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Re: 465B strange ripple
On Tue, Feb 23, 2021 at 07:13 AM, satbeginner wrote:
Interleaved. I did. Hmmm... Maybe I didn¡¯t put the ground in the right place by fear of shorting something... Didn¡¯t I report back on that? I'm not sure if I saw feedback on this, I was busy, so I could have missedMaybe I didn¡¯t report it then. I¡¯m also busy with work (not retired). If you have this temporary ground in place and the ripple is still there andI didn¡¯t see it go. But I probably didn¡¯t put it in the right place.. EXACTLY!! That¡¯s what led me to believe it was trigger related. My long last post talks about that. Now in this last picture: /g/TekScopes/photo/260052/0?p=Created,,,50,2,0,0 You can see a trace with NO INPUT at all. It¡¯s DC coupled, 5mV/Div, 5ms/Div. If you enlarge the image, you¡¯ll be able to see thes little ¡°blips¡± that, put together, seem to form a very low amplitude square wave. Now that square wave moves from left to right, or the opposite way, depending on the position of HOLD-OFF knob. In this picture, it¡¯s at about 9 o¡¯clock, and it¡¯s steady. Otherwise it can get pretty fast... |
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Re: Which probe is better suited for 2465B?
On 23/02/21 16:56, Jean-Paul wrote:
Tom is the 485 input 50 Ohm free of any capacitance? Or just lower than the others?The 485 has two attenuators, the normal 1Mohm attenuator used with traditional "high impedance" probes, and a separate 50ohm attenuator. A relay connects the input from the BNC socket to either one or the other attenuator. In the scanned service manual available at the usual places, have a look as the /two/ diagrams <1> for the "Ch1 attenuators (Hi Z)" and "Ch1 attenuators (Lo Z)". The relay is K151. Other scopes' 50ohm input is merely a 50ohm resistor connected across the "Hi Z" attenuator with 1Mohm/15pF input. |
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Re: Which probe is better suited for 2465B?
On 23/02/21 18:56, Giuseppe Marullo wrote:
A high power through attenuator (probably with cooling fins!), or a 20dB coupler which taps off 1% of the power going through it.For 50 Ohms use a coax like RG/198/U on a BNC.Ok, this is easy...but what about high power stuff, like 100W RF or similar? Probably a T plus a dummy load but then high impedance probe, right? |
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Re: 7A26 no signal
Thanks Tom,
Just repeated my measurements with the 7603 frame. 2.5ns is more correct value (timebase calibration issue with fastest sweep and 10X). This corresponds to 140MHz. I measured also frame 3dB bandwidth and got 140MHz. 7603+7A26 combination gave 3ns rise time (correct timebase). 7603+7A26 3dB bandwidth was 100-110 MHz. These gave 3ns rise time (correct timebase). I am happy with the results. Now I have 4ch 100MHz ¡±old school¡± scope in my lab. Regards, Jouko |
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Re: Which probe is better suited for 2465B?
