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Tektronix 7904 Oscilloscope Repair

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EN0

Member
Hey Everyone,

While testing an IR receiver with my homebrew PSU, I somehow blew my Tektronix 7904 oscilloscope. All the controls aren't working and every time I turn it on, I get a flagrant odor. When I connected the oscilloscope ground to that of my PSU ground, it shorted the PSU. Therefore, I believe that my PSU isn't "true" ground which resulted in the basis of my oscilloscope problem. At the moment, all I see on the screen is a bright dot, which isn’t adjustable. I “sniffed” out where the odor was coming from, and it’s on the left side near the CRT. I have no idea as to what is specifically causing the problem, and I’m hoping that perhaps any of you can help me find it.

Complete Manual:
https://bama.edebris.com/manuals/tek/7904/

Pictures: Oscilloscope :: Right Side, Front Panel Controls picture by ElectroNerdy - Photobucket




I’d appreciate any help!

Thanks,

Austin
 

Reloadron

Well-Known Member
Most Helpful Member
It has been a good number of years but unless the vertical plug ins you were using were differential then the BNC shell is ground, as in chassis ground. This can cause bad problems is anything with power is connected to the shell as the path to ground is through the scope chassis.

I would start troubleshooting with the power supply in the mainframe. If I recall the Tektronix manual for that scope (mainframe) was a good manual and detailed.

Ron
 

BrownOut

Banned
Because you have a dot on your screen, I'd say your power supply is OK. My guess is you've blown your vertical amp, which is in line with the measurement you were taking. On the other hand, your power supply might have different outputs for the vert/horz amp and the CRT curctuirty, so you'll need to verify the voltages. You can do that with a DMM. The great thing about your TEK is you can troubleshoot the circuity, as it's mostly discreetes. The bad thing is you need another scope to see what's going on. I had the same conundrum a couple years ago, so I broke down and purchased a 2nd scope.
 
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Hero999

Banned
After you bought the second 'scope, did you get the original one working?
 

BrownOut

Banned
OH yeah, baby. Though I didn't repair it until I had time on my hands. It turned out to be just some bad solder joints. I even created this thread about it.
 
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