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Advice on buying a 'scope

 

Advice on buying a 'scope

Post by RobJKMannering » Sat Sep 10, 2011 10:32 pm

Hi all,
In my youth, I owned a big valve scope, seperate power supply, spring delay line, 2kv PDA option ect.
Then I went to college and the scope went to the dump as my parents complained about the size and weight.
I then had a couple of slow, single trace scopes whilst at college but I think these were exchanged for beer tokens.
Now I find myself needing annother scope and a trawl through the internet has provided many options.
I do not require anything special or even fast. Single trace would be fine.
Please can anyone reccomend a relaible option that I can find fairly cheaply secondhand? I have a budget of about £100 so limits me a fair bit!
I note that dual trace usb scopes good for 40Mhz can be obtained for about that price but are they any good?
Many thanks for your thoughts,
Rob

 
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Post by Michael Watterson » Sun Sep 11, 2011 12:27 am

Forget about a cheap USB scope. They are toys.

You'll get a good S/H scope about £50, but  £50 postage!

A "40MHz" dual trace USB scope is good for signals up to 2MHz if both channels are used. If it's a good one.

A USB scope equivalent to 20MHz analogue Hameg of pre 1990 vintage needs about 400MHz sampling rate and maybe 10bits. Most cheap USB scopes are 8bit ADC so about 1% resolution if trace fills screen.

Also most have about 1/4 sensitivity of a typical Analogue scope, or less, so not good with 10:1 probe.

You need to spend about £800 to get a Digital or USB scope comparable to a £40 S/H analogue

My ancient Tek 10MHz analogue scope could "show" 144MHz signals, just not accurately. A rule of thumb a digital scope is useful to see shape of a waveform up to about 1/10th of sample rate. Most quote a sample rate including both channels, so is thus useful to 1/20th of sample rate when in "dual" trace!
Also your measurement is usually 1bit worse than ADC number of bits. Thus an 8 bit ADC while in theory 256 levels is "good" for about 128 levels.

 

Post by octal » Sun Sep 11, 2011 9:33 am

For a good all rounder but still serious scope have a look for a Telequipment D83. All discrete transistor, plug in time base and amps, dual beam, 50 Mhz, wide range of features and a nice big screen.
Can be had for peanuts on ebay, I bought a really nice one with the original manuals for £22 !

 

Post by RobJKMannering » Sun Sep 11, 2011 10:12 am

Thanks for your replies and thanks David for taking the trouble to look on E-bay.
Yes I agree, will keep it simple and hopefully something close enough to collect!
I am in no hurry so can spend a while looking at whats on offer. I have not ventured into TV (YET!) so anything is likely to meet my needs for radio and the odd analouge circuits I make at home.
Regards,
Rob

 
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Post by Mark Hennessy » Sun Sep 11, 2011 10:43 am

I've got a Telequipment D83 and also recommend it. The DM63 is also excellent; analogue storage, and in the most commonly found configuration, these come with 4 channels. Only 18-20MHz though - but it's the 'scope I use for most things.

Be aware that with these old Telequipment beasts, some maintenance will be required from time to time. They suffer from high-value resistors failing high or o/c, which upsets the EHT generator, also electrolytic capacitors will be tired by now. I've also had lots of random transistor failures. The DM63 has a couple of custom-made Tek ICs in the timebase, and my first DM63 came with a faulty one - luckily, this doesn't seem to be a common fault, and I suspect that there was actually another fault elsewhere that the previous owner somehow missed. I can help with manuals if necessary, and there's a bit more about the DM63 here: http://www.mhennessy1.f9.co.uk/dm63/index.htm

These are fairly big and heavy, but take up less bench space than a more typical 'scope like the Hameg above. But, if you put your 'scope on a shelf above the bench, then perhaps the more conventional "form factor" is better.

I like the Hameg 'scopes. We had them at university, and they are in countless TV repair workshops that I've seen. Easy to use, helpful LED indicators to show trigger and overscan. Never looked inside one, but from the pictures in the service manual that I have, it all looks well thought out. The one on eBay now might be worth a punt, but of course, the price 3 hours before close could be radically different to the final price ;)

I'd also recommend older Philips 'scopes - these are often forgotten about, but IME they work very well and are reliable - though you'll need to clean the switches from time to time. Examples include the PM3215 and PM3217 (both 50MHz, will do 2mV/div at full bandwidth, and the latter is dual timebase). Newer Philips models had the microprocessor controls, but I prefer these older ones with real controls. Very intuitive layout too, which makes them great for teaching (we still have some at work for that reason).

I like Philips test gear in general - there is a fair amount of "brand snobbery" when it comes to this, but I've found their stuff to be well designed and reliable. Like Tek and HP, they have some custom-made parts, but unlike Tek and HP, I've never had problems with these parts failing. We have a lot of Tek at work, and we've been disappointed at how quickly some of these custom parts have been obsoleted by Tek. When working, they are good, but I can't see why people get quite so excited by them.. I've got a 2215 with a dead power suppy to repair, and although this was a budget 'scope by Tek's standards, I think the Philips PM3217 is a better 'scope, and wouldn't be surprised if it was cheaper at the time (I powered the 2215 from a bench PSU to check there were no faults elsewhere, and have sort-of lost enthusiasm since then).

I might be willing to part with one of my Telequipment DM63s...

All the best,

Mark

 
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Post by rob t » Sun Sep 11, 2011 11:16 am

we had 2 phillips PM3215 both ended up with psu failure but thay were on 8 hours a day failure was caused  by caps .usual problems with anything that uses a smps of that age
rob t

 
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Post by Mark Hennessy » Sun Sep 11, 2011 1:09 pm

Hi Rob,

That's reminded me; those Philips 'scopes have a DC input socket on the back - 24V IIRC, also they are class 2 (no earth). Both these things make them useful when investigating low-level stuff where earth loops become a problem. The possibility of portable operation is potentially useful, but not something I've ever needed...

