EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Curtis on July 10, 2016, 03:14:17 am
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A friend of mine gave me a old Tektronix 422 oscilloscope, the problem is channel "2" does not work. I can see the trace off the screen,and i can bring it up onto the screen by using the variable calibrate knob on channel two, but the calibration is way off.. Someone had mentioned that the "Nuvistor" tube that controls the vertical amp was most likely gone bad, which was common for this model. When I opened up the scope, I could not find any tubes anywhere inside. I later found out that the later model 422 scopes discontinued to use these "nuvistor" tubes and switched to use a "FET" instead. I can not find any schematics for this scope on the net that uses a FET instead of this tube.
If anyone knows where i find this information or what FET number they used for the vertical amp I would really appreciate it.
Curtis
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There's a good few manual repositories in this thread and grab the pdf linked in the first post while you're at it.
https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/ (https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/)
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Try this one
http://elektrotanya.com/tektronix_422_r422.pdf/download.html (http://elektrotanya.com/tektronix_422_r422.pdf/download.html)
134.4 MB
Wait for "processing" to change to "get manual".
I haven't downloaded it just to check but you'll notice it's called 422 and R422, R422 might be the revised model you seek.
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You want the manual for serial numbers 20,000 and up, available here:
http://w140.com/tekwiki/wiki/422 (http://w140.com/tekwiki/wiki/422)
Link to manual:
http://w140.com/tek_422_sn_20000_up.pdf (http://w140.com/tek_422_sn_20000_up.pdf)
It's a very good scan.
Edit & FWIW: the input FETs are Tek part #151-1011-00 and they seem to be available. Probable cross is 2N3822 (from http://www.sphere.bc.ca/test/tek-parts/tekparts2.html (http://www.sphere.bc.ca/test/tek-parts/tekparts2.html)).
I have the earlier version with one nuvistor and one MPF102. Some just plug an MPF102 in place of the nuvistor, which is supposed to sort of work. I found a different circuit (I have no idea where it came from now) which adds an extra resistor in series with the source of the MPF102. If the other channel goes, there are plenty of nuvistors available on ebay, not to mention that MPF102s are getting harder to find - can't get them in the local Radio Shack any more (and yes, my local Radio Shack still lives).
I just had to fix the horizontal sweep on mine - "the tunnel diode, of course" (D455, a 1N3713 - you won't find these any more).
Orin.
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Thank you for the link, and the information. I think that is exactly what I was looking for. I also heard the a "MP102" could be used directly to replace the "nuvistor" tube, so I purchased some, but when I took the scope apart I was surprised there were no tubes!! O-well. Q114a,b looks like the one I need. I could pull out Q14, and plug it in to channel two to see if that fixes the problem before buying a new one. :)
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Thank you for the link, and the information. I think that is exactly what I was looking for. I also heard the a "MP102" could be used directly to replace the "nuvistor" tube, so I purchased some, but when I took the scope apart I was surprised there were no tubes!! O-well. Q114a,b looks like the one I need. I could pull out Q14, and plug it in to channel two to see if that fixes the problem before buying a new one. :)
I didn't notice the FET was socketed or I'd have suggested swapping Q14/Q114. With these old sockets, reseating all the socket transistors in the bad channel may be all you need.
Vernonia Northern (http://www.vernonianorthern.com (http://www.vernonianorthern.com)) seem to have a great deal on the FETs. I've never dealt with them though.