H/P 1745A Oscilloscope Clipping Signal  - Page 1
Forum section: electronics / repair

author: DanH
author_rank: Newbie
timestamp: December 16, 2015, 04:21:48 pm
content: I just got this scope and just testing it out and seen the sine wave is being clipped off. I am sure it a adjustment somewhere, before turning any screws I think it best to ask for help if anyone could point me in the direction to start.A second issue the beam does not seem to be very sharp, I have focused it to the best sharpness it will do.I have attached a picture of the screen for viewing.The probe is a H/P 10/1 #100738.The signal is off a low voltage AC transformer.Any help or comment are appreciated!

author: tggzzz
author_rank: Super Contributor
timestamp: December 16, 2015, 05:22:08 pm
content: That might actually be the mains "sine wave"! I was fooled by that not long ago.Change the scope's DC offset. If the waveform is unchanged as the waveform moves up and down the screen, then the scope is pobably OK.The cause of the flatness is that many low-power appliances take most of their current from the mains only at the peak voltage. To visualise it, think of a low-voltage DC power supply consisting of a transformer plus rectifier plus capacitor. Zero current is taken when the transformer's output voltage is less than the voltage across the capacitor. If the mean DC current is X, then the peak current through the recifiers and transformer is probably somewhere near 5X, since current is only taken for (say) 20% of the time that the transformer's voltage exceeds the capacitor voltage.

author: Anks
author_rank: Frequent Contributor
timestamp: December 16, 2015, 05:43:00 pm
content: More than likely saturation if the transformer as no load on it.

author: tggzzz
author_rank: Super Contributor
timestamp: December 16, 2015, 06:16:43 pm
content: Quote from: Anks on December 16, 2015, 05:43:00 pmSaturation of what?Saturation of a magnetic core is related to the field which is related to the current, not to the voltage. If there is no load then the current will be very small.

author: tautech
author_rank: Super Contributor
timestamp: December 16, 2015, 06:23:46 pm
content: Quote from: DanH on December 16, 2015, 04:21:48 pmWelcome to the forum.tggzzz's comments are accurate.Quote from: tggzzz on December 16, 2015, 05:22:08 pmDo this^The mains sine waveform is commonly affected be the various loads placed upon it. Minor distortions to its sine wave purity are not uncommon.Post a pic of the probe Cal square wave output with amplitude settings so it nearly fills the screen and repeat the above.Improvements of beam sharpness will require some internal rework/replacement/adjustment and if you can live with the scope as it is I suggest you do unless you have the skills and tools for such work.

author: DanH
author_rank: Newbie
timestamp: December 16, 2015, 07:18:42 pm
content: Thankyou all for responding so quickly!I am adding two more pictures, picture of the sine wave is the output of the AC transformer loaded to its rated current of 1.5A the wave form is reduced a bit from being loaded through a wire wound resister, I even ran it up to 3 amps and waveform remained basically the same.The second picture is the square wave produced through the Cal port of the scope.

author: tautech
author_rank: Super Contributor
timestamp: December 16, 2015, 07:26:26 pm
content: Did you adjust the trace "position" up and down looking for clipping or distortion while doing so?It is what tggzz asked you to try as a basic performance check.Trace brightness when set too high as yours is will impact in apparent focus and sharpness. Turn brightness down.

author: edpalmer42
author_rank: Super Contributor
timestamp: December 16, 2015, 07:59:33 pm
content: Quote from: tggzzz on December 16, 2015, 06:16:43 pmIf the transformer is poorly made, the primary could be on the edge of saturation just from the line voltage, regardless of output load.Ed

author: DanH
author_rank: Newbie
timestamp: December 16, 2015, 08:29:03 pm
content: Sorry for the sideways pictures, on my cell phone, On the DC offset I am a little rusty on some of the term's I assume you mean there is a 3 position lever that has AC - GND - DC on the Volts/Div control. When using the square wave test signal in the GND the trace is Flat I move it to the Center of the screen and Move it to the AC position, the trace splits the center line 50% on top and 50% beneath it. In the DC position it go Up 100% , I don't notice any change in the wave form at all.On the low voltage Xmf I took a picture of the wave form using both channels. It looks identical on both channels.I also hooked the scope to a 110V AC coming into a signal generator showing the waveform it is still distorted on both sides of the waveform.The Beam intensity in both of the new pictures is completely turned down, the focus knob is dead in the middle of its rotation capability. I can't shut the beam off with the intensity knob. When I turn it up it looks like it is blooming (Old TV Term when High Voltage has a problem)

author: DanH
author_rank: Newbie
timestamp: December 16, 2015, 08:50:33 pm
content: One thing more I did move the trace up and down with no real effect on th wave form.

