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Ok, Q879 collector is solid.? When things go sour, it stays.? The base voltage is moving, as is the emitter.?? So it's being told to compensate for the change.? However, under normal conditions, the collector voltage is lower by a few hundred mv than the schematic calls for while the base and emitter are within a few mv.? Under normal conditions there's a 300mv drop across the shunt.

Going to take a closer look at Q879's feed.

Vince.


On 09/01/2020 10:03 AM, Siggi wrote:

On Mon, Aug 31, 2020 at 8:36 PM Vince Vielhaber <vev@...> wrote:

The 5V jumping around is the 5V regulated that C494 is on.?? There it
drops a volt or so when I switch below 200ns.? According to my Fluke,
when it goes out completely it holds at 3.93v.

Oh, interesting. I assumed you were talking about the 5VD, which is the
main switch-mode regulated supply that feeds all the digital logic.

You're talking about the linear regulated 5V supply. Same deal though, go
back to the first decoupling cap, see whether the supply is jaggy and
whether the raw supply holds up when the regulated supply drops. Most
likely - as others have noted - this is due to geriatric bulk caps in the
secondary, so check that first.
If the raw supply is holding up OK when the regulated supply drops, then
you move downstream and see what's up. This supply is current limited, so
you can infer the current it's using by measuring across the shunt.

On Mon, Aug 31, 2020 at 8:36 PM Vince Vielhaber <vev@...> wrote:

The 5V jumping around is the 5V regulated that C494 is on. There it
drops a volt or so when I switch below 200ns. According to my Fluke,
when it goes out completely it holds at 3.93v.

Oh, interesting. I assumed you were talking about the 5VD, which is the
main switch-mode regulated supply that feeds all the digital logic.

You're talking about the linear regulated 5V supply. Same deal though, go
back to the first decoupling cap, see whether the supply is jaggy and
whether the raw supply holds up when the regulated supply drops. Most
likely - as others have noted - this is due to geriatric bulk caps in the
secondary, so check that first.
If the raw supply is holding up OK when the regulated supply drops, then
you move downstream and see what's up. This supply is current limited, so
you can infer the current it's using by measuring across the shunt.

Acquisition stopped, doesn't happen. I can go all the way to 2ns/div.

Normal trigger but trigger set to +or- 90v (not triggering), it happens.

Single, doesn't happen, can go all the way to 2ns/div, can even trigger
it (in single) and it doesn't happen. The probe was hooked to the
calibrator for the triggers. Normally doesn't matter if there's a
signal there or not.

Thanks!
Vince.


On 08/31/2020 01:43 PM, Siggi wrote:

Hey Vince,

I have no idea what's going on there, though I'd suggest looking at the
power supplies as a first step.
You can also - perhaps - diagnose this a little ways by playing with
trigger and acquisition settings. My guess would be that a power supply

is

collapsing when the sample clock exceeds some particular rate, though
that's of course just a guess.
Does this still occur when the acquisition is stopped? If not, you can
maybe narrow this down by e.g. going to NORM sweep/trigger mode, and
setting up a trigger that never hits. Does it still happen when

acquisition

is started but no trigger fires? How about under a single-shot

acquisition

in AUTO mode, etc.

Siggi

On Sun, Aug 30, 2020 at 5:37 PM Vince Vielhaber <vev@...> wrote:

I picked up a 2440 on ebay that had the dead nvram battery problem. Got
that fixed up with an external battery. Ran thru the self cal and ext
cal and all seems fine, except when the sweep rate is turned above 100
or 200 ns.

In trying to research this a bit, the only thing I could find was on one
of the Tek forums back in 2017 but there were zero responses. Here is
how the other person described the problem:

---------------------------------------------------------------------
The 5 volt regulated rail is getting crowbared when the scope Sec/Div is
set to 100ns or faster. If save is pressed at a slower setting the
Sec/Div can then be set to any value without the 5 Volt rail getting
pulled down. It appears the issue only occurs during an active
acquisition cycle at 100ns Sec/Div or faster.
----------------------------------------------------------------------

While I didn't measure the 5v rail, the screen shrinks a couple of times
as the waveform changes, the inputs get switched to GND and the display
goes blank. Sometimes, if I'm quick enough, I can turn the sweep back
down (1us or slower) and it'll recover, but I still have to go to the
input coupling and switch the inputs off of GND.

Anyone seen this or have any ideas what's going on?

Thanks!
Vince.
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K8ZW

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K8ZW