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Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

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    Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

    I was given a Trio CS-1562 for next to nothing, along with an old TO-3 valve-based CRO and some other stuff and figured - why not, right?


    I decided to start with the Trio CRO. I was told it was working fine then the display quit with the waveform shrinking down to nothing and never coming back. I was also told the HV was tested as OK, which didn't seem to make a huge amount of sense.

    Well, I plugged it in and flipped the switch. The power LED came on but for sure the display was dead. I was getting a squarewave from the CAL output though so I figured that was a good sign.

    With the case open, I verified the heater comes on. Then I noticed some cracked joints on the J310\P109 board interconnect, one of which was on the 180v rail. I reflowed the lot, but it made no difference. I also noticed the CAL signal comes straight from the main transformer and simply squared up, so that working doesn't prove much. Time to dig deeper...

    I unscrewed the shield from the HV circuitry and had a look. Nothing seemed untoward at first, but diodes D109\D110 do look a bit funny on top. They tested OK in-circuit though, maybe it's just the red paint(?) has gone over time with heat or something? Any ideas?

    The HV is made from a transistor based oscillator and transformer and nothing was visibly blown or overheated. Without an actual HV probe\meter, I tried waving my scope probe over transformer T101 and got nothing, and probing the primary winding showed only a stable 15v DC. At one point I also waved around a neon globe on a stick, to no avail. So it looks like the HV oscillator is well and truly dead. So much for "The shop I took it to said the HV is working"

    A quick look at the underside showed a lifted and (not too well) repaired trace on Q142. A good clue! Though how someone trashes a big pad on a single layer board is beyond me.
    I pulled Q142 and tested it. The B-E junction seems OK but the B-C junction appears to be open. That would explain the no oscillation and no HV. Perhaps the tiny amount of dried up heatsink paste explains the dead transistor. Maybe there is a bad component elsewhere which killed Q142...


    Anyway, now for the fun part.

    As expected, the 2SD401 isn't manufactured anymore (except by some Wun Hung Lo outfit) and naturally there is nothing on a 2SD401M. I did find in a Mospec 2SD401A datasheet that 'M' corresponds to the lowest (40-80) hFE, and that's it. But a 2SD401A datasheet from NEC has nothing about hFE grades at all.

    The only new source was NTE Components who claimed to have a 2SD401M replacement - the NTE375. On paper it looked OK. It has the same voltage (or slightly higher) ratings, a little bit faster, and can handle more current. I figured that sounded great, and they're available at RS-Components.

    However, in a post by rhomanski I recently read, he explained NTE components just relabel parts they get from elsewhere. Google, and NTE's own website seems to confirm this. OK, well that's not awesome, but if they work and are in spec, well... that's not too bad right?
    Then I looked at the NTE375 datasheet again, and noticed the spec for cutoff currents are much higher than any of the old datasheets I could find. Particularly the IEBO is maximum 5mA for NTE375, and in the NEC 2SD401A datasheet it's a maximum 50 microamps - that's up to 100 times worse!
    Even the Chinese 2SD401 only has 0.2mA maximum for both IEBO and ICBO.

    Of course, this may not matter. I have no idea how much or if this difference in maximum cutoff current will make in the circuit here. But it does make the NTE375 seem less appealing.

    I see some various N.O.S. 2SD401s and 2SD401As on eBay, which are more tempting, but none are 2SD401M. I can't find out what the M stands for.
    The 2SD401A at least seems to just be one with higher power handling.
    There are some Fairchild 2SD401-Y but no datasheet to explain what the 'Y' means.


    Any ideas?
    I'm happy with NTE as long as the higher cutoff current isn't going to be a problem, but I have no experience or knowledge to tell me if it will be OK or not.
    Attached Files
    "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
    -David VanHorn

    #2
    Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

    beat it with a big stick.
    this is what is common in old flyback drivers.
    Attached Files

    Comment


      #3
      Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

      btw, maybe M means isolated tab or internal reverse diode.

      Comment


        #4
        Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

        Hmm. Well it looks like the order went through already and I don't think I can cancel it so I guess we'll just see how the NTE375 goes...
        "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
        -David VanHorn

        Comment


          #5
          Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

          Well the NTE375s arrived and I must say I like the fact they come individually with insulating washers and pads. Aside from that they do look rather cheap, probably some Asian clones (stamped\dimpled leads - if you know what I mean) but hey, if they work.