Mark

 

Post by tunneldiode » Sun Sep 11, 2011 1:15 pm

With your budget, you're really looking at a second-hand analogue scope.
I won't even bother discussing USB scopes, waste of time.

I wouldn't bother with a single-trace scope (unless it's free)
Go for dual-trace, so you can look at (say) the input and output of (say) an amp at the same time.

I would strongly suggest that you collect it, unless you are buying from a dealer. Why?
No-one knows how to pack a scope (IME, no-one knows how to pack anything)
I think out Glorious Leader has some experience of this as well.......

You mention not being in a rush, so watch ebay and friday-ad.

Be prepared to have to do some maintenance and repairs, using old equipment it goes with the territory.
Manuals can be had for free from bama, ebaman, etc.
At least these old scopes are repairable.
I wonder how easy it would be to repair a Rigol.  

As for makes, I've had good expereinces with: Tek, HP, Gould/Advance, Hitachi, Telequipment.

I've also had some bad experiences with Gould/Advance.

Good luck!

 
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Post by Michael Watterson » Sun Sep 11, 2011 2:59 pm

I think a lot of good advice there.

Some of the Hameg have useful component tester. My Hameg was "free". I'd not pay much for a Hameg, but they work ok and and the manual usually has schematic. No exotic parts on the 1980s models at least. I also often use the 200mV and 2V calibration square waves as "test" signals. Overshoot or rounding of the square wave is a quick and dirty AF frequency response and negative feedback test.

X - Y mode is useful. The cheap USB scopes basically are rubbish at X-Y.

Uses for X-Y
Home made curve tracer or component tester.
Home made spectrum analyser
Tweaking an oscillator to match an oven or GPS reference
Looking at Modulation (carrier as Y , modulation as X)

Dual trace lets you rapidly see the gain and phase of a stage.

 

Post by octal » Sun Sep 11, 2011 3:44 pm

tunneldiode:3007 wrote: Why?
No-one knows how to pack a scope (IME, no-one knows how to pack anything)
!


So true !  Although both my D83's (thought it would be useful to have a source of parts if one fails !) were ebay purchases and arrived ok, despite not being packed as I would have myself but at least they tried. Someone should put up a vid on youtube showing just how much abuse parcels suffer in transit, sellers may take more care after seeing it !
Received a book purchased from the US off an Amazon affiliate seller last week, described as virtually as new by them, it arrived packed in a grey plastic bag with no other packing what so ever !!  Apart from being beaten to death in the post, all the corners, both ends of the spine and surface indentation on both boards, it was obviously not in the best of condition anyway, before it was mailed !!
Needless to say a refund will be requested !!

 
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Post by Michael Watterson » Sun Sep 11, 2011 6:46 pm

The Tek I got shipped was double boxed. Scope in inner box separated from outer. Maybe I bought it from Telfords? I know the postage was more than the scope.


I got a replacement PC Case and PSU in 1991 and it was squashed from 7" to 2" high at one side. The outer box had sprung back into shape but had a tyre mark on it, otherwise looked fine.

 

Post by RobJKMannering » Mon Sep 12, 2011 9:12 am

Wow, well thanks again everyone!
There's a bit to digest in all that lot but pretty much all of you are of the same opinion.
Think I will follow Davids suggestion and keep an eye for an ebay purchase (makes as reccomended by others) untill one at the right price/collection distance becomes available.
Maybe if I don't blow the whole budget, SWMBO will allow the purchase of a signal generator?
Regards,
Rob

 
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Post by Mark Hennessy » Mon Sep 12, 2011 10:01 am

I'd also add Kikusui to that list - we have a lot of cos5020/5040 and similar models in our electronics funds lab, and they have proven to be very reliable and excellent value for money. Nice and simple to use. The only caution I'd add is that I've never seen a schematic for them, and would never want to own test gear without the full documentation. I haven't tried looking for the last couple of years, so you never know - perhaps it's out there now...

I've had disappointing experiences with Gould 'scopes - I gave away the one I owned.

Which reminds me; back at uni, some wag had written "and chips" on the front panel of a Hameg 'scope. Made me chuckle at the time ;)

Mark

 
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Post by sideband » Mon Sep 12, 2011 10:15 am

Mark Hennessy:2986 wrote:I'd also recommend older Philips 'scopes - these are often forgotten about, but IME they work very well and are reliable - though you'll need to clean the switches from time to time.

I like Philips test gear in general - there is a fair amount of "brand snobbery" when it comes to this, but I've found their stuff to be well designed and reliable.


Just to back this up, I have an ancient Philips PM3110 which is actually hybrid using valves for the X and Y input stages (ECC83's with DC heaters) but transistors everywhere else. It only has a bandwidth of 10MHz which is not much use for digital work but it is good for most TV service. I've had this about 25 years and it probably gets used once a year if it's lucky! I also have a complete box of spare panels including a CRT if it's ever needed which is probably very unlikely.

I rescued this when Philips were throwing out all the old test gear and it's been totally reliable when required. I may start using it more regularly now I have another signal generator with a wobbulator output so I can now display the I.F response of radio's easily.

SB

 

Post by octal » Mon Sep 12, 2011 11:39 am

Philips stuff also appeals to me, I have to confess a bias though, at one time I worked for them !  It was the test gear I cut my teeth on during my training in Croydon.  Some of the features of Philips gear are good quality components and professional standard construction.


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