EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Gunsanddaisys on June 11, 2018, 11:59:59 pm
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This is my first post in the EEVblog forum, so here goes...
A few weeks ago, I got an old DSO from a neighbor of mine who said I could have it if I could fix it. He was always going to tear into it and repair it, but that was 10 years ago. According to him, the problem was probably in the vertical and horizontal amps since the calibration signal only took up a small portion of the crt. I turned it on after bringing it home and it functioned just like he said. All of the controls did what they should, the waveform was just very small with some strange artifacts on the leading edge.
I began to read up on the scope with the service manual and began to pull the cover off. I checked each of the supply voltage test points and thought I may have found my problem! The rails are as listed: +6, -6, +12, -12, +100, +170, -60, and -120. All of the probed voltages looked good, except for the 100, 170, and -120. The 170 was about 110V, the -120 was at about -136v, and the most curious was the 100v testing at -1.15v. Upon checking the schematics, each of these voltages appear to be generated from individual taps on a transformer on the power supply board (A7). I could not find any faulty caps with a visual inspection or checking capacitance while in circuit. However, when I probed tap leads 15 and 16, the 100v winding, I found it to overload the ohmeter. The adjacent 170v winding reads 22.4 ohms while still in circuit.
I haven't got much experience diagnosing this complex of equipment so I feel slightly in over my head. Am I looking at a bad transformer, hence a bad board since I cannot find this tranny by itself at all? I would really like to get this scope functioning and make it my personal first oscilliscope!
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Hi, Guns and?...
I also have a Sony/Tek 314. It sort of works, but not really, so I don't use it. I also have a working Sony/Tek 335 and a 336 much more advanced sets in the same size. My first guess would be to me look for bad diodes, or faulty connections, there is a lot of stuff in a small space inside these sets. Transistors can lose gain (hfe) over decades and then will not work, but don't test as shorted either, you have to know what the spec of a given part is, to know if it is good or bad. Sometimes more test equipment is needed than just an ohm meter or DMM, or a few DMM's to test a circuit.
Those rail voltages I guess they are from the service manual, are they are AC or DC voltages? Big difference! Ohm readings won't tell you much, but "diode test" of diodes may uncover a shorted part or two. Diodes read real low on diode test like .188, .250 or .567 depending on whether they are Schottky, Germanium, or Silicon types, in one direction and usually infinite when the leads are reversed. In circuit may complicate things, but nearly shorted in both directions either means a bad part or you are reading the transformer winding. Be extra careful around caps, they can hold a charge for a long time if "bleeder resistors" are open or way out of spec higher. I never tried to work on my 314, too many other complex (make your head explode level) projects so I may not be the best source for troubleshooting advice.
-Steven
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http://w140.com/tek_314_service.pdf (http://w140.com/tek_314_service.pdf)
I should be sleeping, page 39 is PSU
Some day soon I need to have another go at a tek 422, I have / want to learn how to calculate what it's linear PSU should be doing. And my main scope had a low voltage but I don't know why, but it works.