          I checked out the primary side of the HV circuit in case of a blown snubber capacitor or something, but all was OK. Except for choke L107 which was cooked. It should be 470uH but measured about 0.57uH. I guess Q142 shorted, burnt L107, then went open circuit. That also explains the lifted pad on Q142. I got a new choke from Jaycar.

          I decided to remove and test D109 and D110 as well. They measured OK with diode check, but I don't like the look of them. In fact, on close inspection you can see where the glass has actually bulged out on top, in the area where the paint has gone. Running my fingernail over it I can feel a slight bump, too. Another W06C diode elsewhere on the board in a much less demanding position shows no such thing.
          It would be interesting to see what they look like on a curve tracer. I would be a bit surprised if there was nothing wrong with them. I replaced them with 1N4004s which have a higher voltage rating anyway.

          Then I installed the new NTE375 for Q142, tidied up its pads as best I could (and removed that bodge wire), and of course put on some fresh thermal paste too.

          With all that done, it was time for the smoke test! I fired it up and there were no immediate problems. The trace came up on the screen crisp and clean. The focus control has plenty of adjustment in either direction. I waved a probe over the HV oscillator and picked up a nice clean sine wave. After running for about half an hour I switched off and finger-tested Q142. It was barely warm Everything looks nice.


          Well, apart from the trace rotation! The schematic says this is handled by Q1, Q2 and VR4 and that there is a front panel adjustment for it. Well, maybe on the CS1562A there is. But I have the CS1562, there is no control, and I can't find a Q1, Q2 or VR4 anywhere. So it looks like I just have to unclamp the tube and rotate the whole thing. Right?
          Attached Files
          "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
          -David VanHorn

          Comment


            #6
            Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

            Looks like it unless they put those adjustments on the inside for this model. There might also be 4 metal tabs around the tube you can rotate to adjust the magnetic alignment of the beam. It been over 30 years since I played around with a CRT so this is an old memory. If there is these tabs just remember where the original position is in case you need to rotate it back.

            Comment


              #7
              Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

              it's too far out to be normal - what's on the rear of the unit?

              Comment


                #8
                Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                Originally posted by keeney123 View Post
                Looks like it unless they put those adjustments on the inside for this model. There might also be 4 metal tabs around the tube you can rotate to adjust the magnetic alignment of the beam.
                Aren't those the convergence magnets for a colour TV tube? I wouldn't expect any on a CRO tube? I don't see anything like that on this one anyway. Unless they're buried under that shield.

                Originally posted by stj View Post
                it's too far out to be normal - what's on the rear of the unit?
                Mains socket, fuse holder, voltage selector plug, and a Z-Axis input. I wonder if you are right about it being another fault. I tried rotating the trace on my other CRO and found that the rising and falling edges of the square wave rotate as well, but on the Trio, the edges are actually vertical and parallel with the graticules, which suggests some weird non-linearity in the vertical drive?
                Attached Files
                "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                -David VanHorn

                Comment


                  #9
                  Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                  Aren't those the convergence magnets for a colour TV tube? I wouldn't expect any on a CRO tube? I don't see anything like that on this one anyway. Unless they're buried under that shield.

                  You have to understand this is old knowledge. I remember in school we had a black/ white crt and those tabs/magnets aligned the the N/S/E/W of the beam. So with no signal there would be a dot in the center of the tube. The magnetic effect would pull that dot one way or another. My experience is that it would align the beam. If one ramped sawtooth pulse to the horizontal magnets the beam would pull a line from one side to another. If one ramped a sawtooth wave to the vertical magnets a line would pull from top to bottom.

                  Comment


                    #10
                    Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                    i expect a coil around the tube, driven by a regulated current.

                    Comment


                      #11
                      Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                      Too me it looks like the vertical is fine as the square waves are where they are suppose to be. It seems the horizontal ramp wave timing is off compared to the vertical wave ramp pulse. Perhaps what needs to be done is to rotate the screen and then use the vertical adjust pot to bring the leading edge down to a square wave. On a Tektronics they have a screw in the front panel to adjust the square wave for this type of calibration. In your post eight you can see that the pulse is even over the center line perfectly symmetrical just the edge of the pulse needs to be adjusted. As I remember some probes have a screw adjust for this.
                      Last edited by keeney123; 03-22-2016, 05:09 PM.