Hi Jim,
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Lower frequency scopes merely throw a 50 ohm terminator across the input to provide a 50 ohm mode, but the 485 has two separate attenuator sections, one for 1M, and a different one for 50 ohms, with a relay switching between them, as necessary. Because the entire 50 ohm signal path is different, the capacitance can be (and is) designed to be inherently lower, as it has to be to make the bandwidth spec. Cheers Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 2/23/2021 11:00, Jim Ford wrote:
Well, 20 pF is about 22.7 ohms at 350 MHz, so that would affect the 50 ohm input.? So I assume Tek left the 20 pF shunt cap off the 50 ohm input. |
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Re: 465B strange ripple
Mike,
I DID all the recommended tests except for the one involving a diff amp, which I don¡¯t have. Ozan suggested that I use the ¡°INVERT¡± and ¡°ADD¡± trick, although it was not as good. I asked where I should probe for that, but got no answer. But that¡¯s fine; everyone has their own stuff to do, and I understand that; that¡¯s perfectly normal and understandable. Anyway, I did all the other tests. And as far as the magnetic issue, I did mention that the problem occurs all the time. Even when the scope is inside it¡¯s box. Even in broad daylight, when nothing is powered on, no incandescent lights, NOTHING is even up and running besides a FG that is not even plugged into the same wall socket. All the shields are also in place, and all the ground connections have been checked. There is absolutely no visible ripple. Some are even a bit less than what the manual suggests. Some filter caps have been replaced as well. The rectifiers have been checked and are all good. I have done everything that I could or knew how. I have also been carefully documenting within my posts and most of my pictures, all the voltages and sweep times. I don¡¯t know how much clearer I can be... Now, something I did notice earlier today, is that at lower sweep times.. I can somehow considerably slow down the ripple to ¡°almost¡± nothing, when I turn the ¡°HOLDOFF¡± pot clockwise... But it¡¯s still there, and it makes the calibration of anything X10 related, very difficult, as I can see the pips from my Type 184 rocking horizontally back and forth... Or When trying to adjust ¡°High Speed Timing¡±, for Example.... BTW, I¡¯m not an EE either. Far from it even. Also self taught. |
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Re: Which probe is better suited for 2465B?
Well, 20 pF is about 22.7 ohms at 350 MHz, so that would affect the 50 ohm input. So I assume Tek left the 20 pF shunt cap off the 50 ohm input.
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Jim Ford ------ Original Message ------
From: "Jean-Paul" <jonpaul@...> To: [email protected] Sent: 2/23/2021 10:41:46 AM Subject: Re: [TekScopes] Which probe is better suited for 2465B? Rebonjour, the 485 spec is 50 ¦¸ /1 M¦¸//20 pF |
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Re: Which probe is better suited for 2465B?
Hi Jeffrey,
For probing 50 ohm signals it's best to use a simple passive low resistance probe. This also works well for fast logic signals. This will outperform all standard probes. Links to a couple of designs here: >probe/First link inside EEVBlog is dead, but the second seems really good: For high voltage you need a probe that is rated for that voltage.The P4100 looks suitable. Just take care around high voltages. If the high voltage comes from a low impedance source, such as a transformer direct from mains/line then it can kill you. I >survived the experience but it hurt. A lot.Already bought, hope it will work thanks. Giuseppe Marullo IW2JWW - JN45RQ |
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Re: Which probe is better suited for 2465B?
Non, non et non.
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I gave you the upper bound on the input capacitance implied by Tek's input VSWR spec. That number is 5pF. It's irritating that I do this work for you and you simply ignore it. That diminishes my willingness to help you going forward, as it would be a waste of time. It should be obvious that the 20pF number cannot possibly apply to 50 ohm mode. Do a calculation as I did. If you find an error in my button-pushing, let me know. Otherwise, accept the implications of my calculation. -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 2/23/2021 10:41, Jean-Paul wrote:
Rebonjour, the 485 spec is 50 ¦¸ /1 M¦¸//20 pF |
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Re: Which probe is better suited for 2465B?
Hi John,
Hello: Yes P6137 is the specified for the BW and Cin. Hard to find good one, many junkers out there.Trying to get them from Ebay, I asked the seller specific conditions/photos and I will claim the money back if they are not in good order. Anything specific to ask to be checked, like the button or something similar? For 50 Ohms use a coax like RG/198/U on a BNC.Ok, this is easy...but what about high power stuff, like 100W RF or similar? Probably a T plus a dummy load but then high impedance probe, right? For probing wideband we use a Zo probe P6156 eg 1K Ohm resistance but very wideband.Twice the price of a used P6137. Will have to wait a little. At 1 kV for HV we use the old P6015, the Freon filled units are all degassed but still can work to 10 kV. Available on epay ~ $50 but check if all parts present.I checked but prices are way different at the moment. I bought the P4100...like 10EUR shipped. How I will not die using it. General info is in the Tek Circuit Concepts book, Oscilloscope Probes, highly recommended.Great reading, thanks! Enjoy, Jon |
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Re: Which probe is better suited for 2465B?
