content/TeK2445_3.rst

Thu, 21 Jul 2016 10:41:14 +0200

author
David Douard <david.douard@logilab.fr>
date
Thu, 21 Jul 2016 10:41:14 +0200
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[why] typos

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

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