Re: A 2465 teaser...

 

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.  
Mac

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎07‎:‎25‎:‎58‎ ‎PM‎ ‎CST, machine guy @Mac [TekScopes] <TekScopes@...> wrote:

 
Glad you got it fixed.  Glad I could help.  Only the returning the many times you have helped me.

Mac

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎04‎:‎53‎:‎00‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

 
According to tektronix, U485A, U485B, U485C, and U475, form an
analog switch that routes either the readout at the base of U485A,
or the ground at the base of U485C to the output at the emitter of
U475C. The selection voltages are developed by voltage dividers
on the complementary outputs of U975C, and U975A.

Which pretty much fits your description exactly...

Also interesting is readout can only happen when the CH3 and CH4
inputs are deselected (eg. VS3 and VS4 are high). So, readout only
steals the beam from CH1 and CH2, or when no channels are being
displayed.... never from CH3 or CH4.

Why didn't I notice that phrase earlier?

I checked all of the signals once again, and they match what the manual
describes, however, the signal examples are only for when the readout
is turned off. Hmmm?

The nand gate in U975A, that creates signals 51 and 50 are indeed inverses
of each other, and look clean... and they match the waveform examples.

Hmmm? Hmmm?

Ok, why am I in AC mode, and why didn't I notice that the DC signals
at 52 and 53 were straddling ground?

Ah! U485 can do that if the "C" section is broken... Let's replace
it. Now where are my CA3046's? I only bought them a couple of years
ago... Seems like only yesterday....

Found them, replaced U485, and.......

The problem is gone!

Seems to me like I am always tagging a 2N3904 on the side of an open
circuited CA3046.

One of these days I should start recording which section of the
CA3046 pops.

Thanks machineguy for forcing me to think about that ugly mess
tagged on the side of the Channel Switch hybrid.

And thanks to everyone else for their hints, and being a sounding
board.

-Chuck Harris



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