Which isolated o'scope for mains SMPS repair?

[crystallattice]

Hi,

I need an oscilloscope for SMPS, VFD and other high voltage repairs, so isolated inputs are a must. I'm looking at the Tektronix THS720 with its isolated inputs.

Is this the best option or are there other options like a cheap handheld scope + some isolated differential probes added on for isolation?

The THS720 is 20+ years old already and I was wondering if there is anything newer that can replace it or if I should just buy the THS720 and stop looking for anything else? The schematics and service manual for the THS720 is available which makes it more attractive than a similar priced alternative of which these will not be available.

It seems the THS720 can range from about $300-800 depending on what version you go for. I was hoping to find something cheaper but also don't want to waste my money on junk and buy again later, but also not invest a lot of money in an outdated piece of kit when something newer/better is available.

What would you guys suggest?

[kripton2035]

I found some years ago a used Metrix OX8042 that is just suited for that . you can find some on ebay sometimes, but mainly in europe.
there was also the ox8062 with better bandwith.

[HKJ]

Siglent has some handheld isolated scopes, I do not know how good they are:
http://www.siglent.com/ENs/pdxx.aspx?id=93&T=2&tid=2

[tautech]

Siglent has some handheld isolated scopes, I do not know how good they are:
http://www.siglent.com/ENs/pdxx.aspx?id=93&T=2&tid=2

Yeah.....nah !
For the OP's needs he should get a 4 channel scope......but if he's serious about it, it won't stop there. 

100:1 probes
Differential probes $ $
Current probes $ $ $
And maybe a Power Analysis option for the scope too. $ $ $

[BravoV]

I was hoping to find something cheaper but also don't want to waste my money on junk and buy again later, but also not invest a lot of money in an outdated piece of kit when something newer/better is available.

What would you guys suggest?

Without upper budget limit, this discussion is meaningless and pointless.
As people will start to bombard you by suggesting tons of various products without concern the depth of your pocket. 

[BravoV]

Regarding Tektronix THS72xx series, we have a huge thread on it, and hacking too, that turns the low end model into full featured top end model, and free bandwidth upgrade too.

Imo, the Tek THS72xx series, even its 20+ years old, the price for used ones are still quite high is for a reason. 

[capt bullshot]

If you can find one for a reasonable price, I'd prefer the TPS2024 (or one of the other TPS 20xx models) over the THS7x0 - it's colour screen and way more easier to operate, more responsive. The TPS series has the same isolated inputs as the THS series, swappable internal battery, basically all the features but better. Still very small sample memory, even the latest modern ones have 10k (or maybe 20k) samples only.

BTW, IMO the power analysis option (THS720P) is pretty much useless (line frequency only, and slow as molasses). Didn't check this option on the TPS2024.

[crystallattice]

I thought I mentioned that the THS720 was around $300-800 so that is roughly where my budget lies, or less...

The TPS2024 and the SHS1062 are more than double the THS720 price, and then you still have to buy the fancy probes!

So the THS720P is not worth it? rather buy a THS720A and upgrade it? Recently bought a Flir E4, and upgraded it, nice when these options are available!

[capt bullshot]

So the THS720P is not worth it? rather buy a THS720A and upgrade it?

THS720P is the same as the THS720A. You'll just have to change an internal jumper to make the "P". IMO it's not worth the effort.

Using a float scope is always a good way to meet your creator. Get a real differential voltage probe and plug it to a grounded scope so you can go home alive.

These are proper isolated scopes, not "just open the earth wire" floated scopes. Voltage applied to the input BNC shell will not appear anywhere else on these scopes, and the channels are isolated per channel. If you use the proper probes (with the isolated BNC connectors), you're safe within the specified limits.
If used properly, they can do a way much better job than a diff probe, especially for floating small signals.

[BravoV]

Using a float scope is always a good way to meet your creator. Get a real differential voltage probe and plug it to a grounded scope so you can go home alive.

If you mean the Tek THS7xxx series, how ?  Its designed from ground up to be float scope "AND" isolated.

From the manual ...




You just can NOT do with your ordinary scope like this ... but the Tek is designed to be used like this example condition.

[BravoV]

Using a float scope is always a good way to meet your creator. Get a real differential voltage probe and plug it to a grounded scope so you can go home alive.

These are proper isolated scopes, not "just open the earth wire" floated scopes. Voltage applied to the input BNC shell will not appear anywhere else on these scopes, and the channels are isolated per channel. If you use the proper probes (with the isolated BNC connectors), you're safe within the specified limits.
If used properly, they can do a way much better job than a diff probe, especially for floating small signals.

+1

An example of the Tek THS7xxx , watch the BNC probe's inputs, no exposed metal at all.

[tautech]

The TPS2024 and the SHS1062 are more than double the THS720 price, and then you still have to buy the fancy probes!

I'm unsure about SHS1k probes. 
The online images suggest they come with good HV probes but there's some inconsistencies listed between standard accessories and optional.
If they come with the PB925 style 10x probe:
Safe voltage levels: CAT II 1000 V, CAT III 600 V

From the datasheet, confirmed:
Two special 10:1, CATII 1000V, CATIII 600V, 100MHz oscilloscope probes

Obviously the PB925 are the optional 250 MHz probes.


