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:
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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 ¨C 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
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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 ¨C 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
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Hello, I am looking for a volts/div knob for my 485. Plastic has degraded and skirt is loose.
Thanks!
steve_tech@...
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Re: 2235 - high pitch noise
You can use some small flexible plastic tubing to locate the noise. Hold one end to your ear, and use the other end as a probe to localize the source.
Michael A. Terrell
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-----Original Message----- From: Dmitri Shuev <dshuev@...>
Sent: Jul 30, 2018 11:03 AM
To: [email protected]
Subject: [TekScopes] 2235 - high pitch noise
Hello:
I am checking out my 2235 (US/AN version), and apart from some other idiosyncrasies, that I will leave for later, the unit is making a quiet, but very unpleasant high pitch "whine" after about 5-10 min of warm-up. The best I can say it is coming out of the power supply portion, but the source of the sound is difficult to pinpoint for certain...
What would be you suspicion? Is it a PSU showing signs of fatigue? Recommended steps? Time to recap?
Not sure if this is relevant, but 2 out of 3 small neon bulbs close to the PSU are briefly flashing, when the power is being turned off... Switch-off voltage spike protection possibly?
Advices would be appreciated.
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Re: 2235 - high pitch noise
Vincent,?
Try replacing the FET first and see what happens.?
Sent from K5JLR
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-------- Original message -------- From: Vincent Trouilliez <vincent.trouilliez@...> Date: 07/30/2018 11:47 AM (GMT-06:00) To: [email protected] Subject: Re: [TekScopes] 2235 - high pitch noise Hi Dimitri, Had a horrible high-pitch noise in the SMPS of my 2215, though was from cold, unbearable (I am only 40 so I still hear this kind of noises...sadly). Power rails were all spot on and ripple well within spec.? Recapped it anyway the other day... now silent as a dead body. To be honest, at the same I recapped it, I also replaced the MOSFET, with a different/more modern part.? So possibly the recap did not fix the issue but the new/different FET did... but I found reports of other "singing" SMPS from other gear of a different design (for example an old 8 bit computers like the TRS-80), where recapping fixed the issue as well. So I would put my money on the caps rather than the FET... My 2232 is also singing... so I will be recapping that one as well fairly soon. FWIW... Regards, Vincent Trouilliez
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Re: 2235 - high pitch noise
Hi Dimitri,
Had a horrible high-pitch noise in the SMPS of my 2215, though was from cold, unbearable (I am only 40 so I still hear this kind of noises...sadly).
Power rails were all spot on and ripple well within spec. Recapped it anyway the other day... now silent as a dead body.
To be honest, at the same I recapped it, I also replaced the MOSFET, with a different/more modern part. So possibly the recap did not fix the issue but the new/different FET did... but I found reports of other "singing" SMPS from other gear of a different design (for example an old 8 bit computers like the TRS-80), where recapping fixed the issue as well.
So I would put my money on the caps rather than the FET...
My 2232 is also singing... so I will be recapping that one as well fairly soon.
FWIW...
Regards,
Vincent Trouilliez
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Re: 2235 - high pitch noise
Same here. Dumont black-n-white TV was particularly loud to me.
Funny, I can't hear that my LED TV. :)
Thanks,
Barry - N4BUQ
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----- Original Message ----- From: "Craig Sawyers" <c.sawyers@...>
To: [email protected]
Sent: Monday, July 30, 2018 10:50:51 AM
Subject: Re: [TekScopes] 2235 - high pitch noise
2) Most likely your hearing the whine of the power supply oscillator ( at
my age I couldn't hear it
anyway) When I was a kid the line whine from the TV at 15.625kHz (UK) used to drive
me nuts. Now in my early
60's those days are long gone!
I think I still have a good chance of hearing the older 405-line whistle at
10.125kHz.
Craig
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Re: 2235 - high pitch noise
2) Most likely your hearing the whine of the power supply oscillator ( at my age I couldn't hear it anyway) When I was a kid the line whine from the TV at 15.625kHz (UK) used to drive me nuts. Now in my early 60's those days are long gone! I think I still have a good chance of hearing the older 405-line whistle at 10.125kHz. Craig
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Re: 2235 - high pitch noise
Look through the archives for "singing power supply," and related links and discussions. I posted quite a bit of info on this a few years ago. Ed
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Re: Desoldering Iron vacuum
Oops - one more thing. While looking at the old vacuum unit some more, I saw a second solenoid valve and remembered I added it for vacuum-hold, to prevent run-back through the pump. The pump has no valves, so in cycling applications, the vacuum on the inlet would pull air back and run the pump backwards after shutoff, loosing the vacuum. The solenoid acts as a check valve and disconnects the pump inlet from the receiver at shutoff. Any sort of passive check valve would have forward pressure drop (analogous to diode Vf), which would waste some of the vacuum potential. A little is lost though, due to the response time of the valve, but much less than the forward drop of a check would cause.
When you have at best, one atmosphere to work with, you can't afford much pressure drop in any part of the system.
Ed
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Re: 2235 - high pitch noise
DS
1)As long as the Supply Voltages and Ripple is in spec too soon to talk about recapping
2) Most likely your hearing the whine of the power supply oscillator ( at my age I couldn't hear it anyway)
3) Could Be corona leak on the HV lead going to the CRT
4) Could be the power supply transformer getting "loose" with age the switch mode oscillator runs at around 20khz probably what your hearing.
5) Normal safety disclaimer about discharging the HV etc and then I would give the power supply and the CRT HV lead a REALLY good cleaning. ( Alcohol, a paint brush, compressed air etc) .
After a good (and I mean spit and polish.. after all as youi said this is an old MIL version scope)? cleaning and Voltage & Ripple check if whine still there check back in here for more ideas if still there
-DC
manuals@...
