| General > General Technical Chat |
| Hey, wanna come over and checkout my O-scope, baby? |
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| Andrew:
I don't think a calibration will fix your problem. |
| TrentO:
I think you're right about that-- I went back and documented all of my tests, and the trend is as follows-- After a 20-30 minute warm-up period, with the scope set at all recommended settings for calibration, probes attached to the Calibrator, Ch.2 is way off scale on .5v, .2v and .1v v/div ONLY. 5v, 1v, 50mv, 20mv, 10mv, 5mv and 2mv are accurate. I noticed that this seems to correspond to a series of relays 'clicks' entering/leaving that range. I would assume that these relays are contained in the attenuator hybrid units... I also assume that a resistor is being switched-in when the ch.2 div knob is rotated CW from 1v to .5v... Page 225 shows the "Auxiliary Control Register" section controlling these relays-- I'm in the process of trying to figure out what outputs are generated through IC U120 based on vDIV and/or coupling settings. Prior to the 20-30 minute warm up period, Ch.2 will show what I can only describe as a 'sharks tooth' wave pattern in the .5v, .2v and .1v v/div range, also off-scale high. After the 20-30 minutes, it settles into a nice square-wave (but still unreasonably-high amplitude.) Ch.1's amplitude remains low, over the complete scale-- the trigger won't pick it up on the calibrator input until 20mv v/div. All test point voltage levels are within 1% of spec. So still, based on my observations, I believe the attenuator for Ch.2 is bad, worse when cold; Ch.1 suffers from generalized attenuation issues. My Rigol 1052E shipped today from China-- assuming that Customs doesn't hold it due to my H-bomb comments earlier, my first task would be to validate the test waveform exiting the attenuator hybrid. What a DORK, huh? Buying a new, cheap-o oscilloscope to fix another pre-Madonna scope. <--- get it ?!! ;D -Trent P.S.: Andrew, I owe you a beer (or two.) |
| TrentO:
Thank you to all that responded, especially Andrew! I got my Rigol 1052E today, and after taking it apart (and putting it back together again) I used it to once again troubleshoot my aged Tektronix 2445. As suspected, the pre-amp unit on Ch.1 is bad, and the analog-switching unit on Ch.2 is bad. Either way, I did confirm that both Ch.1 and 2 have problems. Although this Tektronix was essentially given to me, I feel compelled to give it back to the Italian-guy source. Otherwise, I would have put a bow on it and shipped it off to Andrew for spare parts! What I learned from all this-- 1. They don't make 'em like they used to. 2. There's a reason why they don't make 'em like they used to.... <--- that's actually a joke. My pre-Madonna Tektronix has craftsmanship beyond belief-- as finely engineered and tuned as an Omega watch; and most of it was probably built by hand! My new Rigol is much more advanced, but assembled worse than my Black-and-Decker toaster. Analogous to that cheap $5 no-name digital watch that keeps better time than any mechanical Omega, the Tektronix 2445 does have it's place, but the Rigol is simply better suited to my needs. But then again, if I set my watch down for more than 30 hours, it simply stops. The $5 Timex "will take a lickin' and keep on tickin" for years and years. I did learn a lot doing this and feel a whole lot wiser for going through the process, inherent risk of electrocution aside. Thanks, everybody! -Trent |
| TrentO:
Not that anyone gives a rat's-ass, but I fixed my Tektronix 2445 today!!! Contrary to Andrew's and the Tektronix Service Manual's recommendations, I disassembled the "Hybrid" assemblies and cleaned them-- Ch.1 actually had a moth cocoon-case inside, limiting the travel of one of the relays. Ch.1 now works fine! Ch.2's Hybird was fine, but I noticed someone along the way dropped a solder ball onto the mainboard (next to it)-- it looked like someone replaced the Ch.2 hybrid before, but screwed up in the process-- it was the only crappy re-soldering job on the entire unit. So 1/2-hour with solder-wick and Ch.2 now works fine! Ah yes!!! The sweetness of having fixed something yourself-- --- End quote --- Sorry-- my camera focused in on the probe wires and not on the screen-- but for the first time in who-knows-how-long, the traces from all four channels are in full sync! As Dave would say-- "Whoo-hoo!" I originally didn't plan on taking it this far, but I had nothing to lose-- it was either trying, or taking to the range to shoot-up. Thanks for everyone's help. -Trent P.S.: Notice my shiney-new Rigol in the background-- what I used to fix my Tektronix! |
| EEVblog:
Sweet!, nice work! Dave. |
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