Probes for Tektronix 2445A - Page 1
Forum section: electronics / testgear

author: shotgun25
author_rank: Contributor
timestamp: March 26, 2015, 05:48:53 pm
content: Hi,As you may have seen from my previous posts, I'm interested in repairing some TVs, monitors and SMPS computer PSUs I have acquired and stored in the loft as a learning exercise and with a long term view to possibly repairing as a hobby.I have managed to buy a Tektronix 2445A oscilloscope from a member of this forum but I need to source some suitable probes.After speaking with Tektronix it seems they only offer one type of probe for this scope/my needs and it happens to be the P6139B at a hefty price tag of £353 direct from Tektronix. It's not much cheaper elsewhere.I remember from Dave's "$50 ebay scope" video that he said you could get some cheap probes for about $10. I have no problem doing this if they are going to be suitable for the task I have in mind and I wont end up with something improperly rated that will give me a shock or melted probes.I've found these compatible probes on eBay,http://www.ebay.co.uk/itm/Ez-Probe-TEX250-DC-150MHz-x1-x10-Oscilloscope-Probe-Kit-TEX-250-TEX-250-/221539248456?pt=LH_DefaultDomain_0&hash=item3394c4a948,but I'm unsure if there are any problems with the specifications, anything that will bite me.I'm not keen on buying from abroad because I won't have a realistic price for return shipping should I need to use it.Any probe recommendations or advice for not buying something wildly dangerous. I guess I should mention I am planning to learn how to use a scope on Plasma and LED/LCD Tvs and a few Dell monitors in the loft if that gives you any ideas of the voltages/level of danger I am getting involved with (novice here).Please help!

author: grumpydoc
author_rank: Super Contributor
timestamp: March 26, 2015, 06:01:43 pm
content: The Tek probes are high bandwidth and have detection of the x1/x10 setting via the readout ring. ISTR that the front end of the 'scope was designed with the performance of the probe in mind.Of course a 500MHz probe on a 150MHz 'scope is overkill.If you don't mine the loss of the auto detection of the probe attenuation then you have a wide choice - the Texas probe that you linked to is fine. I have some 100MHz probes that I sell for £6 each (but they're flat to well over 150MHz).Second hand Tek probes are not uncommon on ebay either if you keep an eye out.

author: tggzzz
author_rank: Super Contributor
timestamp: March 26, 2015, 06:03:24 pm
content: Quote from: grumpydoc on March 26, 2015, 06:01:43 pmWhat's the tip capacitance and the grounding arrangements?

author: grumpydoc
author_rank: Super Contributor
timestamp: March 26, 2015, 06:55:36 pm
content: QuoteNow you're askingTip capacitance is listed as 15pF in x10 mode, driven from my sig gen with no termination they were pretty flat to 150MHz, I've discussed them before if you look over old posts.They don't come with a ground spring so you'd have to roll you own - they just come with the typical 4" long croc clip attachment but, then, they are a cheap low bandwidth probe so I don't think not having a ground spring is all that much of an issue.In fact they are some NOS Texas 100 probes - so, basically just the cheaper 100MHz cousin of the probes the OP linked to.

author: TimFox
author_rank: Super Contributor
timestamp: March 26, 2015, 07:07:45 pm
content: When selecting a scope probe, be sure to verify that the actual input capacitance at the CRO (should be specified on the front panel near the BNC, next to 1 Mohm resistance) is within the range of the probe compensation adjustment (should be specified in the probe data sheet).

author: tautech
author_rank: Super Contributor
timestamp: March 26, 2015, 07:31:37 pm
content: While it might be nice to have probes with at least the same BW as your scope, in reality you will probably never need to.Many use the YPioneer P6100 probes for everyday use. Search for P6100, so cheap it's unbelievable.They are rated voltage wise much the same as any other similar spec probe.Then you will be up and running in short time and can the keep a watch out for some Tek fixed 10x should some come up.These P6100 probes are recognised by many here as perfectly OK for GP work.I understand your caution for your intended use, yep these things bite but might I suggest you also get a 100:1 probe for a better voltage "derating with frequency".

author: mskobier
author_rank: Regular Contributor
timestamp: March 26, 2015, 08:16:25 pm
content: Shotgun25,I am in a similar situation in that I need probes suitable for a 2445B I recently acquired and repaired. I have seen the generally recommended P6139 probes, and even on -bay, they can be quite expensive. I found a web page that lists probes by which series of scopes they were designed for. There is actually there are six probe models that are suitable for your scope. The least expensive (on -bay) is usually the P6133 (150mhz) on up to the P6139. Anyway, take a look at the attached link to see the listing of suitable probes. Hopefully you will find a model number that is useful for your purpose. As other posters have stated, even a generic probe of suitable bandwidth will work and give you useful measurements when properly used and adjusted. Anyway, here is the link.http://www.barrytech.com/tektronix/probes/tekprobes.htmlMitch

author: tggzzz
author_rank: Super Contributor
timestamp: March 26, 2015, 11:32:46 pm
content: Quote from: grumpydoc on March 26, 2015, 06:55:36 pmSo with a 4" ground lead I would expect them to ring at ~135MHz, which is just about OK. 6" would resonate at ~100MHz.

