that stage specially transistors and diodes, even if in-circuit measurements do not point at the part being faulty before you even apply power at all. You need to pull every single transistor, diode, etc in the preregulator and test it, and all the lower value and/or higher power resistors as well for open circuit. Of course it goes without saying also check the power input components, like the 75uF cap and the associated rectifier, etc. Double check the Mosfet and driver transistor. Even if it was fine last time you checked, you need to double check everything in one batch before ever applying power. Check the regulator IC as well, I seem to remember it was a TL494 that sometimes just quits. Pull it out and place a socket then put in a new one. The socket will also allow for easier future testing & replacement. You might also have a faulty T906 switching transformer, which will be difficult to verify, so leave that possibility aside for last when you are sure everything else is ok and the preregulator is still not working. As a matter of fact my scope happened to have a bad T948 switching transformer, and the only ay to make sure it was the problem was to replace it. But I digress as this particular trasformer is located downstream from the preregulator circuit.

Unfortunately dealing with a switching power supply you will usually be able to follow a trail until you hit the fault because it works in a closed loop where everything either needs to be right, or all (or most) will be wrong. You will have to test every active component and make sure it is not open or shorted, leaky, etc. The fact that the fault happened while you where using the scope would suggest that it is not related to some component slowly going out of tolerance, but rather of something popping, usually at start up which is the (very short) time interval of highest stress for a switching regulator until after a few milliseconds when things quickly settle down.

This prereg stage, as with usually most other switching power supplies is very temperamental and will absolutely not work unless all is good, and will also tend to blow freshly replaced components during the intermediary "let me check if that was all that was bad" situation, and then you might be quickly sent back to square one (or even further back) with one or more of the higher power parts blown and/or smoking.

Again good luck with the hunt, and don't be discouraged. You will soon be rewarded with that green power LED on the front panel coming back on a fully working scope. And please don't even consider using some sort of permanent external power supply as a hack to operate the scope instead of fixing the problem.

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