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On 26/03/19 20:37, Brendan via Groups.Io wrote:

I have a few questions about power supplies and would like some opinions. Are switching powers supplies inherently harder on components than linear power supplies? If you picked up a 70's-80's vintage scope with a SMPS would you replace power supply components before using it as a daily driver? Or do you treat all power supplies the same and visually inspect, check for ripple and call it good? From my reading it seems that when a SMPS melts down the chain reaction damage has the possibility of being bad.

Replace Rifa mains interference suppression delayed action smoke generators on sight; if there is any sign of cracking in the transparent case, don't even turn it on.

Where there are tantalum beads that are operating near their rated voltage (e.g. a 15V tant on a 13V rail), replace those with a higher voltage.

Visually inspect, measure, replace if there's a problem.

Otherwise, if it ain't broke, don't fix it.

Tough question.

Yes, switching supplies are inherently harder on
components than linear, but they also use much better
parts. And they are more likely to be designed using
math, rather than rules of thumb. And they are more
likely to have extensive protection devices to prevent
catastrophic failures from occurring.

Linear supplies are heavy, are more likely to break your
toes, and make copious amounts of heat. That heat soaks
into everything around them, and causes component failure.

Tektronix used some really uber parts in their 70's vintage
supplies. In some cases, I cannot find modern parts that
were as good as what they used.

I would take it through calibration, testing the ripple
and voltages, and just use it. If it fails, fix the failure.

Odds are very, very, good that in prophylactically replacing
parts you are going to introduce failures, now and in the
future. Replacing a 10,000 hour rated part with a 1000 hour
part, isn't going to improve reliability.

-Chuck Harris

Brendan via Groups.Io wrote:

I have a few questions about power supplies and would like some opinions. Are switching powers supplies inherently harder on components than linear power supplies? If you picked up a 70's-80's vintage scope with a SMPS would you replace power supply components before using it as a daily driver? Or do you treat all power supplies the same and visually inspect, check for ripple and call it good? From my reading it seems that when a SMPS melts down the chain reaction damage has the possibility of being bad.


Brendan

Hi Reginald,

It is in the archives:

/g/TekScopes/topic/7659008#141241

Enjoy.

Egge Siert

1. why is an impedance match between output tubes and the output transformer primary so
important given the short physical differences. Or, maybe the tube specs are not showing the actual
impedance, but rather the recommend Hi-Z on the load end to offer the optimal Low-Z to Hi-Z ratio?

Because there is a mismatch between the high plate resistance of the output pentodes or tetrodes
(about 4.5k-ohms in push pull) and the loudspeaker. The impedance transformation goes as the square
root of the turns ratio. So to match 4.5kk to 8 ohms needs a turns ratio of root(4500/8) = 24:1 turns
ratio. To deliver 30W into 8 ohms (typical for 6550's in push pull) needs 21V peak, times 24 = 500V.
The anode/plate voltage will be 560V - which is consistent with a 500V signal peak.

The details are of course more complicated than that - so you'll just have to read around.

2. Why is a Low-Z to Hi-Z ratio desired in audio applications vs. an impedance match?

Because if you do an impedance match at audio you lose half the signal. So your signal to noise ratio
goes down by 6dB. Which is why no-one does it - there is absolutely no upside.

We're kind of off-topic here. If you want to discuss audio, try
or . There are lots more out
there.

Craig

Gentlemen... have a look here:
/g/TekScopes/message/155394?p=,,,20,0,0,0::Created,,egge+siert,20,2,0,30465052
Albert

At 09:58 AM 3/26/2019, Craig Sawyers wrote:

Going way back long distance telegraph and telephone lines were significant fractions of an audio wavelength in the cables, and they needed to impedance match.

