Tektronix 466 oscilloscope HF response - Page 1
Forum section: electronics / repair

author: lyonsk
author_rank: Contributor
timestamp: May 09, 2017, 06:24:34 am
content: Hello guys. I bought Tek 466 from fleabay as my first scope. My experience with scopes is limited to highschool lab work.Probes were not included so I decided to buy those that were originaly supplied with the scope - P6062B switchable 1x or 10x. I managed to find pair of NOS also on fleabay.Scope is in good cosmetic condition but had few issues:1.) Siemens hall-controlled fan motor - I have cleaned the bearings from old dried lube by dripping isopropyl alcohol on the shaf (there is no way how to get to them unless you rewind the motor) and replaced transistor array by some junk box NPN transistors sticking then directly in the socket of transistor array. Now it works. Current went from initial 240mA when the shaft was seized and 2 of 4 transistors dead short to 200mA when I replaced the transistors and finaly 40mA when I have cleaned the bearings.2.) Delayed time base switch mechanical failure - When I pulled the knob to set delayed time it was freely spinning. I have removed the board and discovered that the plastic key inside the timeselecting drum is broken into 6 pieces. I played 3D puzzel with it to put it together and aproximated dimensions so I could make similar one made of brass. Now it works as well.3.) Strange display when probes switched to 10x - it looks like the probes are overcompensated (too much capacitance). It is the same on both chanels and both probes and all settings of vertical attenuators.I attached 2 images. First one shows the scope's calibrator 300mV square wave output on both channels with probes set to 10x. Top signal is with probe compensation set to the best I could and bottom is the worst I could set. Second image shows calibrator output using 1x probe setting. I red in maintenance manual that there is something called HF frequency response compensation which can be set by variable capacitors in the attenuator. To me it sounds very unprobable that scope would be that much off and pretty much the same amount on both channels.Could it be that there is some other issue? Dirty AC-GND-DC coupling switch? Bad grounding or something like that since it is same on both channels?Thank you for any help.Tomas

author: tggzzz
author_rank: Super Contributor
timestamp: May 09, 2017, 08:24:57 am
content: Congratulations on your work so far, and on the description and pictures.There'a "HF" and then there's "HF". For compensating probes "HF" is above a few kHz. Inside the scope "HF" means above a few MHz.The scope looks fine with a probe *1 setting (as far as can be seen with that signal and that timebase). The *10 display looks like either the scope has a very low input capacitance (unlikely!) or the probe is faulty. Cheap probes are sufficient; any probe with a sufficient bandwidth and calibration range that includes the scope's input capacitance should be OK.Personally I dislike *1/*10 probes; sooner or later you won't notice the switch is in the wrong position.In most scopes the cal out is the minimum necessary to enable the probe to be calibrated. They must have a flat top, but the amplitude is only roughly right, and the risetime is only fast compared to kHz. (The 485 is an exception, where the calout has a very nice <1ns risetime)

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 10, 2017, 03:00:33 pm
content: I would agree that its seems unlikely that both input amps would have exactly the same problem and ditto the probes unless the price was too good to be true and you have been sold a couple of faulty ones. Do you know anyone that might be able to try them on another scope?If the 466 is anything like my 465 (there is a lot of external similarity), then internally there are a at least a couple of wire mesh straps inside connecting modules and boards to ground/chassis. On mine, one was corroded to the point of breaking the connection which introduced a bit of noise into the trace. It might be worth having a look inside and checking that all such straps in yours are OK and connecting the relevant modules.It seems rather unlikely, but the other common point is the actual cal signal itself. Is there a possibility for someone to have a look at the cal signal generated by your 466 using a different scope? Have you tried looking at a square wave signal from a signal generator? Does this square up OK?

