Re: R7903 Intensity control and readout loss

Brent Watson <brentleew2003@...>
 

Dave,
Thanks much for the advice. I especially enjoy Tek history and your insights.
I went ahead and replaced the 47/20v tantalums on the rectifier and Zaxis boards. I think they were fine but I had good replacements. So at this point I must have a bad component somewhere. Good to know the PS wont start without load. I just can't seem to get out of burst/tick mode, with load.  Is it possible that the large filter caps are somehow inadequate? They are much smaller than the originals and while they worked for a couple of weeks, could they be showing too much resistance because of their smaller size ?  My waveforms don't match the 9704 Power supply Servicing Tek note. The Tek note suggests that if resistances are normal and you're still in burst mode you may have one of the semi-regulated voltages overloaded. And to check you look at the waveforms as shown. As I said, mine don't match, therefore, you need to check the rectifier tantalums for shorts or leakage. I replaced them all.

If the power supply is OK, then the problem is in the mainframe- getting back to readout loss and intensity control loss. This should be a clue to the overloaded +15.. I also replaced the carbon comps on the axis, but I want to find the problem, not just change parts.

Still hunting!
Brent
From: "dadhills@... [TekScopes]" <TekScopes@...>
To: TekScopes@...
Sent: Wednesday, June 21, 2017 6:28 AM
Subject: [TekScopes] Re: R7903 Intensity control and readout loss

  OK. If C1893 and C1897 are solid blobs then they are tantalum
electrolytic capacitors. I know of no tantalum capacitor that is not an
electrolytic. Search on "tantalum capacitor", there is a wealth of info
out there from manufacturers, NASA, and others. In this case, the "K47"
gives it away, definitely a solid-tantalum electrolytic capacitor.

The Tek parts lists and schematics can be confusing, as sometimes they
will call out "tantalum" and sometimes not and the schematic may be
missing the polarizing "+" symbol. Keep in mind that these documents
predate CAD and the originals were hand drawn with parts lists created
on a typewriter or rudimentary word processor. Errors are common, it's
something we lived with back then.

As to the 47/20, 100/25, 220/25v conundrum, in the mid 70's through
80's the massive growth of electronics technology created many shortages
of components, especially ceramic and tantalum electrolytic capacitors.
Parts were allocated by manufacturers and often different value parts
were substituted in order to keep production going. Changes were
coming so fast, documentation could not keep up. Using a much larger
aluminum electrolytic cap in place of a tantalum electrolytic was
necessary in many cases to maintain the required ESR value. Your axial
tantalum 47/35v caps should work just fine as long as you observe the
correct polarity orientation.

Tantalum electrolytics almost never fail for capacity or ESR change,
they either short or go completely open.

I believe your Fluke has you covered for capacitance measurements, so
next would be leakage current. All you need is a few 9v batteries, your
DMM or scope and a resistor. Create a series circuit with the
batteries, capacitor and resistor with the battery voltage close to the
circuit operating voltage and less than the capacitors spec voltage.
Measure the voltage across the resistor take a reading after it stops
decreasing. Apply Ohm's law and there you have your leakage current.
*Be very careful with polarity!! Even a brief moment of reverse polarity
can kill a tantalum electrolytic!!*

With a pulse/function generator ESR can be measured, but you don't have
that. Not likely a problem with tantalum electrolytic capacitors anyway.

Tick/burst mode is normal with no load.

Good Hunting!
Dave

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