Through the tube era and the discrete solid state era professional balanced audio lines in radio and recording studios were also impedance matched at 600 ohms (earlier 500 ohms) because they were transformer coupled and power sourced. The constant impedance was implemented for maximum power transfer. It was not until the advent of high quality op amps that voltage sourced balanced audio lines became the norm in studios.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA

The user manual I quoted has a revision date of May 1989. The August 1988 service manual states the calibrator is < 35 ps. The February 1989 revision page states the same rise time.

As I noted previously. I also got a 300 ps rise time from the calibrator due to something not being set properly. This persisted when I used autoset, but eventually went away after I power cycled it a couple of times and used the autoset again. The behavior suggested an off by 10x error in the timebase generator strobe line. But I have far too little experience with the instrument to be sure of anything.

Thanks Craig... so, if I were to summarize what you wrote, at such short
distances, there's really no opportunity for a reflected signal to go out
of phase with the incident signal?

In watching the EEVblog videos, he's clearly using pretty high frequencies
(well out of the audio spectrum). So, your explanation is consistent with
that.

So, two questions remain.

1. why is an impedance match between output tubes and the output
transformer primary so important given the short physical differences. Or,
maybe the tube specs are not showing the actual impedance, but rather the
recommend Hi-Z on the load end to offer the optimal Low-Z to Hi-Z ratio?

2. Why is a Low-Z to Hi-Z ratio desired in audio applications vs. an
impedance match? I understand your point that it doesn't matter at low
distances, but Low-Z to Hi-Z appears to be an objective (iow, the objective
is to avoid a match, by orders of magnitude). Does the higher resultant
voltage (amplitude) at the load spread the signal out in a way that give
the amp more to work with from a fidelity POV?

On Tue, Mar 26, 2019 at 9:59 AM Craig Sawyers <c.sawyers@...>
wrote:

That was a decent video, thank you. It doesn't seem like he had problems

with the terminators

themselves. In fact, he uses them to address mismatches. I'd be curious

to know (from anyone who

knows) why maximum power (matched impedance) is ideal in some

situations, but a low-Z to high-Z

arrangement is the ideal in other situations (ie: guitar to amplifier or

microphone to PA). I

realize the
outcome in the latter situation; you preserve the audio highs and lows.

But need some schooling on

why to maximize voltage at the load (vs. maximizing transfer power) in

those situations and why the

mismatch isn't destructive to the signal. And correspondingly, why the

same isn't true of the

impedance match between output tubes and the primary of an output

transformer since its a similar

audio application. Again, I understand the requirements and outcomes...

but am confused about the

underlying physics.

It is to do with the frequency range. As soon as the length of the cable
becomes a significant
fraction of the electrical wavelength in the cable, you need to impedance
match. That is because
energy is reflected at an impedance discontinuity, so you end up with
standing waves along the length
of the cable. For a 1 metre long coax the wavelength becomes a significant
fraction of the cable
length by about 10MHz, so you need to impedance match.

With audio, the wavelength is so long (at 20kHz it is about 10km in a
typical coax cable) you
absolutely do not need to match. Hence you guitar example.

Going way back long distance telegraph and telephone lines were
significant fractions of an audio
wavelength in the cables, and they needed to impedance match.

Craig

They must have changed the cal output circuitry on the later models.

That was a decent video, thank you. It doesn't seem like he had problems with the terminators
themselves. In fact, he uses them to address mismatches. I'd be curious to know (from anyone who
knows) why maximum power (matched impedance) is ideal in some situations, but a low-Z to high-Z
arrangement is the ideal in other situations (ie: guitar to amplifier or microphone to PA). I
realize the
outcome in the latter situation; you preserve the audio highs and lows. But need some schooling on
why to maximize voltage at the load (vs. maximizing transfer power) in those situations and why the
mismatch isn't destructive to the signal. And correspondingly, why the same isn't true of the
impedance match between output tubes and the primary of an output transformer since its a similar
audio application. Again, I understand the requirements and outcomes... but am confused about the
underlying physics.