                      Comment


                        #12
                        Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                        Originally posted by stj View Post
                        i expect a coil around the tube, driven by a regulated current.
                        Exactly what is shown in the schematic - but in this earlier (non 'A') model, this circuitry does not appear to exist.

                        In any case, I found how to rotate the tube - you loosen the two left and right screws on the back round metal cover which goes over the tube base. Then just rotate it with your hand. The top and bottom screws hold the neck clamp and the whole tube turns easily.

                        I can get the trace perfectly horizontal by turning the tube very slightly.
                        But! - then the square wave edges lean left slightly and the vertical position control sends the trace up and down the screen on a slightly left leaning axis, just as it does on my other scope when I adjust the rotation control off-centre.

                        So it does not seem that rotation is the problem, rather there is some other issue.

                        Originally posted by keeney123 View Post
                        Too me it looks like the vertical is fine as the square waves are where they are suppose to be. It seems the horizontal ramp wave timing is off compared to the vertical wave ramp pulse. Perhaps what needs to be done is to rotate the screen and then use the vertical adjust pot to bring the leading edge down to a square wave. On a Tektronics they have a screw in the front panel to adjust the square wave for this type of calibration. In your post eight you can see that the pulse is even over the center line perfectly symmetrical just the edge of the pulse needs to be adjusted. As I remember some probes have a screw adjust for this.
                        There are various trimpots in the schematic, many are not marked as to their purpose. I don't know what I would be looking for. There are no front panel adjustment controls aside from the usual and the focus\brightness controls.
                        "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                        -David VanHorn

                        Comment


                          #13
                          Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                          So I would look for a pot in the circuitry that would be going towards the BNC connector, then you will have to turn it and see if it changes. Before you do this mark the pot position with a fine tip marker. When you return the pot to it's previous position, turn it beyond the mark and then come back to the mark. Scratch that. VR301 adjust the calibration for the square wave.
                          Last edited by keeney123; 03-22-2016, 08:29 PM.

                          Comment


                            #14
                            Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                            Oh, no wonder I was confused. I don't think it's the internal calibration signal that's the problem.

                            I get the issue with the cal signal from my other 'scope, and also with a square wave from my Arduino.
                            "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                            -David VanHorn

                            Comment


                              #15
                              Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                              So if you can rotate the scope so that the trace is even on both sides of the O Volts but has an over shoot on the leading edge than yes it is the calibration pot that will straighten it out. What you are doing would be to tune in that probe to that scope. This has to be done with every new probe. As I have said some probes have this calibration with-in the base of the probe. Perhaps your O-scope came with such probes when new and that is why they do not have a calibration pot on the front face of the scope. Instead of using dual trace select channel 1 and see what you get and switch to channel 2 and see what you get. Also, make sure your sync sources are in the same way on both scopes + or -.
                              Last edited by keeney123; 03-23-2016, 12:22 AM.

                              Comment


                                #16
                                Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                                Ahh I see the confusion. The thing is, it's not overshoot. Well there may be some there but I can adjust the probe for that, as you say.

                                The problem I have is that the square wave edges are not perpendicular to the trace. I have uploaded attachment drawings to show (in an exaggerated style) the problem.
                                Attached Files
                                "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                                -David VanHorn

                                Comment


                                  #17
                                  Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                                  have you got another scope?

                                  Comment


                                    #18
                                    Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                                    Originally posted by stj View Post
                                    have you got another scope?
                                    Yes, the Metrix OX860.
                                    "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                                    -David VanHorn

                                    Comment


                                      #19
                                      Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                                      then get the trio diplaying a flat-line and use the metrix to trace the vertical drive back from the scancoils through the circuits till you find where it's getting cut/boosted

                                      Comment


                                        #20
                                        Re: Trio CS-1562 oscilloscope repair - 2SD401M replacement transistor?

                                        According the service manual page 5, the INTENSITY knob #14 is also used for the TRACE ROTATION adj #24 page 6. So can the Intensity knob can be pulled out to let you do the TRACE ROTATION adj?
                                        Last edited by budm; 03-28-2016, 11:30 AM.
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