author: lyonsk
author_rank: Contributor
timestamp: May 10, 2017, 04:15:07 pm
content: Hello guys, thank you for the answers. I'm very busy these days so I was not able to have another look.The probes were packed in original packages and I was opening them myself so I believe that they were "brand new" (NOS).Unfortunately I don't know anyone with the scope or function generator. :-(What I have done so far is:I have found one burned elyt. cap on the main board which was filtering -8V supply voltage in the horizontal amp and I have replaced it with 2 low ESR caps in paralel since I didn't have that value on hand. (see attached pic.)Cleaning of all the contacts in the time base switch using IPA.Measured the testpoints - voltages are all extremly close to nominal values so perhaps someone has calibrated those in the recent past.I have also checked the output voltage of the calibrator circuit. It requires shorting two test points so output goes to DC and then measuring it with DMM which should show 300mV. It was 299.9mV...There is a switch that limits the bandwidth to 20MHz on the vertical board which I have cleaned and measured the resistance. It was originaly more then 5ohms and now it shows about 0.1ohm.If I had accurate square wave gen I could connect it to the input with proper 50 ohm termination and see how the waveform looks like. Would it rule out the probe issues?

author: David Hess
author_rank: Super Contributor
timestamp: May 10, 2017, 11:55:45 pm
content: Quote from: lyonsk on May 09, 2017, 06:24:34 amThe first analog storage oscilloscope I used was a 464 which is a 466 without reduced scan mode. These oscilloscopes are nice but more complex than the non-storage models and their storage CRTs are not as bright or sharp.QuoteProbes are not all that critical for 100 MHz oscilloscopes so there is little advantage to using the stock ones although one nice feature of the original switchable probes is that the readout on the volt/div controls indicates the actual setting.QuoteThe original transistor arrays do not seem to be very reliable for whatever reason. There are modern drop in replacements but I would replace them with discrete parts.I think you can access the rear thrust bearing through a hole in the printed circuit board. The thrust bearing has threads so it can be removed, oiled, and adjusted. IPA is fine for cleaning the bearings but they should also be lubricated with a heavy oil; motor oil or gear oil is suitable but light oils are not.QuoteHF frequency response is *very* HF response and has nothing to do with the probe compensation. This is adjusted without the probes with the high frequency test signal applied directly to the oscilloscope vertical inputs through a feedthrough termination.Unless someone deliberately rebuilt the input circuits for a different input capacitance, there is no way the probe compensation could be that far off for both channels and all input attenuator settings and the x1 photograph shows that the problem is not with the oscilloscope or probe compensation signal. Each attenuator modules has an adjustment for compensation and an adjustment for input capacitance; I do not recommend messing with these until you have no other choice.Do you have some way to test it at 5mV/div where none of the input attenuators are used?QuoteSometimes an open in the high impedance circuits causes a high frequency response but your second photograph shows that that is not a problem.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 11, 2017, 02:56:57 pm
content: Quote from: David Hess on May 10, 2017, 11:55:45 pmAgreed, and my apologies. I missed that in the second photo. The problem is clearly not the cal signal which the photo shows is perfectly fine. I also agree that the attenuator modules should not be adjusted unless everything else has been thoroughly checked and ruled out first.I had a couple of tantalum caps fail short in my 465, as well as an intermittent fault in the smoothing cap on the -8v line. This is to be expected with equipment of this age. Still, it sounds like the power rails are all OK.

author: StillTrying
author_rank: Super Contributor
timestamp: May 11, 2017, 07:34:36 pm
content: Bit of a mystery!Is it the same on DC coupling.Going by the X10 photo, the curves seem to be leveling off at the right level, 0.6Div X 0.5V= 0.3V p-p, which would make me think the X10 probes's 9M resistance and the scopes 1M resistance are correct.Something to try:Connect the 1kHz 0.3V directly to the scope's BNC through a 1M resistor to check the input capacitance of the scope on a couple of Y sensitivities, if the scope is 20pF the RC time of the edges should be around 10us, and the risetime around 18us.Disclaimer: I'm no expert on these things!

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2017, 08:48:12 pm
content: Quote from: StillTrying on May 11, 2017, 07:34:36 pmThat gives me an idea.In addition to the test above, with the oscilloscope powered and DC coupling selected, measure the input resistance at all vertical attenuation settings. It should *always* be 1 megohm. Any ohmmeter should be able to make this measurement.

