Looking for a used inexpensive scope - Page 1
Forum section: electronics / general

author: BillWojo
author_rank: Regular Contributor
timestamp: July 01, 2014, 04:04:22 am
content: Hi everybody, my first post here. As the subject line says, I'm looking for a good used scope. Before everybody yells at me to read and search, well I have and I'm not coming up with the answer that I need.A little about myself, I make my living repairing machine tools, a master mechanic, machinist and can fix most electrical issues. CNC control problems are more hit and miss if it requires more than using my DVM. Usually I can work my way through most control problems if I have decent documentation and tech support on the phone. Sometimes I need to tune an old analog DC drive, requires a scope, so I have my buddy stop by and give me a hand. Never been a real pain but I should have one in my kit of tools, even if I just load it on my truck when needed.Recently I purchased an old lathe for my shop and it has a Ward Lenard DC drive. Uses thyratrons on both the field and armature control. I have a problem with instability on the armature circuit and after replacing all the caps, resistors and swapping out tubes, it has gotten no better. Tubes are know to be good, checked all the smaller control tubes on a Hickok tube tester along with a pile of used and new ones I bought from a friend, swapping tubes to no avail. Inspected and reflowed solder joints etc.I know with a scope I could trace the trigger circuit to the C16J and see whats going on. One C16J fires normally, the other surges, swapping them has no effect, one side is always surging. Since one side fires properly, I have a good circuit to compare it to.So I'm looking to get a scope that I can use to troubleshoot this drive but also use it for testing encoders and adjusting older analog drive systems. Mostly old tech stuff.Will some of the cheap scopes I see coming from china be good for this or should I look for an old school scope? Also, I know some of the voltages can be in the 300vdc range that I need to look at, what kind of probes should I get?Thanks for reading this and any advise you can give me.BillWojo

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 01, 2014, 04:21:59 am
content: I skimmed your post, you haven't given your location. Could be critical for a post like this.

author: theatrus
author_rank: Frequent Contributor
timestamp: July 01, 2014, 04:22:00 am
content: As you're on high voltage and high energy components, you should pick up a differential probe. Scopes (except for the rare battery powered models) are earth referenced, meaning the ground lead is directly connected to earth, which would not be ideal.Any scope should do (a working used analog can be had really cheap). The differential probe will likely cost more than the scope.

author: BillWojo
author_rank: Regular Contributor
timestamp: July 01, 2014, 04:34:44 am
content: Fixed my location, thanks.What's a differential probe? As you can see, I have a lot to learn. Keep it coming.ThanksBillWojo

author: tautech
author_rank: Super Contributor
timestamp: July 01, 2014, 04:39:06 am
content: Yes, I'm sure a buddy would help you with his scope, but would he lend it to you?It is a bit more complex than just clipping probes on, for your safety, the scopes and the DUT.If you are comfortable and competent with this sort of fault finding, get yourself a reasonably modern DSO with an input rating of 400V. There are several battery powered handhelds that could be useful for portability.Price might not be to your liking though.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: July 01, 2014, 10:29:09 am
content: You may want to look at a couple of these, I personally have experience with these and they are all pretty decent scopes, I placed them in the order from the best top down. The Siglent offers two different models, one of them has electrically isolated inputs which is more expensive. That model not listed here.Siglent SHS806http://www.amazon.com/gp/product/B00GQNNQTM/ref=wms_ohs_product?ie=UTF8&psc=1Owon HDS1022M-N Series HDS-Nhttp://www.amazon.com/gp/product/B006LR5ARE/ref=wms_ohs_product?ie=UTF8&psc=1Hantek® DSO 8060http://www.amazon.com/Hantek%C2%AE-Handheld-Oscilloscope-Multimeter-Channels/dp/B00CTHC0O6/ref=sr_1_14?ie=UTF8&qid=1404210157&sr=8-14&keywords=handheld+Oscilloscope

author: xwarp
author_rank: Frequent Contributor
timestamp: July 01, 2014, 10:38:23 am
content: Any chance there might be mechanical binding or maybe worn brushes?

author: BillWojo
author_rank: Regular Contributor
timestamp: July 01, 2014, 11:10:08 am
content: Terabyte, Those handheld scopes look really nice but I can't afford that right now. Looking in the 100 dollar range. As I find more uses for the scope and want to carry it with me in the truck, than I'll be looking for something like that.Xwarp, no mechanical binding, motor was rebuilt before I got it. Problem is definately in the trigger circuit, one thyratron fires properly and the other surges. Both tubes are used for the armature control. Field control is ok, uses a much smaller set of thyratrons for that.ThanksBillWojo

author: xwarp
author_rank: Frequent Contributor
timestamp: July 01, 2014, 12:38:36 pm
content: Can you post a schematic for that?

