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| 1 ========================================== | |
| 2 Tektronics 2445 scope PSU repair: part 3 | |
| 3 ========================================== | |
| 4 | |
| 5 :Author: David Douard | |
| 6 :Category: Electronics | |
| 7 :Tags: Tektronics, 2445, analog scope, repair, test equipment | |
| 8 :series: Tek2445 PSU repair | |
| 9 :series_index: 3 | |
| 10 | |
| 11 | |
| 12 After having mostly `destroyed the PSU of my Tek 2445 | |
| 13 <{filename}/TeK2445_2.rst>`_, I've decided to be a bit less foolhardy | |
| 14 and I've subscribed to the `yahoo Tek group`_ and discovered that such | |
| 15 a failure has been reported there, also after a full recap of the | |
| 16 preregulator and regulator of the power supply. | |
| 17 | |
| 18 I found that I was not so wrong when I built my completely guessed | |
| 19 LR1060. | |
| 20 | |
| 21 It seems to be built on a 75Ω resistor. Mine was not so far, but I | |
| 22 decided to rebuild one with closer specifications, so I took a new 68Ω | |
| 23 resistor and I wrapped some 34 gauge wire. The result is a bit | |
| 24 cleaner, from: | |
| 25 | |
| 26 .. image:: {filename}/images/tek2445/lr1060.jpg | |
| 27 :alt: first attempt of a hand made LR1060 chocke. | |
| 28 | |
| 29 to: | |
| 30 | |
| 31 .. image:: {filename}/images/tek2445/lr1060_v2.jpg | |
| 32 :alt: a better hand made LR1060 chocke. | |
| 33 | |
| 34 | |
| 35 But I was stuck because of my dead TR1050. Due to the lack of sucess | |
| 36 getting the main oscillator starts, I ended by suspecting TR1050 | |
| 37 suffered when Q1050 exploded... And I was right: one of the two | |
| 38 primary coils was short. | |
| 39 | |
| 40 I've found a reasonnably cheap replacement part on `QService | |
| 41 Electronics`_ shop. It took a bit long to arrive (12 days from Greece | |
| 42 to France, but to be honest, it seems the longest part of the transit | |
| 43 was in France), but it arrived. So yesterday evening I've been able to | |
| 44 replce it. | |
| 45 | |
| 46 | |
| 47 The TR1050 transformer | |
| 48 ====================== | |
| 49 | |
| 50 This transformer is a small input transformer with 4 coils around a | |
| 51 common ferrite core: | |
| 52 | |
| 53 .. image:: {filename}/images/tek2445/tr1050.jpg | |
| 54 :alt: The TR1050 input transformer. | |
| 55 | |
| 56 I was not sure of the specifications of each coil, since mine as (at | |
| 57 least on one coil) defective. From the schematic, it looked natural | |
| 58 that both the primary windings should have the same charactritics. So | |
| 59 when my replacement transformer arrived, I took a few measurements. | |
| 60 | |
| 61 The result is: | |
| 62 | |
| 63 .. list-table:: TR1050 windings | |
| 64 :widths: 15 10 30 | |
| 65 :header-rows: 1 | |
| 66 | |
| 67 * - Winding | |
| 68 - R | |
| 69 - L | |
| 70 * - 1-2 | |
| 71 - 4Ω | |
| 72 - 6mH | |
| 73 * - 4-5 | |
| 74 - 4.5Ω | |
| 75 - 6.5mH | |
| 76 * - 6-7 | |
| 77 - 0Ω85 | |
| 78 - ~ 10µH | |
| 79 * - 6-7 | |
| 80 - 0Ω8 | |
| 81 - ~ 10µH | |
| 82 | |
| 83 I've also quickly checked the voltage ratios using a 42kHz signal: | |
| 84 | |
| 85 .. image:: {filename}/images/tek2445/tr1050_ratio_measurement.jpg | |
| 86 :alt: Measuring the turn-ratios of the TR1050 transformer. | |
| 87 | |
| 88 I use CH2 of my scope to probe the signal produced by the `HP890A4 | |
| 89 <{filename}/hp8904a.rst>`_ linked to pins 1 and 2 (thus a primary | |
| 90 winding), and CH1 on a secondary winding (pins 6-7), which resulted | |
| 91 in: | |
| 92 | |
| 93 .. image:: {filename}/images/tek2445/DS1Z_QuickPrint17.png | |
| 94 :alt: Measuring the turn-ratios of the TR1050 transformer. | |
| 95 | |
| 96 .. image:: {filename}/images/tek2445/DS1Z_QuickPrint18.png | |
| 97 :alt: Measuring the turn-ratios of the TR1050 transformer. | |
| 98 | |
| 99 As one can see, the turn ratio seems to be 10. Both the 2 secondary | |
| 100 windings (5-6 and 7-8) which provide power for the preregulator | |
| 101 control (U1030) and the inverter drive (U1062, U1064 and U1066), are | |
| 102 identical to each other. | |
| 103 | |
| 104 Testing the new T1050 transformer | |
| 105 ================================= | |
| 106 | |
| 107 After having soldered the "new" T1050 transformer, I followed one part | |