author: lyonsk
author_rank: Contributor
timestamp: May 12, 2017, 07:18:29 am
content: Thank you guys! I'm very busy right now and the scope is packed in the closet but I will check what you are suggesting during the weekend.In the meantime I have checked the probes. As I mentioned they are NOS with all original accessories, paperwork and bags. When I looked in one of the bags there was a loose sticker floating around that says: CA Non Standard Probe. Hmm... I opened the compensation box and I found that one of the resistors is shorted using the wire on top of it. This does look like it was done in the factory but I can not confirm. Both probes have the same modification. I did not open the box on the tip of the probe.I have measured the resistances inside and they match the schematic for 3.5ft probe:From tip of the probe to the BNC pin in 10x setting: 9.003MohmFrom tip of the probe to the BNC pin in 1x setting: 278.88ohmR3 -140ohm: 140.33ohmR4 - 34ohm: 33.97ohmR2 - 11kohm: 12.064ohmR5 - 43.2ohm: shortedWhat do you say? Should I remove the wire?

author: StillTrying
author_rank: Super Contributor
timestamp: May 12, 2017, 08:28:18 am
content: R5 being shorted doesn't explain it for me.It's more like the value of C1 9.7pF in the probe is too high, perhaps the values for the 6 or 9 foot probes.Or the scope input capacitance is well below its ~20pF.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 12, 2017, 09:05:26 am
content: Quote from: StillTrying on May 12, 2017, 08:28:18 amThe smaller you make the series R the bigger the influence of the capacitor, so it does make sense to me.

author: lyonsk
author_rank: Contributor
timestamp: May 12, 2017, 09:34:29 am
content: It looks like the cap and resistance at the probe tip is part of the probe assembly. In case that it has incorrect value I think I will need to buy different probes :-(I'm sorry - in the picture it should be items -2 and -3 ...

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 12, 2017, 10:48:54 am
content: Quote from: lyonsk on May 12, 2017, 07:18:29 amAll your resistance readings seem to be correct, but since there is only a 140ohm resistor in the path on x1, this reading seems rather high. It could be down to a dirty switch perhaps and working it a few times might reduce that. It probably wouldn't affect the 10x mode though.Quote from: lyonsk on May 12, 2017, 07:18:29 amWell it looks like you are measuring 9Mohm on 10x mode, so at least the resistance in the probe tip assembly seems to be correct. Since the input capacitance stated on the sleeve is 13.5pF, which is as per the spec of this probe, I can't imagine that the series capacitance at the probe tip would be incorrect nor is it likely to have changed significantly. I don't think you need worry about that.http://www.barrytech.com/tektronix/probes/tekp6062b.htmlQuote from: lyonsk on May 12, 2017, 07:18:29 amThe effect of the compensation RC network will have been altered by adding that link. Whether this would affect the compensation capacitance to the extent shown I'm not sure, but I see no reason not to disconnect one end of the shorting link and see what happens.

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2017, 12:37:12 pm
content: Occam's razor says the shorting link is the problem. Both probes have it, they were marked special, the shorting link is not in the original design, and both probes display the same broken behavior. Remove the shorting bar on one and see if that clears up the problem.This could be tested, well, the oscilloscope could be tested, by building a normalizer circuit. It just requires a 1 megohm resistor and a parallel capacitor which can be adjusted to match the oscilloscope's input capacitance by placing it in series with the input. These are effectively x2 probes and are used to calibrate the oscilloscope's input capacitance at all attenuator settings. Check the PDF below.I have one Tektronix x10 probe where someone removed the entire compensation circuit and replaced it with a short but I think that was done by the user for some weird reason. It is of course completely useless as an oscilloscope probe.

author: StillTrying
author_rank: Super Contributor
timestamp: May 12, 2017, 12:52:14 pm
content: Quote from: WaveyDipole on May 12, 2017, 10:48:54 amThe center core of a probe cable is made from resistance wire, so a total of 278 ohms would seem about right to me.http://www.dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%20OCt09.pdfQuote from: David Hess on May 12, 2017, 12:37:12 pmDespite what everyone else is saying, I think removing the link (adding 43R back in) will make the overshoot noticably worse!

author: lyonsk
author_rank: Contributor
timestamp: May 12, 2017, 02:09:23 pm
content: At high voltage pulse capacitor behaves like a short so we can ignore them for our purpose. Then it all boils down to one series resistor(Rs = Rwire + R4 ) and one paralel resistor (Rp = R4 + R5). What scope sees is voltage at Rp. If we make it bigger the voltage should go up so I tend to agree with StillTrying - it should get worse.Lets see what happens when theory meets practical experience. I'll keep you posted.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 12, 2017, 02:16:05 pm
content: Quote from: lyonsk on May 12, 2017, 02:09:23 pmYes, remove the wire already!