author: EEVblog
author_rank: Unknown
timestamp: July 01, 2014, 12:59:39 pm
content: Quote from: BillWojo on July 01, 2014, 04:34:44 amOne of these:http://www.ebay.com.au/itm/New-Pro-Differential-probes-DC-25Mhz-Max-Voltage-1300V-Tektronix-OEM-Taiwan-CAT3-/290711961811?pt=LH_DefaultDomain_2&hash=item43afc880d3Lets you measure high voltages or any circuit safely without being concerned about where or how you probe anything.

author: PaulAm
author_rank: Frequent Contributor
timestamp: July 01, 2014, 07:10:29 pm
content: Look for a Tek 422 or 453. They're both older tech, but pretty rugged and relatively cheap. Both of them support differential input if you need it. I found a 422 off of craigslist for $35. Either one of those should be available in the under 50-100 range. I bought the 422 to keep by my CNC stuff.I've had a 453 on my bench for 15 years and never opened it. It just keeps working.You could also look for a 465; those are a little more recent, but will cost more and tend to fail more often. I once bought a pile of dead 465s from a CNC repair company.These are all portable scopes, so will be easy to haul over to the machines.

author: tautech
author_rank: Super Contributor
timestamp: July 01, 2014, 08:11:48 pm
content: QuoteModern DSO's are quite suitable for this in original packaging or the range of soft or hard padded cases available.A CRT scope is designed for bench work and IMOwillget.Entry level DSO's are well under US$300

author: kingofkya
author_rank: Regular Contributor
timestamp: July 01, 2014, 08:19:34 pm
content: Quote from: EEVblog on July 01, 2014, 12:59:39 pmWhat about just using 2 probes? Is there any issue with just using 2 probes and the subtract/diff mode of the scope, without grounds connected of course?I have done it this way before when i didn't have an isolation transformer handy(ac mains connected stuff). just curiouse if there is any issues with that.

author: miguelvp
author_rank: Super Contributor
timestamp: July 02, 2014, 12:36:30 am
content: Quote from: kingofkya on July 01, 2014, 08:19:34 pmWorks great on analog scopes, affordable digital ones tend to be slow on the math end. Maybe bandwidth limiting or other settings can speed them up.

author: BillWojo
author_rank: Regular Contributor
timestamp: July 02, 2014, 03:20:05 pm
content: Been searching Craigslist and Ebay the last few days. Found a scope local to me for 99.00, a Instek GOS 620. Says it was purchased in 2000, never used, still in the box. Are these any good? It's been sitting 14 years, would that effect anything?Thanks guysBillWojo

author: Shock
author_rank: Super Contributor
timestamp: July 02, 2014, 08:02:55 pm
content: Always look for a waveform by connecting it to it's calibration signal and turn the knobs to confirm it's at at least functional. You will want probes and if they don't come with it, that's an additional expense.The Instek GOS 620 is 300V so not suitable, according to your requirements.If you are new to testing 300V with electronics equipment you should get some help with safety aspects. At that voltage a bench oscilloscope without proper input protection can either become dangerous or while your testing, the oscilloscope can act like a short circuit and may blow in your face and hands and damage the oscilloscope or what your working on. This is regardless of the maximum input. Testing with a decent multimeter is probably about 100x safer in comparison.

author: kingofkya
author_rank: Regular Contributor
timestamp: July 02, 2014, 09:29:12 pm
content: Quote from: miguelvp on July 02, 2014, 12:36:30 amAh got it thanks.Also getting back on topic i got a 35mhz Hitachi off ebay for around $60 delivered.This should get you started.http://www.ebay.com/sch/i.html?_odkw=oscilloscopes%20hitachi&_udlo=1&LH_BIN=1&_osacat=92074&_from=R40&_udhi=60&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xoscilloscopes%20&_nkw=oscilloscopes%20&_sacat=92074

