Good basic scope?

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Good basic scope?

Postby Blair » Wed Dec 30, 2009 9:43 am

Hey everyone,

In my attempt to test my output powr, I have been informed that to any degree of accuracy test output power, I need a scope.

I have a friend with a scope, but I think I may need one for other projects as well. These are mostly SS projects, but I still think I need a scope all the same.

I was put in touch with a retired Tektronix service guy that listened to my story, and suggested that for audio and what I'm going to be doing I should look for a Tektronix T922. It is a 15MHz dual trace scope. I have found them for anywhere from $40-80. What do you guys think? Is this sufficient? Is the price right?

Thank you for your suggestions!

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Postby Geek » Wed Dec 30, 2009 4:52 pm

I think that'll be fine.

I use an ancient B+K 1470 with a 1460 as a backup. Never needed more for audio.

Even after a while of using it, you'll be able to see what waveforms do what when they have invisible-to-the-scope VHF/UHF parasitics riding on them.

Cheers!
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Postby Blair » Wed Dec 30, 2009 5:55 pm

Thanks!

What MHz do I need to be able to see these parasitics? 60MHz?

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Postby Geek » Wed Dec 30, 2009 6:16 pm

Blair wrote:What MHz do I need to be able to see these parasitics? 60MHz?


If you play with MOSFETS, try more like 500MHz (srsly).

But here's an example what you'll see on a 10MHz scope...

When designing MooseFET, I had a rather thick leading edge on the square wave response. Unknown to me, the wife left her FRS handheld on in her jacket and the squelch was breaking. The closer I brought the talkie to the breadboard, the stronger the carrier (we're talking 460MHz band here).

So, I put a couple little ferrite beads on the gate of the IRF510. No more UHF carrier and no more thick leading edge on the scope Image

Nelson Pass has also had similar issues with MOSFET design.


I've had RF parasitics with tubes too, but they have been lower... 2-30MHz. I use my shortwave radio to verify those Image

Cheers!
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Postby EWBrown » Wed Dec 30, 2009 10:02 pm

That's what grid stopper resistorss are for ;-) the same will work on moosefet gates, too, as well as the small ferrite beads, and I suppose, conversely the same small FBs would also work well as grid stoppers. I've tried them, and at least found no negative effects. Also good to use on the input grid of any amp, if you are near a strong radio transmitter.

I've also found with the 10M45S current regulators, that using a 330 ohm to 1K CC resistror on the gate really keeps them stable and prevents them from working as nasty little VHF oscillators.

back to the original "scope" topic (what a concept) ;-)

My favorite "old cheap" scope is the venerable Tektronix 465B, it is good up to 200MHz, and these can be found quite reasonably cheaply, and they are still considered as good quality lab grade instruments, and not as cheaper "hobby grade" units. I bought mine about 15 years ago from DEC salvage in Nashua NH (no longer around) when Digital Equipment was deep into its "death spiral". They even threw in a bunch of free scope probes.



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Postby Geek » Thu Dec 31, 2009 1:31 am

EWBrown wrote:That's what grid stopper resistorss are for ;-)


Had 'em... still needed the beads.

Cheers!
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Postby Blair » Thu Dec 31, 2009 7:54 am

Thanks Guys!

I looked at the 465B, but it was much more than this. I'll get this T922, plus probes for about $75. Not too bad for a beginner scope. I'll upgrade later if needed. I am working more with high power mosfets these days.

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Postby Ty_Bower » Thu Dec 31, 2009 8:51 am

I'm guessing the T922 is going to be perfect for what you want it to do. I bought a 15MHz Kikusui off eBay for dirt cheap, and I've been happy with it. It seems to be a relatively unknown brand, but it works more than good enough for what I need.

http://www.diytube.com/phpBB2/viewtopic ... 22&start=5
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Blair » Thu Dec 31, 2009 9:43 am

Cool!

Thank Ty! I'm going to order some decent probes, and I should be good to go. I downloaded the manual today and printed it. 220 pages! Of course that includes the service manual also.

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Postby Ty_Bower » Thu Dec 31, 2009 10:17 am

FYI, there's one up on the 'Bay right now. $50 BIN, plus $13 to ship. No probes.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Blair » Thu Dec 31, 2009 10:23 am

That is the one I purchased. He had three of them. I offered less, and we met at $45 plus $13. $57 shipped to my door. Not too shabby, plus it looks very clean. A friend of mine found some 1.2Kv probes for $35 shipped also. All in all, I think this will be a nice addition to my junk pile. The friend is where I got the calibrated Simpson meter from in my "Testing output power" post also. I would buy a function generator, but my phone does all of that. Square, sine, white/pink noise.

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Postby Blair » Thu Jan 07, 2010 9:15 pm

Ok,

I got my scope, and the probes yesterday. I used the online manual to set it up for initial inspection. I can get my nice internally provided square wave and trimmed the probes accordingly. I can get this signal to sit still fairly well, but any other signal fed from my phone (function generator ap) since I do not have a function generator seems to be almost impossible to dial in on unless it is certain frequencies. Is there a trick to this, or is the pot or a cap bad? It is model T922 for any of you techies that may want a schematic:

http://www.logwell.com/tech/oscilloscopes/Tek_T922R.html

Thanks for any help!

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Postby TomMcNally » Thu Jan 07, 2010 11:31 pm

Mess with the time/div knob (whatever the multi-position rotary switch is called on your scope. You need to get in range for the frequency you are trying to look at.
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Postby Ty_Bower » Fri Jan 08, 2010 7:09 am

Mine has three triggering modes. I leave it in 'AUTO'. There are some triggering options (int/ext, wide/hf rej, +/-) which might be worth fiddling. Finally, there is a 'LEVEL' adjustment knob which must be set correctly for it to find the beginning edge of the waveform. If I set the level too high or too low, it can lose lock on the signal and the trace goes all squirrelly.
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Postby Blair » Fri Jan 08, 2010 8:50 am

I have the triggering set on auto, and the +/- set to +. The level seems to have issues. I can only get certain frequencies to lock stil. Others regardless of what position I'm in on the level knob seem to want to run either wild or slowly across the screen. I think maybe a good deox, and a control cleansing my be in order. If not, then a new pot maybe. The screen kind of flickers with the triggering, or at least that is what it appears to be also.

Thanks for your help!

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