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 12, 2017, 02:29:16 pm
content: Quote from: StillTrying on May 12, 2017, 12:52:14 pmInteresting article. This seems to be the relevant comment:Quote from: The Secret World of ProbesThis is something that I was unaware of and does not seem to be factored into the Tek diagram of the probe, but if true of the P6062B, then it might explain the reading of 278.88 ohm and would then seem to confirm that the probe is as per spec in all respects except for that shorting link.

author: StillTrying
author_rank: Super Contributor
timestamp: May 12, 2017, 02:50:12 pm
content: Quote from: WaveyDipole on May 12, 2017, 02:29:16 pmWhy do you think the pdf is titled "TheSecretWorld Of Probes".There's lots of links to probes stuff in tggzzz's sig. above.Quote from: lyonsk on May 12, 2017, 02:09:23 pmI'm glad somebody does!Before you condem or dismantle the probes, get yourself a 1M resistor, connect directly between the 1kHz and the BNC input's center, and measure the 0% to 63% risetime of the edge to check the scope is ~20pF.I'll post my attempt when I find it. - Found one.I adjust the Y amplitude so that the height of the wave covers exactly 6 divisions and I can then judge the 63% amplitude position from the 4th up 66% grid line.8.6us / 0.5M = 17.2pF which is close enough to the 17-18pF specs. for me!You can also 'measure' the capacitance of probes this way.

author: tggzzz
author_rank: Super Contributor
timestamp: May 12, 2017, 02:59:34 pm
content: Quote from: WaveyDipole on May 12, 2017, 02:29:16 pmEr, no. The explanation below hints at why scope probes are relatively fragile, and why I don't like seeing them compressed into a figure-of-8 with an elastic band.It is easy to simulate the problems mentioned below with LTSpice and a lossless transmission line. Change it to a lossy line and the simulated results are markedly improved.From Tektronix "Oscilloscope Probe Circuits, by Joe Weber in 1969, pp14-15, myemphasisI have ignored one aspect of lossless 50ohm cables.A lossless cable, as its name implies, is a circuitwith a high Q. A high Q circuit will oscillatewhen it is excited by a burst of energy such as afast rise pulse. The situation is shown in Fig. 2-5.The output is a series of sine waves with a frequencyaround 60 MHz for a 3.5 foot 50 ohm cable. Theseoscillations, often called ringing, will eventuallydamp if no further energy is applied to the cable.If the bandwidth of the oscilloscope is relativelylow, say 30% down at 5 MHz or less, the ringingwill not be seen on the CRT display. If the 30%down frequency is 10 MHz or more, the oscilloscopeis capable of processing the ringing. Then the CRTdisplay will include the distortion introduced byprobe cable.We find the circuit of Fig. 2-3 is unacceptable formeasurement systems with 30% down frequenciesgreater than 10 MHz. About a generation ago, 10 MHzwas the state-of-the-art for high speed oscilloscopes.The design of passive probes had to change toprovide an acceptable device for coupling signals tothese oscilloscopes. We can suppress theoscillations by adding resistance to the circuitwhich lowers circuit Q. Since we want goodtransient response, the resistance must providecritical damping. Ergo, let us place an unbypassedresistor in series with the cable. The location ofthe unbypassed resistor will affect transientresponse. Placing the resistor at the oscilloscopeend of the cable results in a slow transientresponse. Placing the resistor at the probe end ofthe cable or using equal value resistors at bothends of the cable gives some improvement in transientresponse.We can obtain the resistance required for criticaldamping in another manner.Let us replace the highconductivity center conductor of the 50 ohm coax witha resistance wire such as a nickel-chromium. Assumethe resistance required for critical damping is350 ohms. Then a 100 ohms/foot wire will satisfy theresistance requirement of a 3.5 foot cable. We nowlabel this coax as an R cable to differentiate witha 50 ohm or Z0 cable. When a Z0 cable is replacedwith an R cable, critical damping is realized andwewill have a faster risetime than is obtained withseries damping resistors. R cable is also called lossy cable to differentiatefrom lossless or Z0 cable