Rebonjour, the 485 spec is 50 ¦¸ /1 M¦¸//20 pF
I assume the 20 pF applies to either Zo. Best measurement of Cin is the classic TEK 130 LC meter, very easy to use and accurate, with a 1, 3 pF scale. We also use the HP 4232A LCR meter. Very interested to hear the 485 Cin actual data. I bet its >0 (:-:) Bon Journee Jon |
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Re: 465B strange ripple
Hi,
A while back I asked you to put a temporary (very short) ground in between the attenuator output and the amplifier input to find where the ripple is picked up. I'm not sure if I saw feedback on this, I was busy, so I could have missed this. If you have this temporary ground in place and the ripple is still there and is still the same, it is save to say the ripple is picked up in the amplifier. If it is gone, it should come from the attenuators. One other thought: In one of your pictures it looks like there are two traces. When you see this signal, can you play a bit with the trigger hold-off to see if that makes a difference? Good luck, Leo |
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Re: Which probe is better suited for 2465B?
I should add that I don't fully understand the reason for your question, though I was happy to answer it. The 485 is a 350MHz scope, and so one would naturally expect the input RC time constant to be small enough not to get in the way of the bandwidth spec in 50 ohm mode. With my 5pF estimate, one sees a time constant of 125 ps, or over 1GHz bandwidth at the connector. So all the numbers fit together sensibly.
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Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 2/23/2021 08:56, Jean-Paul wrote:
Tom is the 485 input 50 Ohm free of any capacitance? Or just lower than the others? |
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Re: 465B strange ripple
Hello Stephen,
Although I don't have a 465B, I have been following this thread because line-frequency noise can be so hard to track down, in many types of things, and was curious about how this all would work out. From what I can tell, you did not do all of the recommended tests, and there is still the idea that the problem could be magnetic versus electronic. The same goes for the HV section versus power supply. The rectifier test via looking at wave forms is very clever and could save a lot of board removal and soldering time. If all of the rectifiers, caps, and grounds were truly okay, that would leave magnetic (I guess). Which is easier to test for? If it's magnetic, do that first. If that passes, then you have a bad part or ground somewhere. There is a lot of advice here on where to look. Sometimes it's good to take a break, take notes (do you keep a notebook to track repair steps, etc.?) and give it another try later. Also, when writing up your observations, it is good to clarify which trace you are talking about -- on the 465B or the scope being used to check the 465B. Sometimes, I had trouble following this, but that could be my fault. These repairs can be like Rubic cubes. Recently, I repaired the B-sweep on my 453. It took a long time, because one of the transistors was apparently faulty at operating voltages (and only after a warm-up, so I missed that it ever worked, because I always waited until warmup to use the scope or do the checks, until the one time I did not wait). The sweep finally failed completely, and with the usual testing procedure, my repair notes, reading about the circuit design and studying the schematic again, I found the faulty transistor in like 5 minutes. Even though it still tested okay (on a Heathkit IT-3120 tester), the B-sweep only worked when I replaced this transistor with a new, original tek part. Now the B-sweep works fine. I am not an electrical engineer, and am largely self-taught on electronics, so please consider that when reading my posts. Good luck with your repair. |
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Re: Which probe is better suited for 2465B?
