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Old 16th Nov 2004, 7:58 am   #1
McIntyre
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Default Purpose of an oscilloscope

Without giving off 'too much' ignorance what is the real purpose of an oscilloscope in the restoration of valve radios, does one really need one? I've got the usual test equipment (CR bridge, Avo meter MK8, VTVM, Signal Generator, valve tester, etc) but have never had a situation arise wherein I could not repair, troubleshoot, align etc, a receiver without these. I was given a Heathkit 5'' Oscilloscope Model 10-18 with another piece of test equipment lately but can't decide if it is actually useful to keep the oscilloscope. The only description I've seen in using one was to test for ESR in electrolytics and in a variety of tests for transistors (gain, shorts etc). As I consider transistors evil (except germanium) and my other test equipment seems to do it all, what am I missing?
Andrew
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Old 16th Nov 2004, 9:35 am   #2
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Default Re: Purpose of an oscilloscope

Hi, probably the only real use would be if you have a tricky fauly with the detector on an FM set. You can also use it with a sig gen as a wobbulator and look at the IF responses. This is useful on some sets.
Main uses are on vintage TV where there are pulse circuits, without a scope it's vey difficult to decide what is going on.
They can also be useful in tracing causes of distortion in tape recorders.

Ed
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Old 16th Nov 2004, 9:42 am   #3
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Default Re: Purpose of an oscilloscope

I think the use of scopes is mainly a matter of past experience and personal preference. But its usefulness for radio work also depends on its specification and mainly bandwidth.The problem with many scopes is that they don't have a high enough V/div for valve work. You can of course use X10 probes, but I always think twice before using them in circuits at 6 - 700v.

My background is microcontrollers, I have a modern 200Mhz Digital scope on my bench and I used to use it quite exclusively. It has a on screen textual display and shows Vpk, Vpk-pk, Vdc, RMS and Frequency Counter and it is these extra functions that make spot measurement quick to do.

I have to admit I am a complete gadget freak and have quite a collection of test equipment, but apart from a good DVM almost none of it ever gets used.
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Old 16th Nov 2004, 11:16 am   #4
McIntyre
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Default Re: Purpose of an oscilloscope

So there is no reall use for valve equipment? Why are they then so 'highly rated' and priced (when new) so much?
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Old 16th Nov 2004, 11:43 am   #5
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Default Re: Purpose of an oscilloscope

I don't have a scope. Valve radio circuits are generally pretty simple and a scope won't tell you much you don't know already. They only became common in workshops with the arrival of TV technology, and have a wide range of uses in modern digital circuits.

They're expensive when new because they're complex pieces of equipment used by professional engineers working for large companies.

HTH, Paul
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Old 16th Nov 2004, 1:09 pm   #6
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Default Re: Purpose of an oscilloscope

Andrew,

A scope is the most generally useful piece of test equipment after a multimeter in most branches of electronics.

E.g. If you are using a frequency counter, you're better off looking at what you are feeding it, as it could be a dirty waveform. I've seen audio amplifiers which go into high frequency oscillation and so on. All this is easy to pick up with a scope.

However, if you are just fixing domestic AM receivers, you can get along very well with a multimeter and a sig gen as many radio repairers did from the 1930s on.

If you want a functioning scope, you can pick up a used solid-state 20MHz surprisingly cheaply, and it will be more stable and useable than most valve scopes. Valve scopes are OK, but may need TLC. I use both valve and solid state scopes, but rarely for domestic radio repairs.

The big Tektronix scopes were pushing valve technology to the limits, with matched pairs of valves in distributed amplifiers, tunnel diode triggers and so on. That's why they were expensive when new. These days they are very hard to maintain, and you can get better performance with less hassles, for surprisingly little.

Of course, some people take the view that playing with old scopes is much more fun than playing with domestic radios..............



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Old 16th Nov 2004, 2:01 pm   #7
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Default Re: Purpose of an oscilloscope

Hi Christian,
Yes, analogue storage; you need to be careful not to burn the tube but it doesn't tell lies as a digital one can, only a pity you can't get pre trigger views.