author: George_Race
author_rank: Regular Contributor
timestamp: March 27, 2015, 11:32:28 am
content: I have purchased 2 sets of probes from the eBay seller listed below.Very satisfied with all my purchases. I have two TEK scopes, one is a 2445.Most of their probes are missing the slip on "grabber" and the ground wire with the push on ring.But, those parts from the low cost probes from China fit perfectly.I ended up with 4 like new TEK probes for less then $100 total cost. And they have the auto 10 to 1 pin in the connectors to auto change the range of of the input voltage. All 4 of them are TEK P6106A type.Here is the link:http://www.ebay.com/sch/fourgt/m.html?_nkw=&_armrs=1&_from=&_ipg=25George

author: shotgun25
author_rank: Contributor
timestamp: March 27, 2015, 06:03:18 pm
content: Hi,Thanks for the responses.I'm only asking about higher voltage measurements because even though I might not be probing anything that high, if I make a measurement somewhere there is upto 300Vs then I don't want to be toast. The Samsung 43" plasma I am looking at had a VS of 208V so I need to be able to safely encounter that. I assume if I am only measuring the Secondary side of most PC/TV/Monitor SMPS that I shouldn't encounter anything too high (>300V)?How would the probes fail should I make a mistake - would they melt, explode or worse?If they only melt I could wear some protective gloves, if explosive, then I would need to be wearing gloves, goggles and maybe a stab vest? Helmet too?I'm still waiting on a price quote from Tektronix used selection for a P6131 so I will see how they fare for price.I've managed to buy a new P6139A which the Tektronix Centre of Excellence claimed would be compatible.QuoteI think the item I have secured on eBay was a total including shipping from a UK seller of around the £70 mark but I don't think it had a grabber on it. Way more than I wanted to be paying, but the auction was ending and I hadn't read this page at that point.So is the general consensus is that most probes will handle up to 300V Cat II regardless of most other specifications, but I should be suspect of the cheap Chinese or no brand probes claiming to be rated that high? Or expect bad things when they do fail?How would the cheap Chinese grabbers attached to Tek probes hold up with upto 300V?Thanks for the help, I'm a novice when it comes to electronics and test equipment, but I'm interested in learning and hope to learn enough to start making a lot of good lasting repairs and maybe even second wage in the long term.

author: tggzzz
author_rank: Super Contributor
timestamp: March 27, 2015, 07:40:21 pm
content: The maximum voltage is strongly frequency dependent. At high frequencies it can be ~10% of the DC value.

author: tautech
author_rank: Super Contributor
timestamp: March 27, 2015, 07:50:55 pm
content: No you won't see 300V or anything like it on the secondaries.But be aware some backlight technologies other than LED have HV, 1 Kv or more.Commonly another inverter is used for this HV.SMPS do fail on the primary side, startup resistors open, controller IC's die, IC PSU caps go bad etc.You will likely have to troubleshoot these, hence my previous warning.All probe grabbers are not made equal., Interchangability is not guaranteed but some do.I don't like to risk anything with the cheap probes and HV, just my nature, and I change to 100x probes for 300VEditSMPS frequencies are commonly in the 30-60 KHz range

author: shotgun25
author_rank: Contributor
timestamp: March 27, 2015, 09:56:28 pm
content: Hi,Thanks again for the responses.I've just successfully bid on another P6139A probe for £57. Here's to hoping they work ok.So you are saying my scope(2445A + 10X P6139A) is unsuitable for SMPS probing, or just that I won't see the full range of frequencies without altering settings or something like that?Would I need active different probes to work on SMPS?From what I've seen on youtube, you are recommending 100x probes because I can see finer detail on waveforms to look for distortion, correct? Does the 100x probe give me any other improvements, does it let me work on different equipment?

author: tautech
author_rank: Super Contributor
timestamp: March 28, 2015, 12:02:54 am
content: Not at all.Understand that a 10x or more correctly 10:1 probe ATTENUATES the signal by a factor of 10 times.That might help you understand.

author: shotgun25
author_rank: Contributor
timestamp: March 28, 2015, 04:42:04 pm
content: Reading on the following,http://www.radio-electronics.com/info/t_and_m/oscilloscope/oscilloscope-probes.php, I see that using 100x or 1000x would allow me to work on higher voltages without damaging the scope? Although this could introduce noise into the signal apparently (I'm sure I'll learn about noise when I can actually use my scope).What I don't understand is, how are these probes able to work with higher voltages?Do they use thicker cables, or does it only relate to having a different compensation adjustment in the end of the probe and thus the cable doesn't change thickness/material?My question really is how does it keep you safer from those voltages i.e. not melt, explode etc the probe?