author: tautech
author_rank: Super Contributor
timestamp: December 16, 2015, 08:59:03 pm
content: Quote from: DanH on December 16, 2015, 08:29:03 pmYou describe correct functionality of Input coupling.Waveform position is adjusted vertically with the POSN pot.Best to not have the waveform adjust too far off screen as it overloads the CRT plate output stages.QuoteBlooming is still a correct term for a trace adjusted too bright.Normally internal adjustments are set to enable the trace to be dimmed out at minimum brightness (Intensity) settings. Adjustments might be needed.Quote from: DanH on December 16, 2015, 08:50:33 pm

author: TimFox
author_rank: Super Contributor
timestamp: December 16, 2015, 10:14:17 pm
content: Quote from: tggzzz on December 16, 2015, 06:16:43 pmThis is a common error. There is amaximumvoltage that can be applied to a transformer.A transformer without load current at 60 Hz can saturate if the applied voltage to any coil is too high.Remember that the EMF in a coil is the time derivative of the flux linking the coil, which in a iron-core transformer is the flux through the iron core. The maximum flux is the product of the maximum B field ("induction") times the effective cross-section area of the core. For transformer iron alloys, this Bmax is somewhere between 1.5 and 2.0 T, where 1 T = 1 V-s/m^2 in mks units. At 60 Hz, the time derivative of a sinusoidal flux is (2 pi) x (60 Hz) x (flux). If the applied voltage is larger than this max EMF, then part of the applied voltage is essentially applied only to the copper loss of the coil, since the back-EMF from the iron is not sufficient to counteract it.Furthermore, when a transformer is operated with a proper load, the flux from the primary current and the flux from the secondary current cancel each other, except for (small) leakage flux. The current rating of the transformer depends more on the core losses and (especially) copper loss, to keep temperature rise below specification.

author: Anks
author_rank: Frequent Contributor
timestamp: December 16, 2015, 10:22:39 pm
content: And the load being reflected back to the primary will be the impedence of the scope. Pretty much an open circuit.

author: tggzzz
author_rank: Super Contributor
timestamp: December 16, 2015, 10:35:25 pm
content: I'm unconvinced.For an unloaded low-voltage mains transformer, what would be the voltages at which such effects would be visible?Why were such effects not visible 40 years ago? Obviously I never took a photo, but I'm sure I would have remembered such gross variations from a sinusoid - and enquired about them.If the conjecture is that the transformers have changed, what are the changes and what other effects would be manifest?OTOH the changes I've mentioned have occurred, but I haven't assessed the magnitude of the consequences.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2015, 10:36:37 pm
content: FYI, saturation doesn't look like clipping.Clipping is very common these days, as mains loads are primarily switching supplies with rectifiers on the input. Lots of big honking diodes and capacitors hanging off the line tends to flatten the waveform, just as seen here. (Be glad it isn't uglier..?)Tim

author: tautech
author_rank: Super Contributor
timestamp: December 16, 2015, 10:52:29 pm
content: Quote from: tggzzz on December 16, 2015, 10:35:25 pmDon't be, you're quite right.SMPS is responsible for much mains sine wave purity contamination, more pronounced in a local environment where many SMPS are, multi PC lab for example, and when the local supply is some distance from the energy providers last transformer/substation.

author: TimFox
author_rank: Super Contributor
timestamp: December 16, 2015, 10:54:48 pm
content: I was referring to your statement that the transformer saturates on the current, not on the voltage, rather than directly to your scope traces.My discussion directly followed the physics of iron-core transformers.To allow a voltage well above the manufacturer's intended rating, he would need a larger cross-sectional area (reducing the B field for a given flux requirement) which would increase both the weight and cost of the unit.