BTW do you still need the Farnell psu circuit details?

Ed
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Old 16th Nov 2004, 7:23 pm   #8
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Default Re: Purpose of an oscilloscope

Andrew,

I wouldn't be without a scope, either professionally or at home!
Whilst I agree that most faults on vintage radios can be repaired with nothing more than an AVO (it will find bad valves, leaky caps, poor insulation, and out of tolerance resistors, with a little ingenuity), and maybe a signal generator, there are a number of times when a scope eases the process, for example, with a x10 probe, you have a high impedance voltmeter (OK, so it's only as good as a cheap DVM), it will allow you to see (and measure) ripple on HT lines, you can see if an oscillator is running, and measure its frequency, you can use it as a signal tracer (with the advantage of being able to measure amplitude as well), and it is invaluable for distortion problems. On top of that, there are the visual alignment processes that have already been mentioned.

As regards modern scopes and old ones, we have modern Philips 100MHz scopes at work, but they won't trigger well below 25Hz. Digital scopes only show you what they think they are seeing (which is not always what is on the input......).

You would have to go a long way to beat the versatility of the old Tektronix 53*, 54* and 55* series scopes, with the large variety of plug-in amps availabe. Although the 545A (30MHz) has in excess of 100 valves, they are easy to repair and maintain, and for the most part, can be calibrated quite well with limited test equipment. I use my 535A (15MHz) both at home, and on occasion, at work, if I need to look at slow pulse trains, or if I need a differential input.

Finally, if you do at lot of work on amplifiers, you will need a scope, and both a sine and square wave generator, so that you can check the response of the amp.

Jim.
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Old 16th Nov 2004, 8:45 pm   #9
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Default Re: Purpose of an oscilloscope

Hi

I have to say I reach for my 'scope before anything else. I have 2, both I am afraid s*licon. The “workhorse” one, and the one I don't mind damaging ('cuase it's easy to repair) is a Phillips PM 3215. It has a bandwidth of 50 MHz. I find that is more than enough for AM valve radios and everything except the " front end " of 405 Line TVs. The thing was bought on eBay for £100 ! My " good " 'scope is a Tektronix '475 (200 Mhz). This is a complex beast, with delayed time base. It’s a real b*stard to mend because of the torturous layout of the internals. It's got a fair few internal sockets, but these are completely negated by hard wired cable runs <ARGGG> It also uses a lot of " custom " chips that maybe difficult to get – although I have a couple of “skip fresh” chassis from a previous place of work for cannibalisation.

WRT digital 'scopes, I have used these in my work on digital video circuitry, but in all honesty I wouldn't say they have a place in servicing valve circuitry. The higher bandwidth ones are still very expensive and as has been mentioned, they can " lie " as the readout is, by definition, an approximation and (depending on the model) often not that good for high frequency sine waves etc. These beasts are really often just gloried PC’s with some specialist hardware attached.

I started off years ago with a valve Telequipment D43 ‘scope (valve) but it annoyed me because of an inherent lack of stability and inability to hold calibration (WRT to later silicon technology). Although I wouldn’t argue with folk who like operating valve ‘scopes because they are themselves “retro technology” I myself have never had a real interest in such things, preferring to concentrate on the “target” technology




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Old 16th Nov 2004, 11:09 pm   #10
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Default Re: Purpose of an oscilloscope

Hello there,
I find that a digital storage oscilloscope is invaluable at work especially for digital electronics.

Even today my PIC programmer failed and I could not find anything wrong with it.
Using the digital storage scope enabled me to take a " snapshot " from the programmers signal lines, hold it and then analyse it.
I was able to see that the rise time from the programmer's output was around 10 milliseconds. Being as the pulse widths from the programmer can be as low as 4-5 milliseconds you can see that you would have had problems programming.
10 milliseconds rise time being extremely poor for a logic chip and a replacement 74LS05 inverter restored the programmers operation.
Without a storage oscilloscope such a problem would be nearly impossible to analyse except for trial and error of replacing components.