At least these HH's have a reasonable amount of memory depth, sure only 2Mpts but that's a heap more than the Teks.

[BravoV]

I thought I mentioned that the THS720 was around $300-800 so that is roughly where my budget lies, or less...

The TPS2024 and the SHS1062 are more than double the THS720 price, and then you still have to buy the fancy probes!

So the THS720P is not worth it? rather buy a THS720A and upgrade it? Recently bought a Flir E4, and upgraded it, nice when these options are available!

When it comes to isolated "AND" floating scope, its always expensive, even used. 

Should you decided to pick THS7xxx series, watch out for the isolated probe, as usually the one that included with original probes is much more expensive than the one without any probe or with some random grabbed ordinary one.

As the standard isolated probe is fully isolated with no exposed metal, watch closely the BNC connector below, its fully encased in a strong black plastic, while ordinary scope's probe usually have some sort of metal part exposed.

It will become pointless, if you bought this expensive scope but used with a probe that with exposed metal parts, as this defies the original purpose to have the protection, so be careful before deciding it.

[crystallattice]

Yeah, the isolated probes are expensive!

Whats the difference between the P5102 and the P6117 probes?

It would appear that the THS720 is the scope that I should get/can kind-of afford.

[crystallattice]

Just found a TPS2024B 4ch locally for about $1300

Waiting to hear what probes it has...

A bit more than I was planning on spending but maybe this is more worth it, than the THS720 for around $300-800?

[capt bullshot]

Used scopes most often come without probes.

< $1000 IMO would be a real bargain on a TPS2024B (without probes), if I was in the need of one, I'd rather buy the TPS2024 for $1300 than a THS720 for $300.

The user interface of the THS is really annoying ... plus 4 channels instead of two, 200MHz BW and a decent colour screen instead of a lousy B/W LCD. And I like the TPS form factor more than the handheld THS. Annoying with both is their external wall wart / power brick.

At work, I've used both (TPS2024 and THS720A), I'd never touch the THS if I can get the TPS.

[JPortici]

I agree. the TPS was built with this specific application in mind (altough we use ours to probe high and low voltage sensors in cars)
Four isolated channels are very nice! and battery operation!!
but i wish there were more options.. this scope is just another ancient, very noisy tek which is still being sold (i think the only difference between variants is that the CF card slot was replaced with USB at some point)

I suppose other manufacturers prefer to sell differential and current probes

[Hydron]

Testec do some fully insulated probes, see here under "Probes for battery-powered Oscilloscopes SP Series": http://www.testec.de/en/products/product-categories/standard-probes/
The same probes also seem to be available from Multi-Contact - no idea who is the actual manufacturer: http://ec.staubli.com/products/hfline/1
Prices from RS for the insulated probes start from about 40GBP (e.g. https://uk.rs-online.com/web/p/oscilloscope-probes/5180971/ )

[nctnico]

Hi,

I need an oscilloscope for SMPS, VFD and other high voltage repairs, so isolated inputs are a must. I'm looking at the Tektronix THS720 with its isolated inputs.

Is this the best option or are there other options like a cheap handheld scope + some isolated differential probes added on for isolation?

What would you guys suggest?

IMHO the main problem is that scopes with isolated inputs are also very limited when it comes to features because they are geared towards field/mobile use. I would recommend to get a regular scope and differential probes.

[mos6502]

The Fluke 190 series has input channels that are fully isolated from each other. Also, the scope is rated CAT III 1000V and CAT IV 600V. If you're working on live circuits and your life is worth anything to you, that's what I would use. They're a bit pricy though.

[artag]

Or you can float the power supply with an isolating transformer.
Much easier.

[schmitt trigger]

A good insulation transformer. Some ITs have so much interwinding capacitance that at the switching frequencies they allow hundreds of milliamps of leakage.

Thee way the windings are arranged, plus any shielding, will determine how much leakage one gets.

[crystallattice]

Thanks for the input guys!

I am concerned by spending all that money for a "special purpose" scope but I also have an old CRT scope(s!). I have used these with the earth disconnected and also with a isolation transformer but feel like the proper thing is in order, besides I do not own a digital scope although I have access to them.

The chap with the TPS2024B has not replied so not sure if it is still available, seems like I have to wait a bit and keep my eyes peeled.

[tautech]

I just remembered this thread for you to get a closer look at the SHS1000 models:
https://www.eevblog.com/forum/testgear/review-siglent-shs-1602-isolated-scopemeter/

[BravoV]

Or an alternative, find an used Tektronix probe isolator, basically this gadget turns your ordinary scope's probes into a fully isolated and floated ones.

At the market, these used isolators are "relatively" much-much cheaper than a full blown used isolated scope you're looking for, but alas, it has the same problem as we discussed before, as it came with its own special prorietary isolated probes standard accessory, so if the seller doesn't include it, again, then probe isolator will become an expensive paper weight. 

As an hobbyist, I had been thru the path like yours actually, in the search of scoping mains related signal, and at some gadget, I had to wait for years to get a decent priced one, so I think I can feel your pain.