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Old 15th October 2004, 12:50 AM   #11
Enzo is offline Enzo  United States
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For audio you don't need a lot of bandwidth unless you are working on non-audio stuff like digital sections. If you work with tubes, the input voltage rating can make a difference. Many more modern scopes only go up to 5v/div, while older scopes go up to 10v/div. WIth a x10 probe, I can see the whole range of a 500VDC B+ on the screen with a 10v/div scope, but unless the screen is huge, you will not on a 5v/div scope. Just a thought.
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Old 15th October 2004, 07:59 AM   #12
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Default The Eagle has landed........

Hi all,

Thank you for all your kind advise which is sincerely appreciated.

Last week the scope hunt ended when I came across a Plilips PM6260E. Still old (early eighties I guess) but with a extremely clear and contrast full tube, and with 2 channels at 120 Mhz plenty more than what I personally need. Got the scope inclusive an Exact 7030 function generator for 120 €, so in this light the old Gould was overprized....... I shyed away for remote buys (E-bay) and recommend that you physically examine the scope hands-on.

Just to add a few answers to my initial own questions, as well as some useful information that I picked up along the way (Might come in handy for future users of the 'search' button)

1) For analogue audio work 30 Mhz will work OK - and as shown by this thread many get along just fine with less.

2) Frequency ratings refer to pure sine waves. I reality it's all about rise times. Hence as square waveforms (digital equipment) has steeper slopes; you need a higher rating to follow the signal. Rule of thumb is 10 times the frequency (120 Mhz scope will show 12 Mhz square etc.)

3) Unless the majority of your measurement are well below 1 V get yourself a x10 prode and shy away from those cheaper switchable probes - the switch itself may cause problems or degrade the performance of the probe.

Again
Thanks to all of you
Henrik Juhl

PS: Anyone who knows about the Exact Model 7030 Function Generator (age?). User manual and schematic would be very nice. Its working and can produce a fairly good base form, but some of the 'advanced' features like symmetry and DC offset adjustments seems to ruin the signal completely.........
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Old 15th October 2004, 08:29 AM   #13
tekman is offline tekman  Germany
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2 Geek: Yes, 10 MHz already help a lot. But don't forget:

1. times have changed. Today, I won't do less than 30 MHz. Even in TV. I did and do many TV repairs.
2. The buyer might be interested to do the invest once, so why pay 50 euros for a 10 MHz (just example price; but not uncommon - e.g. ebay)

2 Henrik: Perfect buy! The 3260 (i think there was never a 6260 scope made by Phil) is a classic. Switches might be a little bit failure prone, but the trace is the BEST SHARP I have ever seen in scopes.

All the best for DIY!
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Old 15th October 2004, 08:40 AM   #14
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Right on: PM 3260E it is (That's what you get for trying to type a post with the left hand, while eating breakfast with the right......)
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Old 15th October 2004, 08:46 AM   #15
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Try to get an used but not so old oscilloscope. This oscilloscope may not be so healthy, I'm thinking of all switches etc. If you get it for free, that's OK but 110 EUR seems a bit much.

10MHz is a bit low these days. It was low even 20 years ago! It gives you not much overhead to pick oscillations and micro details.

If you were in Göteborg I could probably dig up a working oscilloscope for free and give it to you.

EDIT: You have got a 120 MHz scope that's better
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Old 17th October 2004, 09:48 AM   #16
Thomas is offline Thomas  Denmark
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I just bourght a scope too!

It's a Philips PM3218 35MHz. I think 35MHz is a 'good' minimum for my hobby which is audio repair and building. I bourght it from Helmut Singer. They have a awfull lot of used instruments. They also have a few other interesting things. If you need the taperecorder from a patriot misile, Helmut Singer is your man!

I have used scopes before, but I never had to buy one before now, so I have just one or two questions about probes. The input impedance is 1Mohm and 20pf on the scope. Should the probes be matched in some way to this, or...? Is there any reason to buy 'faster' probes than 35MHz?
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Old 17th October 2004, 12:28 PM   #17
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Default Fast probes ?

Hi Thomas,

The combined 'Rise time' of the measurering system (scope + probe) is the sum of the rise times of the probe and the scope itself. So it all depends on how close to those 35 Mhz you want to be operating. My Philips PM 3260 has a rise time of 3 nS and is rated at 120 MHz.

Rise time is typically defined as the time of the trace to go from
10% to 90% of the 'true' value.

Capacity wise I don't know what is wise, but I guess you balance that out when adjusting the capacity of the probe with the calibration output of your scope.

PS: Reichelt in Germany sells probes at resonable cost (17 € 150 Mhz/ 2.3 nS or 23.15€ for 250 Mhz/1.4 nS)
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Old 17th October 2004, 01:27 PM   #18
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Default Re: Fast probes ?

Quote:
Originally posted by Henrik Juhl

PS: Reichelt in Germany sells probes at resonable cost (17 € 150 Mhz/ 2.3 nS or 23.15€ for 250 Mhz/1.4 nS)

The current issue of Elektor (yes we do read it in the States !) has a great article on a FET probe which should cost less than $10 to make -- it uses a dual-gate FET, one gate of which is biased -- performance should be good above 1 gHz.

FET probes and Differential probes never come cheaply on EBay -- not an area in which to be cheap.
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Old 17th October 2004, 06:04 PM   #19
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Default Probe bandwith

Thomas and others,

I just stumbled over a rule of thumb for probes, stating that the probe should have double the bandwidth of the scope (or half the rise time)
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Old 18th October 2004, 06:59 AM   #20
tekman is offline tekman  Germany
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2 Thomas: Yes, Singer has a lot of stuff. He is in the business since more than 30 years.
My personal (and friends) experiences:
The prices are higher, and the quality is not better than average ebay sales.
But if you are happy with your scope: No problem, go for it!

2 Henrik:
Well, things are a little more complicated with the rise time. It would take a litte long writing to put i down here.
So, if you are willing to go deeper into this matter, I made good expereinces with these two:

www.tektronix.com
tutorials: abc of probes AND abc of scopes.

The risetime rule of thumb I can't state as valid. Usually, a probes risetime of e.g. 3.5 ns is alright for a 100Mhz scope (which is a 3.5 scope risetime, considering:

T (rise) = 0.35 / freq. (scope bandwith)

The reichelt ans RS probes are a real good "bang for the buck". I especially like the ones from RS: solid contruction, good behaviour up to 200 Mhz.

Okay, that's muc hmore than you really need for audio.
but, refering to Jim Williams artciel in edn in 1995 about taking care care of personal equipment: A good probe last's appr. a year i nthe industry, bt most times more than 20 years in your personal diy lab. So, it does not matter much to me if I bay 20 $ for a no-brand probe or a 60 $ fora well working used Tektronix P6137. That's lees difference than a billet for cinema per year -- but I am shure I am better of with the Tektronix.

So fellows - you are wiling to pay lots of money and tiem for DIYing a real good amp -- why do you scrifice on chap probes or other test equipment?

sorry, just my opinion..
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