I have a Tek 545B at home and it is very useful for problem solving. You can probe the grid of an output valve and see if there is any DC present or look at the waveform and see if it looks distorted or has an excessive AC signal imposed on it if hum is a problem in a reciever.
Christopher Capener
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Old 17th Nov 2004, 10:05 am   #11
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Default Re: Purpose of an oscilloscope

I think your being a little hard on digital scopes, you can always switch off the digitising mode and use them as a straight analogue scope. The biggest gripe I have is that it takes ten times longer to setup using a push button menu than just turning a pot. I also have a D43 that I still take on site and this has been a really reliable old work horse.

I mainly use an old transistor HP scope a time delay sweep 100Mhz from day to day. I have been told these scope trigger poorly but have always found mine very easy to use.

Do anyone have any experience with TEK scopes. I have been wanting to service and recalibrate my 459 but have been rather daunting by its apparent complexity and that of the service manual. The scope does work well but I would like to check it properly.

Chris
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Old 17th Nov 2004, 10:14 am   #12
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Default Re: Purpose of an oscilloscope

Chris. Join the TekScopes Yahoo group. Sure they can help you.
http://groups.yahoo.com/group/TekScopes/
Paul E
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Old 17th Nov 2004, 10:17 am   #13
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Default Re: Purpose of an oscilloscope

Quote:
you can always switch off the digitising mode and use them as a straight analogue scope.
Most digital scopes are purely digital. There were a few that were effectively analogue scopes with a digitiser bolted on but these were a passing phase.

My Tek 2465 pure analogue scope is a superb design. I also have a Tek TDS210 digital " lunchbox " scope which is horrible but does all those odd jobs where I need storage or want to look at 5ns every other Tuesday.

I am told that the modern " digital phosphor " scopes are much better than older digitals since they can use brightness in the same way as an analogue scope. Much of my work is still with analogue TV waveforms which are appallingly displayed on most digital scopes.
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Old 17th Nov 2004, 11:04 am   #14
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Default Re: Purpose of an oscilloscope

Hi Chris

Whilst TEK scopes are complex I think that the manuals are written in an exemplary manner, neatly broken down to sections on a page. I don't normally like the American way of drawing schematics/circuits but these manuals are excellent. Having said that you say that your 459 is working OK. Well, if it ain't broke then don't fix it.

Whether one needs a scope at all is a matter for never ending debate. I'm sure that people who were in the repair trade look upon them as " bleedin' science " , but personally I wouldn't be without one. Perhaps thats because I spent most of my working life in R&D on digital systems where a scope is about as essential a screwdriver. In radio repair I feel that its nice to be able to " see " signals, power lines etc and besides you've got a high impedance voltmeter and frequency measurer all rolled into one. Of course if you need to service the scope then you'll probably need a second scope to do it with.

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Old 18th Nov 2004, 12:30 am   #15
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Default Re: Purpose of an oscilloscope

Hi all.

I am considering buying a scope at the moment.
So, what would the forum suggest is a reasonable spec for a general purpose scope for use in valve sets etc.
I have been watching a few on Ebay, but there are so many to choose from.
It needs to be reliable, user frendly, etc.
What make?
These are lots of Hammeg - any good?

Richard.
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Old 18th Nov 2004, 10:29 pm   #16
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Default Re: Purpose of an oscilloscope

Quote:
Hi all.

I am considering buying a scope at the moment.
So, what would the forum suggest is a reasonable spec for a general purpose scope for use in valve sets etc.
I have been watching a few on Ebay, but there are so many to choose from.
It needs to be reliable, user frendly, etc.
What make?
These are lots of Hammeg - any good?

Richard

I don't really know about Hameg, they maybe OK, but I think they were always sold as " budget " equipment. Probably better to get something of a more professional nature if you are buying second hand.

Tops for analogue 'scopes are undoubtedly Tektronix, but good silicon ones fetch quite a bit of money, even the old 475 type.

For a good " bomb proof " 'scope, I would recommend something like a Phillips. A 50 MHhz type is more than adequate for most vintage type technology. I imagine it will probably go for £80- £90, as it is calibrated and verified working. Much less than that and it would be a bargain. N
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