Re: A 2465 teaser...
Chuck,toggle quoted messageShow quoted text
The only thing I see wrong with that description is I have to read it more carefully. I am embarrassed to admit that I only skimmed that description in the past and wrote my description without even looking at the manual (except for the schematic). So I agree, lets use the manual. Duhh
I, too, am an engineer, retired after 35+ years. Worked during my college years as a radio/TV technician and repairman. So nearly 40 years around oscilloscopes before assembling my home lab with three Tek scopes and a parts mule. Ive been retired 18 years now. I hate shotgunning for repair. It leaves me wondering if I fixed the problem or covered it up. Worse, like you said, I never learn how it was designed to work. That's why the amplifier/switch arrangement for Ch5 input to U400 stands out to me. It just doesn't fit with my expectation for quality design work. Your explanation of why its that way (someone wrote the spec wrong) is as good as any.
Anyway, glad you got it fixed.
On Sunday, November 12, 2017 10:05:39 AM CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:
Your description looks to be ok. But what is wrong with the description
in the manual? It is available for our use, and seems pretty clear to me.
My shame is I should have read it earlier as it contained lots of clues
that would have gotten me off of the bad power boondoggle.
2 or 3 years ago, I did read the description, and concluded that U475 or
U485 was likely bad. I replaced U475, but hesitated to change U485 when
replacing U475 had no effect. I don't like replacing parts shotgun style,
much preferring learning over efficiency. Shotgunning when done rationally
is very fast, but you never really learn what the failure was, just the
area where the failure occurred.
I think the difference in the way I approach repairs is because I am an
electrical engineer, not a technician. The failures a technician fixes
are circuits that once worked. Replace the broken part, and operation
will be restored. The failures an engineer fixes may never have worked
correctly. You have to replace the broken idea to fix an engineering
I don't believe the complexity in feeding CH5 is a mistake, but rather
an issue of partitioning... I believe the channel switch hybrid works
exactly the way it was supposed to work.
Partitioning was the job of the 2465 system engineer.
What I think they did is reason that the display needed to be updated
often, without fail, or it would flicker, or wink out. You can tolerate
a little of that in a trace, but in the display it would look ridiculously
broken. The display has to be stable. Display has to be priority 1.
So, what they did is make a 4 channel MUX for CH1-4, and give it individual
selection lines that mostly mirrored the state of the channel selection
buttons on the front panel.
CH1 and CH2 were special in that tradition required there be a function
where the MUX output is CH1 + CH2... the "ADD" function. That means there
is an adder gate before the CH1/CH2 MUX. I think the designer simply made
that adder gate a 3 input adder that added CH1+CH2+CH5, and presented
the sum to the channel switch MUX.
For this to work, the way you deselect an input is to change it to a
zero, or ground (Ch1 + CH2 + 0= no display...). The VS1 - VS4 selection
lines automatically do that for CH1 - CH4 input, but CH5 has no such
selection line, so the user of the Channel Switch is required to do it
Earlier I said it was a partitioning problem. I believe the mistake
that happened was the Channel Switch hybrid designer, and the Display
hybrid designer each thought the other was responsible for grounding
out the display input when data was not being displayed. The possibility
is also pretty good that neither designed their part with the 2465, or
the other designer, in mind. They designed to spec.
The simple fix would have been to add VS5 to deselect the display channel.
machine guy @Mac [TekScopes] wrote:
Chuck, I am sure you understand that "complex circuit" on Ch5 input to U400, the Channel Switch. I think I do but I will document it for others and I hope you will correct any errors.
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