Re: A 2465 teaser...


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


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

-Chuck Harris

machine guy @Mac [TekScopes] wrote:
I have a 2465A that once did something I would describe exactly as you have for your 2465. I puzzled out the circuit involving U475, U485 and U975 enough to understand what appears to be its purpose. That purpose is to block readout "noise" from the channel switch when either Ch1 or Ch2 are running. I checked the Ch1 and Ch2 inputs to the channel switch and they were clean, the outputs (waveforms 54 and 55) were not. I tried a different channel switch and got the same results. It seemed to be associated with the readout (turning off readout would stop the noise the "running part" of the trace). So I studied the complex circuit at the input to Ch5 and concluded its purpose was to block the readout when Ch1 or Ch2 are running. In my case, one of the pins of U975 was stuck. I changed U975 and everything got well for me. U975 isn't the only part that is critical to the operation. Its just the easiest to diagnose. A simple test of my theory would be to firmly ground pin 2 of U485. This will prevent all readout vertical from reaching the channel switch. Then if the noise on the trace goes away, the complex circuit is failing. Cant say how without more tests.

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎01‎:‎12‎:‎28‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

Another possibility.

The complexity of the circuitry going to CH5 has made me lift an
eyebrow a couple of times now. I even bought a stash of new CA3046's,
and replaced U475, to no effect, at the time.

I did check 48, 49, 50, 51, 52, 53, 54 and 55 waveforms, and they all
nominally matched what the book shows, when the SUT was set the way the
book said to. But notice I said nominally, the hook was obviously
there on 54 and 55.. 52 and 53 in the manual are just plain wrong as
far as amplitude goes... 150mv, not 5v p-p. I think the rest were ok,
though I may have missed some other clue.

-Chuck Harris

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