author: Paul Moir
author_rank: Frequent Contributor
timestamp: May 12, 2017, 06:32:19 pm
content: The resistors seem to match the 3.5' probe compensation box. Do you have any way of measuring the tip capacitor?Manual with schematics for the 3.5, 6 and 9' probes:http://w140.com/tektronix_p6062b_probe_ocr.pdf

author: lyonsk
author_rank: Contributor
timestamp: May 12, 2017, 10:31:07 pm
content: I removed the shorting wire but it made no visible difference.After that I decided to open probe tip assembly to find out which type of point it has. Suprise, suprise. There was ceramic 51J (51pF?) cap soldered parallel with 9Mohm resitor. I have removed it and then I was able to compensante the probe to calibrator signal.I was not able to determine capacitance of my probe tip since all I have for measuring capacitance is DMM.Thank you guys for all the good ideas. I learned a lot about the probes and my scope is now one healthy puppy.I'm still wondering what was this kind of modification good for. It definitely looked made in the factory. I was not able to google anything ralated to any of the numbers on the probes or pictures of disasembled probes of this type.PS: Picture of calibrator signal rising edge was taken at 50mV/div 200ns/div in storage mode

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2017, 11:58:37 pm
content: Someone over on the TekScopes@yahoogroups.com or TekScopes2@groups.io email lists might know what the modified probes were intended for. The obvious thing is an instrument which had a lot more input capacitance but I am not sure what they would be. Tektronix was not shy however about making special purpose probes for things like frequency counters so this modification must have been very special.You will need a faster edge to verify that the probes are working correctly. The output from a logic gate would be a good place to start. Make a direct connection to the probe tip and ground without the ground lead.I used to have a bunch of these probes but the cables all got stiff and cracked. About half of them had the readout wire which runs through the shield break.

author: lyonsk
author_rank: Contributor
timestamp: May 13, 2017, 07:33:11 am
content: Someone asked to measure input resistance on all attenuator settings:R in Mohm:Channel1:0.99300.99850.99940.99850.99860.99860.99880.99880.99880.9988Channel2:0.99050.99720.99870.99900.99910.99910.99970.99980.99980.9998

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 13, 2017, 08:02:58 am
content: Glad you have been able to identify the problem. So it turns out that StillTrying was right about the tip capacitance. This thread has provided me with an interesting insight into probes!

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2017, 08:56:46 am
content: I asked about the input resistance; so the oscilloscope inputs are completely normal and not damaged.I have never even heard of a probe modified like that before.

author: lyonsk
author_rank: Contributor
timestamp: May 13, 2017, 05:20:14 pm
content: Quote from: StillTrying on May 12, 2017, 02:50:12 pmI tried this approach and result is 12ns -> 12ns/0.5M = 24pf. That should be fine considering that I don't have anything to check the time base calibration with.Now to get my paws on sime nice function gen...Thank you guys for all your help.

author: StillTrying
author_rank: Super Contributor
timestamp: May 13, 2017, 10:33:20 pm
content: 12ns/0.5M = 24pfus24pF is in the right ballpark!I can't think what device would have had 1M+~80pF of input impedance either.Quote from: WaveyDipole on May 13, 2017, 08:02:58 amDid you see my attempts to make a DIY X1 High BW Passive Scope Probe, it's a "work in (slow)progress".https://www.eevblog.com/forum/beginners/diy-x1-high-bw-passive-scope-probe/

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2017, 03:07:26 am
content: Quote from: StillTrying on May 13, 2017, 10:33:20 pmA really good voltmeter input could have that low of an input capacitance. The highest capacitance 1 megohm instrument inputs I have seen are 47pF and operate up to 1 MHz although they could be faster.Maybe we are thinking about this the wrong way. What if the probe was intended for a different input resistance yielding a different attenuation factor?

author: StillTrying
author_rank: Super Contributor
timestamp: May 14, 2017, 11:52:19 pm
content: Quote from: David Hess on May 14, 2017, 03:07:26 amI think the LF and HF would be matched on a3M/20pF scope giving a X3 probe.Maybe all the extra HF capacitance was to be used on a very long probe cable of 10m 35ft, someone thought that was just a misprint and put the probes and boxes on a 1.2m 3.5ft cable!After much googleing the purpose of the mod is still a mystery.