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On 7/30/2018 11:03 AM, Dmitri Shuev wrote: Hello:
I am checking out my 2235 (US/AN version), and apart from some other idiosyncrasies, that I will leave for later, the unit is making a quiet, but very unpleasant high pitch "whine" after about 5-10 min of warm-up. The best I can say it is coming out of the power supply portion, but the source of the sound is difficult to pinpoint for certain...
What would be you suspicion? Is it a PSU showing signs of fatigue? Recommended steps? Time to recap?
Not sure if this is relevant, but 2 out of 3 small neon bulbs close to the PSU are briefly flashing, when the power is being turned off... Switch-off voltage spike protection possibly?
Advices would be appreciated.
DS
--
Dave
Manuals@...
www.ArtekManuals.com
|
Hello:
I am checking out my 2235 (US/AN version), and apart from some other idiosyncrasies, that I will leave for later, the unit is making a quiet, but very unpleasant high pitch "whine" after about 5-10 min of warm-up. The best I can say it is coming out of the power supply portion, but the source of the sound is difficult to pinpoint for certain...
What would be you suspicion? Is it a PSU showing signs of fatigue? Recommended steps? Time to recap?
Not sure if this is relevant, but 2 out of 3 small neon bulbs close to the PSU are briefly flashing, when the power is being turned off... Switch-off voltage spike protection possibly?
Advices would be appreciated.
DS
|
Re: OT: Seeking help reviving a Stanford Research Systems SR760 Spectrum analyzer
John,
BTW, thanks for responding and thanks for your efforts...
Raymond
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Re: OT: Seeking help reviving a Stanford Research Systems SR760 Spectrum analyzer
On Mon, Jul 30, 2018 at 03:03 PM, <johnasolecki@...> wrote: Hi John,
I have it in PDF form so I'll have to figure out where to post it. Doesn't
seem appropriate to put in on a Tek dedicated website...
Are you sure that your version contains the schematics? I've only found versions without them. You may send it to me via the free wetransfer.com service if you want. Please use my address raydfATplanetDOTnl as the destination or use the address that you see in my posts. Raymond
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Re: OT: Seeking help reviving a Stanford Research Systems SR760 Spectrum analyzer
KO4BB.com
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On 7/30/2018 9:03 AM, johnasolecki@... wrote: The link I had for the SR 760 manual seems to be dead. I have it in PDF form so I'll have to figure out where to post it. Doesn't seem appropriate to put in on a Tek dedicated website... Suggestions?
SR 780 manual:
--
Dave
Manuals@...
www.ArtekManuals.com
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Re: OT: Seeking help reviving a Stanford Research Systems SR760 Spectrum analyzer
The link I had for the SR 760 manual seems to be dead. I have it in PDF form so I'll have to figure out where to post it. Doesn't seem appropriate to put in on a Tek dedicated website... Suggestions?
SR 780 manual:
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Re: Pimp my TM5003 ! (swapping fan with a quieter one)
Hi Dennis,
In fact, I've download "Decibel X" right after the first try. It was the highest rated dB meter in the appstore.
My bench has a "noise floor" of around 50dB, it's summer, windows are opened. When cars pass by, it peaks at 55dB.
When I turn on the TM5003 with the new fan, the dB meter stays about the same level at 50dB, maybe fluctuating between 49dB and 53dB.
3dB is double the sound level. But in fact, not that much
But I think it has indeed some perceptual effect.
I mean my fridge when running, makes noticeable noise, it's not loud, but you can hear it. However, it is easy to forget about it.
The TM5003 somehow makes a different kind of noise, more aggressive.
Maybe because it pushes a huge amount of air. Or maybe I didn't screw well the case and it vibrates somehow.
And comparing my desktop PC, with its fans running at full speed (playing games, hot summer), the TM5003 is actually quieter.
So, I may have some too high expectation from the start.
Also, I've probably mis-interpreted the specification. When reading 32dB, I though the end result would be: (new_noise_level = max(ambient_noise_level, 32dB)).
Whereas, it should be (new_noise_level = ambient_noise_level + fan_noise).
In fact, if I place the iPhone at the rear of the TM5003, I obtain 79dB (around 50 + 30), which is actually consistent.
Best regards,
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On Mon, Jul 30, 2018 at 05:33 AM, Tomas Alori wrote:
While triggering on the CRT cable signal (to make the dotting stand still) I
checked TP41120 (with the other Vert plugin) and I could see ripple of 2mV
that aligns exactly with the dotting. Is 2mV enough to cause this?
BTW, the circuit is making a irregular crackling sound. It does not match the
dotting though.
A few volts change is needed at TP41120 in order to vary the trace brightness from just visible to normal. So 2 mV is nothing in my opinion. You might simply verify this. Vary the Intensity control and note the TP level shifts and brightness changes. (I think the Intensity control is far too coarse to obtain 2 mV shift!). It's likely now that the fault is somewhere in the HV circuits, including the grid voltage level shifter. Not the easiest problem to solve. Albert
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Re: OT: Seeking help reviving a Stanford Research Systems SR760 Spectrum analyzer
Hi John,
If the service manuals are in pdf form, can you post them somewhere?
I would be interested too.
Thank you.
Best regards,
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Re: Slightly OT - Bourns trimmer pot failure
I too have had my share of Bourn's 3352 failures. In my case they were inside my Tek 465B. One failed pot had no slider connection, end to end resistance was intact. This post was in horizontal gain adjustment position. The other failed pot was in calibrator output circuit, which was not providing any signal. Here too slider was not making contact with the resistive track.
I suspect there are more faulty pots in my 465B, as I still have a few problems pending for sorting out.
Are there any suggested replacements for 3352 pots?
Regards
Shailendra
I
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Michael A. Terrell
Vince
Raymond Domp Frank