author: DanH
author_rank: Newbie
timestamp: December 16, 2015, 11:51:29 pm
content: I didn't mean to start a disagreement, I did hook my probe to my incoming 110 AC line to remove the transformer from the equation look at the last picture I posted it look the same as the one's with the transformer, I also did adjust the Beam intensity pot to shut the trace off when knob is turned CC.I don't have a HV probe to look at the Focus voltages, I don't plan to get that close to 19,000 volts.I was just trying where to start on correcting the flat top wave form.

author: tautech
author_rank: Super Contributor
timestamp: December 16, 2015, 11:54:13 pm
content: Quote from: DanH on December 16, 2015, 11:51:29 pmBefore you even think of that you must supply the scope with a KNOWN good sine wave...NOT mains or derived directly from mains.

author: tautech
author_rank: Super Contributor
timestamp: December 17, 2015, 12:13:48 am
content: So to recap on what you have done so far:Displayed a distorted (clipped) mains waveform on your scope.Adjusted the vertical POSN control to the edges of the display AND observed no further distortion of the already distorted mains waveform.I believe you thread title is incorrect...your oscilloscope is not clipping the waveform, it was distorted already.Unless you can supply the scope with a "known" waveform....believe what you are seeing....for now.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2015, 01:19:42 am
content: Quote from: DanH on December 16, 2015, 11:51:29 pmYou didn't; we did (and it isn't a significant disagreement) That's the way things go indiscussions.Welcome to the forum, keep observing and asking questions. The old adage is that the most interesting sound in science isn't "Eureka!", rather it is "That's strange.......".QuoteI suspect there isn't anything to correct: the scope is displaying the waveform correctly. As tautech says, find a known good sinewave or triangle wave. Even a square wave with a controllable variable amplitude could be used: change the amplitude and record the amplitude of the displayed waveform, then rinse and repeat.

author: DanH
author_rank: Newbie
timestamp: December 17, 2015, 02:13:30 am
content: That's great, my Wife tell's me I am always in the middle of a disagrement. (lol)I have a signal generator coming, I think I can borrow one to compare. I will reply back to the group when I get the instrument,Just to be plain I just got my General Licence in Amateur Radio (KB9RVY) I wanted to use the scope in my radio room to do some minor repairs on old (Vintage Radios I have)Again Thanks for the input.Dan

author: vk6zgo
author_rank: Super Contributor
timestamp: December 17, 2015, 06:38:46 am
content: Quote from: DanH on December 17, 2015, 02:13:30 amIf you have old radios around,you don't have to wait till your Signal Generator arrives.The various oscillators in radios normally produce clean sinewaves.ORIf you have a Dummy Load with a low level monitoring output,you can put a transceiver in the CW position,key it up on 160m or 80m,& look at the monitor output with your 'scope---this will be a very good sinewave.ORIf your radio has a Crystal Calibrator,turn it on,adjust the receiver for an audible "beat" note,& hang your 'scope probe across the speaker or headphones.Even radio receivers without a Calibrator often have "sprogs" at various tuning positions,so you could locate one of these & use it.The audio "beat' note is a pretty good sinewave,& has the advantage that you can adjust it to 60Hz for a direct comparison with the Mains waveform.

author: MarkL
author_rank: Unknown
timestamp: December 17, 2015, 03:23:30 pm
content: Another easy source for sine waves (and other shapes) is a computer with tone generator software. Connect your probe to the audio output.Its usefulness is going to be limited to audio frequencies, but you can get started with it for this problem.Offhand, this scope looks and sounds sick to me, and not by just the truncated waveform. The cal output from post #5 is also low. It's reading 0.8V pk-pk and it should be 1.0V.Something to note is that there's a pre-amp stage in the front-end of this scope between the 1x/100x and 1x/2x/5x/10x attenuator sections. If it has problems (power, maybe?) it could be responsible for some of the behavior.Also, why would a transformer output go "up" when switching from AC coupling to DC (post #8)? Could be clue.But I agree some stable and known waveforms are needed first.