author: MarkL
author_rank: Unknown
timestamp: March 28, 2015, 06:29:35 pm
content: Quote from: shotgun25 on March 28, 2015, 04:42:04 pmThe input resistors in high voltage probes are specially constructed to be able to withstand higher voltage. This is done by making sure there's sufficient insulation and spacing between the two terminals, and by selecting resistor body and resistance materials to withstand the voltage. It's not your standard resistor.Compared to low voltage probes, the resistor is almost always higher in these types of probes, and use either the same divider scheme, or have a second resistor that shunts and lowers the input resistance on the scope side so that the input resistor doesn't have to be so large.The rest of the probe needs to be designed to withstand the voltage too, and keep the high voltage from anywhere near your hand while you're holding the probe.If you want to measure high voltage, don't go cheap on high voltage probes. Your safety and equipment could be at risk. Get high voltage probes from reputable manufacturers where you know they spent the time to properly design it, and they use quality components and manufacturing.

author: Wim_L
author_rank: Regular Contributor
timestamp: March 29, 2015, 03:41:23 am
content: A warning about high voltage probe use:If you're going to use a high voltage passive probe to look at a signal with a DC component that's greater than the maximum acceptable input voltage of your oscilloscope, made sure that the scope's input is DC coupled. In AC coupled mode, the scope's input circuitry puts a DC blocking capacitor in series with the input. This input capacitor will then be charged to the total value of the DC component of your signal, and it might not survive that.(It's possible to do make probes and/or oscilloscopes that do this in a safer way, but a lot of equipment does it like this)So, for high voltage passive probe use, put the scope in DC coupled mode, and keep it there.

author: shotgun25
author_rank: Contributor
timestamp: March 29, 2015, 03:21:06 pm
content: MarkL -Thanks for the knowledge on how passive probes are made, how they work and keep things safer - I was right to avoid the Chinese stuff and buy Tek probes.Wim_L - Like I've said earlier I'll learn how to use my scope once I have a probe or two to try it with and this hopefully means I wont break it first use so thanks for the warning.If I needed to look at signals on inverter boards in a TV or the high voltage sections of SMPS, I assume from reading around that the voltages can go anywhere up to 1-2.5 KV in an inverter and unsure in a SMPS - would I still be suitable with a passive probe or should I be looking at active probes or differential probes?Also I hear a lot about having one hand behind your back for high voltage.What is the lowest voltage that you would start to put your hand behind your back/in your pocket?What circumstances would I use 2 probes and not use 2 probes?

author: Wim_L
author_rank: Regular Contributor
timestamp: April 01, 2015, 10:37:06 am
content: Quote from: shotgun25 on March 29, 2015, 03:21:06 pmHigher than 2.5 kV. It could be over 20 kV for a CRT. SMPS... Depends on the specific SMPS. The high voltage (flyback) system for a CRT is a SMPS, so it can be very high. Common step-down SMPS designs like a PC power supply often have a few hundred volts on the mains side capacitors (and those store a lot of energy).There exist passive, active and differential probes that will handle these voltages. Which one you pick will depend on specific needs of the application. It's not just voltage that matters. If you want to measure voltages relative to earth only, or on insulated systems (not earthed), single-ended probes will work. Watch out with the ground lead:Otherwise, you need differential probes.QuoteThere are two things to keep in mind here: voltage and current. 10 kV from a supply that can only deliver a few milliamperes is not all that dangerous unless you have a weak heart or other health conditions that make you vulnerable. At least, not dangerous in the sense that it will kill you by electrocution. It will be painful, though, and the shock might make you jump and hurt yourself in other ways. 100 V or even less from a supply that can deliver a few hundred milliamperes can easily be fatal if it goes through your heart. To know what's safe, look up "Extra-low voltage". With anything else, be cautious.Systems may have a capacitance that makes them more dangerous than they might appear. A supply may be limited to, say, 1 mA average output, but have some system on the output with enough capacitance to briefly deliver much more than that. One always has to be very careful when working on unknown systems.

author: shotgun25
author_rank: Contributor
timestamp: April 01, 2015, 07:40:07 pm
content: Excellent response, just what I needed to hear, many thanks.Looks like I still have plenty of reading up/videos to watch/advice to seek before my probes arrive and get used, but this will get me started.I have long heard about the "it's not the volts that kills you, it's the amps" but I also know it's not that clear cut. Your response highlights that and has great detail that should save someone from doing something unknowingly dangerous in the future.I think I need to ask some more questions about:isolation transformerswhen to use a multimeter vs oscilloscopegood practiceI've read/watched on the topics but until I get practical I won't have specific questions and any experience from mistakes.Thanks to all respondents for all your help, I think this topic can rest now, I'll ask other questions in new topics.

author: tautech
author_rank: Super Contributor
timestamp: April 01, 2015, 07:57:21 pm
content: The videos will get you started but experience is the key.Stay with LV until you understand what you are doing, for your and the scopes safety.Hand in back pocket is a very good HABIT to acquire, not applicable to just HV but all probing where 2 hands aren't required.You can't see the risk, but you can reduce it.Remember many devices have mains and LV present, sometime quite close to each other.Be safe not sorry.