It is to do with the frequency range. As soon as the length of the cable becomes a significant
fraction of the electrical wavelength in the cable, you need to impedance match. That is because
energy is reflected at an impedance discontinuity, so you end up with standing waves along the length
of the cable. For a 1 metre long coax the wavelength becomes a significant fraction of the cable
length by about 10MHz, so you need to impedance match.

With audio, the wavelength is so long (at 20kHz it is about 10km in a typical coax cable) you
absolutely do not need to match. Hence you guitar example.

Going way back long distance telegraph and telephone lines were significant fractions of an audio
wavelength in the cables, and they needed to impedance match.

Craig

Attachments are not allowed here Reg. Via the website you can create a new Album in the Photos section and put your picture(s) there. If you do so, it's handy for "us" if you insert a link to the album in your messages.
Albert

That was a decent video, thank you. It doesn't seem like he had problems
with the terminators themselves. In fact, he uses them to address
mismatches. I'd be curious to know (from anyone who knows) why maximum
power (matched impedance) is ideal in some situations, but a low-Z to
high-Z arrangement is the ideal in other situations (ie: guitar to
amplifier or microphone to PA). I realize the outcome in the latter
situation; you preserve the audio highs and lows. But need some schooling
on why to maximize voltage at the load (vs. maximizing transfer power) in
those situations and why the mismatch isn't destructive to the signal. And
correspondingly, why the same isn't true of the impedance match between
output tubes and the primary of an output transformer since its a similar
audio application. Again, I understand the requirements and outcomes... but
am confused about the underlying physics.

Thanks in advance for this help.

On Mon, Mar 25, 2019 at 8:09 PM Roy Thistle <roy.thistle@...>
wrote:

Hi:
If you haven't watched EEVblog #652, he demonstrates a couple of problems
that can arise when using coax, terminators, and Ts, to connect a sig gen
to a scope.
Cheers.

Thanks. It will be a while before I take it apart again, but I'll make a note in my service manual.

Fortunately, most of my HP gear lists the parts in the manuals, so those will be easier to do.

I've attached a photo showing the calibrator output trace. The measured rise time is shown in the lower left at 32 ps. I've got smoothing on.



--------------------------------------------

On Tue, Mar 26, 2019 at 10:49 AM, zenith5106 wrote:

I tried to copy the A5-U300 ROM but I'm not sure that succeeded. The
TNM5000
said it failed to identify it. Can anyone tell me what type of EPROMs are
used
in the scope?

27C010 and 27C512

I may have to take part of that back.
In my F/W EPROM collection I don't have 11801 but there is a note saying that 11801 can also use 11801A F/W
which I have and it uses both types. I now begin to think that is wrong and if so 11801, just like 11802 no letter,
use only 27C512.

/H?kan

On Mon, Mar 25, 2019 at 10:52 PM, Reginald Beardsley wrote:

I tried to copy the A5-U300 ROM but I'm not sure that succeeded. The TNM5000
said it failed to identify it. Can anyone tell me what type of EPROMs are used
in the scope?

27C010 and 27C512

/H?kan

Far as I know he still works at a university in Portugal -he's been there for some years and is well known in scientific and amateur (especially EME) circles.
73
Dave, ZL3FJ

11801 User manual p 42.

"The signal from the calibrator output us a 250 mV square wave with a rise time of approximately 20 ps and a period of approximately 10 microseconds."

After I did the Enhanced Accuracy cal process I got results similar to yours. I eventually got it to do something sensible after a bunch of semi-random button pushing. Pulse amplitude was not stable on the display among other weirdness. It is now behaving as one would expect.

Someone else has stated that the calibrator is the same as the pulse generator in the SD-24.

I am now getting the same results I got before I replaced the NVRAM.


--------------------------------------------

Hi:
If you haven't watched EEVblog #652, he demonstrates a couple of problems that can arise when using coax, terminators, and Ts, to connect a sig gen to a scope.
Cheers.

Great!

Calibrator risetime? Isn't the cal output specified at about 250ps? Both of my instrument measure it about 300ps.