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Snubbers are usually applied to dissipate the energy in reactive transitions. If you use a capacitor only, it will tend to form a resonant circuit that can ring. It's good to have some resistance (loss) to convert it to heat, and quickly damp it out. The resistance also limits the peak currents in the loop.

Ed

Kevin

Thanks!
My goal is to reduce/eliminate arcing in a mechanical switch (or across relay contacts) - that's all.


1.
You've stated between 1-10nF is suitable - is there a definite method to calculate the optimum capacitance?

2.
What about the voltage rating of that optimum capacitor?
Should it be a DC-rated (up to a few thousand volts), OR an AC-rated Class X or Y capacitor?

I don't know why one would look to a snubber to protect a switch. Snubbers, RC, RLC, are typically employed to control dv/dt on semiconductors. I don't consider a capacitor alone as a snubber. It is pure reactance and therefore non dissipating. A snubber needs resistance to dissipate energy.
A switch should be able to sustain far more transient voltages than semiconductors. Switches in old equipment can be worn out in several ways. Spring fatigue, contact erosion or oxidation, lubricant contamination, time, all contribute to ware out.

Switch failure on closing a normal load, even a cap input load, would seem to indicate a warn out contact. This could be worn springs that allow excessive bounce an arcing.

Switch failure on opening a load can be a different issue. If the switch is overloaded, arcing will eventually erode the contacts and lead to some kind of failure or weakening of springs.
For normal switches in "typical" electronics, a 1000-10000pf cap across the switch should suppress most contact arcing. (~300-3000K). I'm sure I'll get some flaming on this, but mechanical components work different than semiconductors.

Kevin

"the PTC is in parallel with the resistor, not in series"
Just to confirm that it is definitely an NTC, not a PTC, in the 2465

"EMF is a high voltage phenomenon, not a high current phenomenon.
The current cannot be higher than the operating current of the supply.
The voltage can rise to thousands of volts."

To clarify:
If the voltage can rise to thousands of volts, then the capacitor which bypasses an on-off switch should rather be a DC capacitor rated to 5KV, and NOT an AC Class X1/Y1 unit.
Is my thinking correct here?

Would a metal oxide varistor be more suitable?

On Mon, 30 Jul 2018 15:52:29 -0400, you wrote:

The supply has to deal with (effectively) a ramp up of the input
voltage rather than an abrupt step. May confuse some chips that need
a minimum dv/dt, but I'd guess that depends on the design of the
regulator and the rest of the circuit.

Harvey

A snubber capacitor is a capacitor, used in conjunction
with a series resistor, to dampen, or eliminate the back EMF
transient caused when an inductor abruptly loses its
current source.

They are useless in capacitor input power supplies, like
used in the 2465 family of scopes, because they have no
appreciable inductance to make a back EMF transient.

However, capacitor input filter type power supplies have
their own switch eating problems, in that potentially,
they could draw extremely high inrush current when the
power is first applied to the discharged filter capacitor.

This inrush will quite nicely burn, or weld, the contacts
of an otherwise properly sized switch.

To cure this inrush problem, tektronix put a thermistor,
with a shunt resistor in series with the power switch.

The thermistor starts out at a high impedance, causing
much of the initial charging current to be limited by
its resistance. As the current passes through the
thermistor, I2R losses cause it to self-heat, which
because of its negative temperature coefficient (NTC),
will cause it to lower in resistance to a point where
it is near zero ohms. At that point, it self regulates,
never getting hotter, or cooler than is necessary to
supply the current to the scope.

Back to snubber capacitors: The snubber can be sized
such that it completely eats any back EMF transient.
To do this, it would be chosen to have a capacitive
reactance equal in absolute value to the inductive
reactance of the load it switches. Its resistance
would be chosen to equal to the DC resistance of the
inductive load.

This is rarely done, because, it is not necessary to
eliminate the back EMF peak voltage, only to dampen it
down to a manageable value that will not burn out the
switch. Usually 10 to 100x the load's resistance is
adequate. Similarly, the capacitance can be reduced
too... but either reducing the capacitance, or increasing
the series resistance will cause the EMF spike to rise.

