Re: A 2465 teaser...


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

machine guy @Mac [TekScopes] wrote:
Here is what I think, and I'm pretty confident. The channel switch (U400) design "leaks" Ch 5 into Ch1 and Ch2. Tek decided (for some reason) to fix this by adding logic external to the channel switch. That external logic is implemented in U485, U475 and U975. These parts function so that any time horizontal A or horizontal B (A sweep or B sweep) are operating the readout into U400 is clamped to zero at the emitter outputs of U475C and U475D. In your case, they are not functioning, most likely a fault in U975. So, the readout vertical reaches U400 and leaks into Ch1 and Ch2, just like Tek always knew it would.

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎11‎:‎47‎:‎44‎ ‎AM‎ ‎CST, machine guy @Mac [TekScopes] <TekScopes@...> wrote:

Chuck, I think its from the readout. Most likely the readout DO circuitry. Probably in U485, U475, or U975. There is a complicated switch using emitter coupled logic implemented by U485, U475, and U975. If this switch and logic fails, readout is coupled to the vertical channels of the channel switch. I have never tested to see if it affects only Ch1 and Ch2. But I have seen it. Check waveforms 50 and 51 on U975.

On ‎Thursday‎, ‎November‎ ‎9‎, ‎2017‎ ‎04‎:‎51‎:‎43‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

A hook is a trace that goes from vertical to horizontal
with a little curve between. If you could see the whole
thing, it would look like a shark's tooth, or a lazy
saw tooth pulse.

With display off, it looks like this:




When you turn display on it looks like:




The "(" sections are actually a sharp rise with
a slower fall, and look like random grass. They
appear only on CH1 and CH2, not at all on CH3 and CH4.

The "(" sections after the trigger are at least
0.75 div high, and unaffected by vertical input
condition, or attenuator setting. The initial pulse
hook, is undoubtedly just one of the "herd"... only
it appears with or without the display being turned

I haven't tried replacing the CH1 and CH2 preamp
yet. It would surprise me to find that both failed
in exactly the same way at the exactly the same time.

[Note that CH1 and CH2 are otherwise performing normally,
and in calibration, and that the "(" stuff does not
appear in the CH2 output connector on the back panel.
I would think that would rule out the preamps.]

It would seem more likely to me that it would be
a bad bypass cap on a filter section that feeds both
CH1 and CH2 in parallel. But the power looks pretty
clean, and nothing larger than 1 or 2 mv.

-Chuck Harris

hahi@... [TekScopes] wrote:
I am puzzled. Anyone have any ideas?
Not sure exactly what you mean by hooks ...

This may not be the same but I have seen a few scopes with noise
on the trace(s) looking like about one minor div of digital noise
randomly across the trace(s). All fixed by replacing the preamp(s).


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Posted by: hahi@...


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Posted by: machine guy <@Mac>


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