author: BillWojo
author_rank: Regular Contributor
timestamp: July 03, 2014, 01:25:01 am
content: Well, this is why I came to this forum. I didn't realize that the scope had a voltage limitation. I thought that you needed high voltage probes and the probes knocked down the voltage to the scope. I was thinking that the probes marked as 10:1 did that, attenuated the signal. The only place that I would come across anything above 300VDC is on the output of the thyratrons, that I measure with my Fluke 179 meter.What I need to measure is the trigger signal to the thyratrons, figure out where the instability is. Start at the transformer that generates the voltage and work my way from there up through the trigger circuit. At a certain point the voltage is split and goes to a RC network for each tube. I replaced all the resistors and caps in that circuit so far. So what do I need to look at this circuit?Thanks again.BillWojo

author: echen1024
author_rank: Super Contributor
timestamp: July 03, 2014, 02:19:10 am
content: Quote from: BillWojo on July 03, 2014, 01:25:01 amI think you should try and start a new thread in the "Repair" section of this forum. Might be of much more help to you.

author: BillWojo
author_rank: Regular Contributor
timestamp: July 03, 2014, 03:40:37 am
content: I'm already working with a few people on another forum that are very well versed on this particular drive system. It was suggested that I get a scope. I've used scopes before but that was 25 years ago, just looking at linear encoder signals for alignment purposes. That was all 5vdc stuff. What I need to know is what I need. Also a quick primer on scope probes would be helpful as I stated above I had thought that the 10:1 probes attenuated the signal by a factor of 10. So what does it do?I'm not new to industrial electronics, just DC tube drives.Thank youBillWojo

author: PaulAm
author_rank: Frequent Contributor
timestamp: July 03, 2014, 01:24:46 pm
content: They do. Part of what you need to look at is the available attenuator ranges on the scope. The Tek422, for example, had a 20V/div range. If your highest range is 5V/div (that's 50 with a x10 probe) and you have 6 divisions, your full screen will be +/- 150 on AC or 300 on DC.You could use a x100 probe, although they're not nearly as common as x10. Tek made x1000 probes so you could look at KV signals. The probe needs to be rated for the voltage you're dealing with, of course.

author: tautech
author_rank: Super Contributor
timestamp: July 03, 2014, 02:06:10 pm
content: Quote from: PaulAm on July 03, 2014, 01:24:46 pmFurther to Pauls post, you need to be aware that a probe voltage rating is down-rated dramatically as frequency increases.For any work not DC at mains or higher voltage I use 100:1 probes

author: BillWojo
author_rank: Regular Contributor
timestamp: July 04, 2014, 11:28:42 pm
content: Ok, I'll be looking for some 100:1 probes than. Been watching Ebay and it looks like I can find a Tek 465 scope for about 100 bucks or so. I think with that combo I could start looking at signals. Max DCV on the trigger side of the C16J tubes is 165 so I should be covered.Now I need to find where to reference that signal from. I read up a bit on probes and grounds and I see that the probe ground ties into earth ground. I'll ask the experts on the other forum who have done this before and look at the schematic to see if it makes sense to me. If not I'll post a schematic here in the repair section and ask.BillWojo

author: BillWojo
author_rank: Regular Contributor
timestamp: July 05, 2014, 04:45:30 am
content: Would 120.00 bucks shipped be a decent price for a Tek 465M 100MHz scope? It shows waveforms in the pictures, states that it works. Seems everyone thinks highly of the Tek 465 series.ThanksBillWojo

author: elliott
author_rank: Regular Contributor
timestamp: July 05, 2014, 05:12:40 am
content: The 465M isn't really a 465, it is more related to the 455. Still a good scope, but not the legendary 465B.If it is showing waveforms then the vertical is likely fine, those had a hybrid module for the vertical amplifier and it is not the easiest thing to find. I passed up on a local one that had a blown vertical amplifier a few months ago.It is not a terrible price if it is truly working.

author: PaulAm
author_rank: Frequent Contributor
timestamp: July 06, 2014, 02:10:24 am
content: Ditto on that. Hard to tell if the delayed sweep is working from a few pics.Working 465s seem to go for a little higher, use Tek specific ICs, but are relatively repairable. If it breaks though, you are into the need a scope to fix a scope game. I don't know about the 465M, but the 465 power supplies often suffer from open filter caps at this age.Make sure the seller has a fair number of positive ratings (> 100 or so), is rated > 99% and see how he wants to pack it. I've had a couple of 465s bounced on their corners and arrived with smashed knobs and B trigger controls. If the scope has a front cover, that's a plus for shipping.Haven't had a CRT broken (yet), but I've read lots of horror stories from people who did.Just got an hp 54645d packed in peanuts. It had settled to the bottom, but actually arrived intact. Something of a minor miracle I think.