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Electronics => Repair => Topic started by: sidharrell on October 18, 2013, 10:35:37 pm

Title: tektronix 475M repair
Post by: sidharrell on October 18, 2013, 10:35:37 pm
Pulled my 475 out of the storage shed cause I got a new table/space that I want to set up as a tinkering center. It was working the last time I turned it on a couple years ago. Now I have no trace, beam finder shows nothing, trigger ready light doesn't come on. I googled and found the troubleshooting guide (general to tek scopes) and the navy's service manual for it. I got the cover off and the navy manual showed where to test the power supply's outputs. The +110V, +50V, and +5V are good. The +15V, -15V, and -8V are bad.  I started trying to track down the problem by starting with the +15V supply. The navy manual shows it as one regulator with Q1444, Q1446, and Q1448 on it, and a feedback amp U1454A. I found it on the board, but the Q1444, Q1446, and Q1448 are discrete components (three legged, small silver cylinders) and the feedback amp is a 8 legged chip (op amp, I'm guessing). The unregulated 15V feed was going into pin 1 of the op amp, and pin 2 had the same 116mV that was on the +15V test pin across the board, so I'm guessing that's the output.
I stupidly pulled out Q1444 while it was on, and I had a cap pop and the fuse blow, so I have to run up to Radio Shack. I figured I might solicit some opinions on what the root cause of the problem might be, before I spend any more time monkeying around.
I don't have a multimeter that goes over 600V, so I can't check the 2450V high  voltage test point.
Title: Re: tektronix 475M repair
Post by: sidharrell on October 19, 2013, 12:33:22 am
Back from the Shack. They didn't have the right capacitor to replace the one I blew, but I did get the fuse. I think the root cause is past the blown cap, because now all the power supplies test good, +110V, +50V, +15V, +5V, -8V, and -15V.
Title: Re: tektronix 475M repair
Post by: sidharrell on October 19, 2013, 02:44:10 am
put the trigger board back in. I now have scale illumination, triggering, and the beam find makes the beam appear, so I think the CRT and high voltage circuits are ok. Still can't get a trace, but I'm guessing it has to do with the cap that is blown (I removed it from the board), the little coil thingy next to it that looks like it is toast, or something in the circuit past it.
Title: Re: tektronix 475M repair
Post by: sidharrell on October 19, 2013, 07:48:22 pm
I removed the toasted component next to the cap that blew. It's shaped like a resistor, but it has a thin copper wire winding around it, and has an orange dot on each end. (the one's near it in the board have a white dot in the middle, so I suspect the white dot on mine was there but burned off). I'm suspecting it's a small inductor. When I examined it closely, the copper wire connected from one lead to the coil in the middle was open. I put a dot of solder on it to close it, put it back in the board along with a ceramic disk capacitor (marked with a 10, so 10pF?) to replace the blown cap (was a orange drop-shaped with 10-25 on it (10pF, 25V max?).
Shockingly, the scope came on with full functions. Hooked up to the calibration signal on the front panel, and got the trace. I think my probe may be bad, cause the calibration signal is very fuzzy at the peaks. (square wave, 300mV, 1kHz) Does anyone know if that is normal?