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On Sep 2, 2020, at 14:06, Dennis Tillman W7pF <dennis@...> wrote:

Michael Dunn started TekScopes 20 years ago on August 31, 2000. There was
just one post that day from Walter Shawlee who joined that same day and he
has been an active member ever since.

To Walter: Thank you for 20 years of contributions to TekScopes and for all
the parts and instruments you have on your web site!

* Three days later Stan Griffiths W7NI (RIP) joined.
* Three more members, including Tom Gootee, joined in October, 2000.
* By the end of 2000 there were 13 members.
* A year later it was clearly important: there were 68 members and >150
posts each month. John Miles and Morris Odell joined TekScopes in that first
year.

To John and Morris: Thank you for your regular contributions over the 19
years you have been members!

* I was a late comer. When I joined in mid-November 2002 there were already
250 members. I was looking for something like this for a long time and I
jumped right in with my first post that same day. Apparently I have been
busy ever since I joined - I just noticed this will be my 5,000th post since
I joined.

Today we have over 8,000 members worldwide and there are 500 to 1500 posts
per month.

Thank you, Michael Dunn, for realizing the need for a Tektronix support
group, for all you did to keep it going the first 17 years, and for being
there for me when I need occasional help and/or guidance as I continue to
moderate it.

Dennis Tillman W7pF

Hi Albert -
On Tue, Sep 1, 2020 at 03:02 AM, Albert Otten wrote:

You probably tried AQS because I pointed to that. Something very strange is
going on with AQS in my 7854 (or with my brains?) AQS worked at 0.5 ms/div

but

not at 0.2 ms/div and faster. OK. Later on I thought to remember a trick to
mislead the scope: set trigger level for not triggering, then do AQS (the
scope waits...), then switch to faster sweep rate, then turn trigger level
produce a sweep. Somehow it didn't work, error all the time. I switched off
the scope and read the operators guide. AQS: works ONLY with 7B87! But I

used

a 7B80 (no 7B87 at all in the scope). And AQS just worked fine at 0.5

ms/div.

Back to the 7854 to do it once more. And this time AQS gave an error beep

all

the time!??

Are you doing single-sweep Acquisitions with the 7B87? Single-sweep uses the
7B87's internal sample clock, and I just checked on mine and confirmed that at
.5mS, the acquisition works fine, and beyond that, it's clocking too fast to
fill up the memory. If I do a single-sweep acquisition at .2mS, I'll see the
wave, but I have to hit stop for it to complete, and it beeps with an error.
I've got a seemingly working (knock on wood!) 7854 w/ a 7B85 & 7B87, so y'all
let me know if you need assistance checking measurements or behavior.

cheers,
Paul

On Tue, Sep 1, 2020 at 10:06 PM Vince Vielhaber <vev@...> wrote:

Both the voltmeter and the 2430A. There's not much garbage on it but I
did hang another cap on it and it made no difference.

Here is a shot of both sides of R873 on the 2430A. At power off, both

traces are together. The top one is the at the junction of R873, R876,
Q879 E. The lower one is on the C494 side. This was caught just when
the sweep was turned to 100ns/div. I quickly hit Save when it happened.

I guess next up is to capture U870 pins 1 & 7, see what's up there when
your rail collapses.
On one hand, you could be hitting the current limit, which could be because
of actual overcurrent, or because the resistor network on U870A pin 2 has
drifted. Also the -5V rail could be compromised, as the current limiter is
referenced to that. Also check all the other rails, as the +5 needs the +8
to be up and sound as that's where Q870 draws from.

On the other hand it could be that Q870/Q879 have degraded and can no
longer do the lifting required.

On Tue, Sep 1, 2020 at 8:15 PM Vince Vielhaber <vev@...> wrote:

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.

How are you measuring the collector voltage?
The switching frequency is ~25kHz, and it's going to be ugly as heck if the
capacitor is out, so the instrument you're using to measure this and its
bandwidth are quite important. The average or RMS voltage there just isn't
super important :).
What happens if you tack on extra capacitance from Q879 collector to
ground? You probably want a capacitor that's fairly low ESR, the ripple
current is going to be something wicked if the average current is 3A (see
below).

Under normal conditions there's a
300mv drop across the shunt.

That looks sane according to the voltages annotated on the schematic.
That's a 3A nominal load, though.
U870A/B are the current/voltage controls, respectively, or-ed together
through the diodes. You can tell which one is in control by the voltage out
of pin 1/7.

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

Have fun!

My mistake: HP1740A.
I get the impression vintage TEK scopes are ten times the price they were ten years ago, and now rare. My HP1740A was from a recycling centre for next to nothing, just needed switch cleaner.
And thanks for the recent information: very useful.
Regards
David Collier

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.