| 108 of the Power Supply Troubleshooting Procedure from the Service | |
| 109 Manual. In the diagram, there is a path in which there is: | |
| 110 | |
| 111 - Unsolder and lift the end of W1060 near pin 7 of T1060; | |
| 112 | |
| 113 - Connect the primary test load between the lifted end of W1060 and | |
| 114 the sources of Q1060 and Q1070; | |
| 115 | |
| 116 - Power up and check for 120V accross load (plus a 1Vpp triangle | |
| 117 wave). | |
| 118 | |
| 119 The test load consist in: | |
| 120 | |
| 121 .. image:: {filename}/images/tek2445/psu_test_load.png | |
| 122 :alt: Primary Test Load. | |
| 123 | |
| 124 So I built one and connected it: | |
| 125 | |
| 126 .. image:: {filename}/images/tek2445/psu_with_test_load.jpg | |
| 127 :alt: The PSU with the test load in place of T1060. | |
| 128 | |
| 129 Was a little anxious when I switch the PSU on, but it did not | |
| 130 exploded. And the voltage across the dummy load was just fine! | |
| 131 | |
| 132 So I resoldered W1060, reinserted all the jumpers (between A2 and A3), | |
| 133 plugged in a dummy load on J303 (unregulated 5V), a tried again. The | |
| 134 PSU was starting... it was not completely stable though, but I was | |
| 135 pretty sure this was due to the fact the load was not correct. So I | |
| 136 reinstalled the PSU in the scope, and gave it a try: | |
| 137 | |
| 138 | |
| 139 .. image:: {filename}/images/tek2445/back_from_the_death.jpg | |
| 140 :alt: It's alive! | |
| 141 | |
| 142 At last! | |
| 143 | |
| 144 | |
| 145 Summary | |
| 146 ======= | |
| 147 | |
| 148 I've spent a lot of time trying to understand and fix this PSU. I | |
| 149 think I've lost a lot of time and effort because I've been too hasty, | |
| 150 and I should have taken more time reading the service manuals. By the | |
| 151 way, the service manual for the Tek 2465A is a better source of | |
| 152 documentation, for example the question I had about the voltages with | |
| 153 respect to "REF" on the schematic is answered in this later manual: | |
| 154 there are indeed 2 references, as I suspected. | |
| 155 | |
| 156 The main lesson from this is: never test a switching power supply | |
| 157 unloaded. I'm pretty sure I blew up the power transistors, the | |
| 158 transformer and a few other parts because I was switching the | |
| 159 *unloaded* PSU on and off. It's probably a lesson 101 on switching | |
| 160 power supplies... never too late to learn. | |
| 161 | |
| 162 A the end, I've replaced many parts on my A3 preregulator board. Some | |
| 163 are probably not required: | |
| 164 | |
| 165 - T1050, | |
| 166 - Q1060 and Q1070: I replaced with IRF820, | |
| 167 - Q1050: replaced with a IRFP450;the higher gate capacitance seems not | |
| 168 to be problematic finally, | |
| 169 - Q1022, Q1030, Q1040 and Q1063: replaced with 2N4403 and BC337 (the | |
| 170 transistors I found at my local components store) | |
| 171 - almost every capacitor, | |
| 172 - U1029 and U1040 optocoupler , | |
| 173 - U1066: replaced by a DS0026CN bought on ebay | |
| 174 - CR1040: replaced by an unknown germanium "similar looking" diode, | |
| 175 - R1060 and R1070, | |
| 176 - R1069 | |
| 177 | |
| 178 So I'm done for now with this puppy. It should be completely checked, | |
| 179 but that will be another story. | |
| 180 | |
| 181 | |
| 182 Post Scriptum | |
| 183 ============= | |
| 184 | |
| 185 For the fun: | |
| 186 | |
| 187 .. image:: {filename}/images/tek2445/side_by_side.jpg | |
| 188 :alt: Side-by-side: DS1054Z vs. Tek2445 | |
| 189 | |
| 190 Despite the fact the Rigol DS1054Z has an incredible number of | |
| 191 features for which the Tek cannot compete, I still like this | |
| 192 old puppy a lot. Its front panel is much easier to use: every control is | |
| 193 directly accessible. And yet, I don't think the Rigol can measure both | |
| 194 the rise and fall time (with a decent precision) of a square wave: | |
| 195 | |
| 196 .. image:: {filename}/images/tek2445/rise_and_fall.jpg | |
| 197 :alt: Side-by-side: DS1054Z vs. Tek2445 | |
| 198 | |
| 199 Beautiful double time-base! | |
| 200 | |
| 201 | |
| 202 .. _`yahoo Tek group`: https://groups.yahoo.com/neo/groups/TekScopes/info | |
| 203 .. _`QService Electronics`: http://www.qservice.eu/ |