-Chuck Harris


M Yachad wrote:

I have now seen two 2465 PCB blocks with a burnt-out on/off switch.

The damage is caused by the arcing across the now-separating terminals, when the machine is switched OFF.

Replacement is easy with NE182UEEP6AMP, but examining the circuit shows me that this failure could well be preventable with the addition of either a single ceramic snubber capacitor of between 3.3nF to 10nF (example Vishay’s VY1 or VY2 series), OR an R+C snubber network – I calculated a 120 ohm 2W ceramic comp in series with a 33nF X2 film cap, across the Tek switch’s on-off terminals.

What are the considerations for selecting EITHER a single ceramic cap, OR making up a R+C snubber?

I’m not looking at the apparent cost savings of installing a single cap vs constructing a R+C.
The objective is long-term reliability.

Also, how would one select the capacitance value of the single ceramic cap?

I’m thinking of applications on vintage stereos.
I noticed that a new Yamaha hi-power stereo uses a 10nF, and a 20-year old CD-player uses a 3.3nF.
Is the current draw a factor?

Over to the experts….

Menahem

On Mon, 30 Jul 2018 11:50:21 -0700, you wrote:

I have now seen two 2465 PCB blocks with a burnt-out on/off switch.

The damage is caused by the arcing across the now-separating terminals, when the machine is switched OFF.

Replacement is easy with NE182UEEP6AMP, but examining the circuit shows me that this failure could well be preventable with the addition of either a single ceramic snubber capacitor of between 3.3nF to 10nF (example Vishay’s VY1 or VY2 series), OR an R+C snubber network – I calculated a 120 ohm 2W ceramic comp in series with a 33nF X2 film cap, across the Tek switch’s on-off terminals.

What are the considerations for selecting EITHER a single ceramic cap, OR making up a R+C snubber?

I’m not looking at the apparent cost savings of installing a single cap vs constructing a R+C.
The objective is long-term reliability.

Also, how would one select the capacitance value of the single ceramic cap?

I’m thinking of applications on vintage stereos.
I noticed that a new Yamaha hi-power stereo uses a 10nF, and a 20-year old CD-player uses a 3.3nF.
Is the current draw a factor?

I'm far from an expert, but I'll guess that you're talking:
1) energy storage. You have to look at what happens when the primary
magnetic field collapses, that ought to give you a relative idea of
capacitance. Your example indicates more power = more capacitance.
2) I suspect that once you calculate the short circuit collapsing
current through the capacitor, the resistor is there to limit it at
the cost of more ringing.

Harvey

Over to the experts….

Menahem

Several people have mentioned their interest in schematics, others have offered to try and make image files or PDF's.
Being the original poster, I'll collect scans/images and if the package is more or less complete, clean up the files if and where necessary and post them. I guess that'll be at Didier's (K04BB). TekScopes' file area would not be my preference, since the instrument is SRS, not Tek...

Currently, it seems that the images will come from the SR780 manual, since AFAIK, no offers were made from SR760 owners.

Raymond

On Tue, Jul 31, 2018 at 12:43 AM, <johnasolecki@...> wrote:

I'm not at the computer that has the PDF's right now so I can't check.

John,
Yes, please have a look if you have PDF schematics, that would make life a lot easier.

Raymond

You could take the schematics to
almost any Office Depot and copy
them full size to a ThumbDrive.

Then post them in the files area.

73, Dick, W1KSZ

On Mon, Jul 30, 2018 at 3:54 PM, Raymond Domp Frank <hewpatek@...>
wrote:

On Tue, Jul 31, 2018 at 12:43 AM, <johnasolecki@...> wrote:

I should be able to photograph them though and forward the shots.

OK, John, thanks.

Raymond

Everyone - thank you very much for the pointers! I will attempt as described - a "spit and shine" cleaning, then checking rail voltages and ripple, then following the H?kan's article (special thanks), and, - if all else fails - the MOSFET and electrolytics.

