Re: Tek 465 Negative 8 volt rail issues


Dave Peterson
 

I've added photos (and schematic) of waveforms for C1512 and C1513: https://groups.io/g/TekScopes/album?id=258720

What I realized looking at these is that the unregulated 55v top of C1513 is basically the transformer/rectifier output with no filtering at all. This "noise" is propagated throughout the power supply. The 120v unregulated node is being pulled down by this noise via the C1512 coupling (i.e. it has some remaining capacitance), and the other supplies are also affected because the 55v regulated output is going to the + input of all of the other supplies' comparator input. The 55v regulator cannot compensate for the full swing unfiltered noise of the unregulated node. Note that the 120v unregulated noise is not pulling to ground where the 55v unregulated node is pulling all the way to ground. No cap at all. Thus the conclusion that C1513 is the culprit. And FWIW, the 120v regulated output is reducing the noise pretty well. It's just still too much to remove entirely.

Pulling the caps verified that C1513 is suffering massive internal resistance, and all other caps are suffering large internal R and reduced effective capacitance. C1513 has capacitance, but it's behind the large internal cap. It'll charge and discharge _very_ slowly. It shows about 1uF at the lead, nominal 1200uF.

That aside, what these waveforms illustrate for me is the nature of "ripple current" a DC PS filter cap needs to contend with. Through out its lifetime a filter cap is going to be pulling AC current at source frequency to smooth the DC value. The cap must pull current during these rectifier valleys to maintain the DC value. There are a number of ways to measure this current in-situ, but I'm not going to put a measuring R in the circuit - as interesting as that'd be - and I'm not going to buy a current probe just for this. To me the AC supply current the cap is pulling is the true stress on these caps, and transient, while potentially relatively large, likely doesn't contribute to significant heating and the associated reduction in lifetime.

Thought I'd share the images as they give a nice visceral depiction of what these caps are filtering.

Dave

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