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DIY TEK compensation loop for R7844

I spent a couple of hours last night making a little probe compensation loop for my R7844 out of a couple bnc connectors and a cannibalized comp loop from the front of a TEK 7904 I parted out. It didn't work as well as just sticking the probe into the BNC connector out from the square wave reference oscillator of the R7844. I know TEK offered a bnc to compensation loop adapter for the R7844 (haven't seen any available, though.)

What am I doing wrong? Does it matter if I just stick the probe into the bnc without the loop? I was able to compensate the probe for a pretty square output this way, using the highest output voltage setting on the oscillator, but I'm just curious why my mickey-moused compensation loop was useless.

Also, although my output looked like a pretty square wave, it only displayed the horizontal lines.... No vertical lines were visible on the swing from peak to peak. Is this indicative of a fault, or am I just missing something obvious? I am relative newcomer to scopes in general, and I've been slowly acquiring plugins for my mainframe (which I rescued from the dumpster, along with a TEK 7904, all without plugins.) So far, it looks like the power supply is a little out of spec (the character generator output seems to jitter vertically for a few minutes before finally stabalizing. I will recap as many electrolytics as possible just because I figure a 30-plus year old electrolytic is probably suspect. I have a nice supply of spare parts from the 7904, as far as logic chips, transistors, etc.

So far, I probably spent about $100 in plugins (7A26, 7A18, 7B53A, 7B80) and probes. The CRT seems fine, and for a dual beam 400Mhz scope, I guess I don't have much into it, money wise. It is probably overkill for me, since I'll only be doing audio band stuff, but I have fun playing around with it for the moment. I probaly could have found a working 2000 series for a little more than what I've got in my dumpster find, but I seem to be developing an addiction to dumpster diving and tinkering.

Anyway, any thoughts on my rant are welcome.

Tod

Reply to
Tod Treganowan
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"Tod Treganowan" wrote in news:e2831v$baf$ snipped-for-privacy@usenet01.srv.cis.pitt.edu:

They made a JACK for the probe tip to adapt to the BNC.It grounded the outer ring of the probe tip,and had a pin socket for the signal tip.It's not really necessary,though,just stick the tip into the center of the BNC.

Or do you mean a *current probe* adapter? That would be a heavy wire loop with a 50 ohm resistor to ground.Preferably a metal film chip resistor.

You don't need a loop for a voltage probe,the loop was for current probes.

The cal signal rise time is too fast for that sweep speed to display. It's no fault.

You are better off with the 7000 series scopes you have;the 7904 is one of the best of the series.

--
Jim Yanik
jyanik
at
kua.net
Reply to
Jim Yanik

Thanks, Jim. Its nice to know I haven't completely wasted my time! Let me know if you ever need any parts. I may have some.

Tod

Reply to
Tod Treganowan

That's exactly what a good, clean square wave should look like on a CRO. It's definitely not a fault!

--
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Reply to
Lionel

Lionel wrote in news: snipped-for-privacy@4ax.com:

It depends on the rise time of the square wave;some TEK scopes had really slow RT's,like the T900's,and you definitely could see the rise and fall of the signal,even on those low BW scopes.

Also,if your scope has a long-persistence phosphor it may display the rise and fall better.

--
Jim Yanik
jyanik
at
kua.net
Reply to
Jim Yanik

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