Hi Jon,
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It's certainly much lower in capacitance in the 50 ohm mode than in the 1Meg mode, but I've never actually measured it. The official spec from Tek only gives SWR values for the 50 ohm mode: < 1.15 for all ranges except the 5mV/div setting (< 1.25 there). If one attributes all of the SWR to a shunt capacitance, the upper bound on that capacitance works out to be roughly 5pF with these numbers, assuming that I didn't punch the buttons on my calculator incorrectly. That number feels about right, in any case. Given Tek's policy of sandbagging, my guess is that the actual capacitance is probably around 3-4pF. -- Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 2/23/2021 08:56, Jean-Paul wrote:
Tom is the 485 input 50 Ohm free of any capacitance? Or just lower than the others? |
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Re: Continued: 2465B CTT/HPIB/WR TEST 05 fail 42, all CTT fail
Chuck Harris
I think I am missing something:
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Did you try swapping the CTT from the scope with the error message into the scope that has a good CTT board? Did you try swapping the good CTT board into the scope with the error message? -Chuck Harris Jean-Paul wrote: Hello again: The continuing saga ... (see my pervious posts re CTT and vet nonlin) |
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Re: Slightly OT: How can I dissolve Potting Compound? FOLLOWUP
Chuck Harris
I have to say that I made that discovery several years ago.
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The floats in the carburetors of my old engines are all soldered together brass construction. Since the advent of first the oxygenated gasoline, and later the ethanol additive, the floats have been getting squished, like the air was sucked out of them. Never being able to find any leaks, I figured that the brass walls had thinned due to chemical etching, and were now too thin to handle normal barometric changes, so I removed their vent's solder plugs, blew them back up with compressed air, and soldered the vent holes closed. [The vents are there so the floats can be soldered, and not have a partial vacuum occur as the air inside cools back to room temperature.] As an attempt to strengthen the floats, I cleaned them really well, and then dipped them in epoxy. I figured the epoxy would both seal any pinholes, and increase the wall thickness slightly. Well, after one season, the engines started to flood the float bowls, and inspection revealed that the epoxy was left in sheets in the float bowl. So, yes, epoxy is no longer gasoline proof. Its dissolution is a very slow process, though. -Chuck Harris. Cliff Barber via groups.io wrote: Hi,I have only just discovered this thread. So I know I am too late. But.... I can assure you that a very "Common Chemical", one that you are will have in your vehicle will destroy epoxy. That chemical is Unleaded Petrol (Gasoline). Sadly, the addition of Ethanol to automobile fuel has caused the failure of the fuel tanks in some aircraft. Including CFM Shadow aircraft. |
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Re: Modifying W Plugin to use 6DJ8 Tubes
In post #108873 (/g/TekScopes/message/108873), David Hess suggested wiring 6DJ8's in parallel and running them off 6.3VAC from pin 13-14.
Unfortunately, that's out. Pin 13-14 is elevated to +100V and that will exceed the heater to cathode voltage rating on V124. To recap, today I see exactly two approaches. One, reverse a transformer. Unless you get lucky and find one with 6V and 18V windings like the 1A4's 120-0481-00, you have to wind your own, or put up with wire sized for 110V, i.e., at least 20VA even though the load is less than 4W. Two, run a switching power supply off +225V. I will investigate this if the 12AT7 sub fails. |
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Vertical attenuator skirted knobs
walter shawlee
Few things are as frustrating as when the vertical knob on a Tek scope is damaged, by cleaning (hah, you didn't know the printing washed right off, did you?) or mechanical wear or other damage. Many discussions have taken place over printing new skirts, etc. but not many actual solutions have turned up. I get a ton of requests for these, but mostly we are out of stock.
However, I am a bad pack rat, I accumulate many odd but interesting things, thinking "ah, these are really cool, I will figure out what these are meant for later!". well, it doesn't always turn out as intended. I found an ignored fat stock box full of these skirted knobs, many brand new, but almost none with identifying information. Rats... Photography to the rescue, and thanks to many pictures they are now up on the stuff page, and YOU can figure out what they are if you need one. Hopefully these can make some sad looking scopes out there look happy again. They are from 400-series, 22xx-series, 5A-series and SC50x-series units. Please look here: all the best, walter (walter2 -at- sphere.bc.ca) sphere research corp. |
Jean-Paul
satbeginner
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