Re: A 2465 teaser...

Chuck Harris

Hi Mac,

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.

-Chuck Harris

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.
U485A and U485B form a differential amplifier whose operation depends on current to their common emitters through R483. Diode CR484 is fitted to stop the amplifier by "stealing" the emitter current when U975 pin 1 is high. U485C and U485D form a similar amplifier that is disabled when the Pin 3 of U975 is high. The outputs from these two differential amplifers is joined by the common collectors of U485B and U485D. U975A acts to select one or the other (not both) of these two amplifiers. Amplifier One (U485A and U485B) carries the readout signal as presented on pin 2 of U485A. Amplifier Two (U485C and U485D) "amplifies" the ground that is present on pin 6 of U485C. Thus, the signal at pin 6 of U475C is either Readout Vertical or ground, depending on the logic state of U975A. U475C is a simple emitter follower to provide the current that drives the input to Ch5+ on U400 and other circuitry. That other circuitry includes another differential amplifier (unswitched this time) comprised of U475A and U475B that inverts the Ch5+ signal for input to U475D and on to Ch5-. U475D is another emitter follower to drive current into Ch5- on U400.
Bottom line, a somewhat complex yet straight forward "mux" to present either Readout Vertical or Ground to the inputs of U400 Ch5.
I was surprised by the number of components Tek used to accomplish such a simple task and aks the question: "why doesn't the Channel Switch ignore Ch5 inputs when Ch1 or Ch2 is selected?" I concluded that late in the development of the 2465 they discovered U400 "leaks" Ch5 into Ch1 and Ch2. The dies for U400 were proved and mass production had likely begun. The only solution was to block the "leak" by adding a switch in the pathway for Ch5 input so it is "grounded" when Ch1 or Ch2 is running. No time for special IC's and fidelity was important. So Tek built a kludge to mitigate the problem. It was cheap, and it worked, and it prevented a complex redesign of U400, so it stayed for the 2465A and 2465B.

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