Dmitri

How about our hero Hakan!
He comes through with the best information on the planet, time after time!
Also, a big 'Thank You' to everyone who contributes to all of the fine discussions that come across this message board.
The current 'snubber capacitor' conversation is going to turn into another 'winner' before it's done, with many secrets laid bare for all to see.
tom jobe...

On Sat, Jul 28, 2018 at 11:44 PM, Raymond Domp Frank wrote:

Ask Stanford directly - AFAIK they have always provided Service Manuals
with
schematics and still do.

Thanks for responding, Dave. As I said, I'm going to try at SRS but seeing
only SM's for their equipment that stop after the parts list, I wasn't
hopeful.
I'll report back what their response is.

I received an answer from SRS. They only supply schematics with paper manuals @ USD 110,-....

Raymond

On Tue, Jul 31, 2018 at 12:43 AM, <johnasolecki@...> wrote:

I should be able to photograph them though and forward the shots.

OK, John, thanks.

Raymond

I'm not at the computer that has the PDF's right now so I can't check. I know for certain I have the schematics in the printed manual but they're 11" x 17" which is too large for my scanner. I should be able to photograph them though and forward the shots. In the next day or so I'll send you the table of contents and you can tell me which specific ones you want. If I remember correctly there's 20-25 pages of schematics so, to start, please keep the list short!

John

The L we are interested in is the inductance that the
power supply presents to the power line. In the case
of the 2465, that is very low, in the tens to hundreds
of micro Henries. Insignificant, in so far as switch
life is concerned..

In the case of a scope with a big iron power transformer,
the inductance is the primary inductance of the transformer.
It is affected by the secondary circuitry, so it is often
easier to simply measure it.

In answer to question #2, use algebra.

Xc =f x C, -> C = Xc/f where Xc == |Xl|

For the case of XL = 100 ohms, C = XL/f

In answer to question #3, AC current needs a phase angle,
relative to AC voltage to calculate a DC resistance. It is
easiest to simply measure the transformer primary with a DVM's
ohmmeter.

In answer to question $4, the PTC is in parallel with the
resistor, not in series. The PTC tends to fail open with
age, which can cause the scope to burn up the parallel resistor.
Generally, though it will make the inrush lower, not higher.

Have you actually disassembled one of the switches to observe
the contacts? I find the biggest problem is not burning of
the contacts due to inrush, but rather the grease hardens,
preventing good contact.

-Chuck Harris

One note that needs to be mentioned, EMF is a high voltage
phenomenon, not a high current phenomenon. The current cannot
be higher than the operating current of the supply. The
voltage can rise to thousands of volts.

M Yachad wrote:

Thanks a lot for that document Hakan, priceless....


Vincent Trouilliez

These scopes were made to operate over a wide temperature
range. A thermistor's cold resistance can become very high
at the low end of the temperature range, perhaps high enough
to make it unlikely to start charging the capacitor. So,
tektronix added a shunt resistor to help define the maximum
resistance (minimum starting current) in series with the switch.

-Chuck Harris

n4buq wrote:

On Mon, Jul 30, 2018 at 05:03 PM, Dmitri Shuev wrote:

What would be you suspicion? Is it a PSU showing signs of fatigue? Recommended steps? Time to recap?
Advices would be appreciated.

It's a known problem and I think it has been discussed here before.
There were a few WW articles written on the subject. You can find them here:

/H?kan

Hi Steve:

I can help you with your knob problem.? First, remove it and clean out any plastic residue from the skirt on the knob.? Then carefully apply a small stream of super glue around the inside of the knob. Orient the notch in the skirt with the boss on the knob and press it home.? Secure in a vice for several hours.? If the legends on the knob are gone you can use the label I made to replace it.? Take a rag and saturate a corner of it in alcohol and start rubbing the original legend off the skirt.? Cut the new label out and stick it to a piece of single sided tape from the back.? Now apply a piece of double sided tape to the label and very carefully orient the knob in the exact center and press firmly.? Cut the center out of the label taped to the knob.? Trim the edges off and re-install the knob.? The ones I made are close to the original but I could not get the shading to work so I omitted it.? You will have use PhotoShop to scale it properly.