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:
toggle quoted message
Show quoted text
Thanks Chuck
Help me here:
You wrote:
it would be chosen to have a capacitive reactance equal in ABSOLUTE VALUE to the inductive reactance of the load it switches.
1. Using the formula (with 230VAC, 50Hz)
XL = 2Pi fL, how would I determine L - the inductance of the machine of the switch which I want to bypass with a snubber?
And I (current) = VAC / XL
2. How do I choose a capacitor of 'x" nF, after I have calculated an XL of say 100 ohms?
3. Let's say an amplifier is pulling 300mA RMS at 230VAC, as measured on an ammeter in series with the AC switch, how does that info help me to choose a snubber?
Or is XL = VAC/I = 230/0.3 = 767 ohms?
4. If Tek put the 5 or 7ohm NTC in series with the 15 ohm resistor on the PSU, then what's happening on these 2465 PSU switches, which is causing the switch to weld?
This a new area for me to explore, but quite challenging with its significance in helping prolong the life of vintage equipment.
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Re: 2235 - high pitch noise
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:
toggle quoted message
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Hi Chuck,
Where does a shunt resistor go relative to the thermistor? I thought the thermistor was the only component in an inrush current limiter. Is that incorrect?
Thanks,
Barry - N4BUQ
<snip>
To cure this inrush problem, tektronix put a thermistor,
with a shunt resistor in series with the power switch. <snip>
-Chuck Harris
|
Re: 2235 - high pitch noise
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.
|
Thanks Chuck
Help me here:
You wrote:
it would be chosen to have a capacitive reactance equal in ABSOLUTE VALUE to the inductive reactance of the load it switches.
1. Using the formula (with 230VAC, 50Hz)
XL = 2Pi fL, how would I determine L - the inductance of the machine of the switch which I want to bypass with a snubber?
And I (current) = VAC / XL
2. How do I choose a capacitor of 'x" nF, after I have calculated an XL of say 100 ohms?
3. Let's say an amplifier is pulling 300mA RMS at 230VAC, as measured on an ammeter in series with the AC switch, how does that info help me to choose a snubber?
Or is XL = VAC/I = 230/0.3 = 767 ohms?
4. If Tek put the 5 or 7ohm NTC in series with the 15 ohm resistor on the PSU, then what's happening on these 2465 PSU switches, which is causing the switch to weld?
This a new area for me to explore, but quite challenging with its significance in helping prolong the life of vintage equipment.
|
Hi Chuck, Where does a shunt resistor go relative to the thermistor? I thought the thermistor was the only component in an inrush current limiter. Is that incorrect? Thanks, Barry - N4BUQ <snip> To cure this inrush problem, tektronix put a thermistor,
with a shunt resistor in series with the power switch. <snip> -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 ¨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
|
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:
toggle quoted message
Show quoted text
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
|
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
|
Hello, I am looking for a volts/div knob for my 485. Plastic has degraded and skirt is loose.
Thanks!
steve_tech@...
|
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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Show quoted text
-----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.
|
Re: 2235 - high pitch noise
Vincent,?
Try replacing the FET first and see what happens.?
Sent from K5JLR
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Show quoted text
-------- 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
|
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
|
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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Show quoted text
----- 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
|
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
|
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
|
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
|
Vince
zenith5106
Michael A. Terrell
Raymond Domp Frank