content/TeK2445_2.rst

Tue, 28 Aug 2018 15:05:01 +0200

author
David Douard <david.douard@logilab.fr>
date
Tue, 28 Aug 2018 15:05:01 +0200
changeset 104
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parent 45
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child 115
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permissions
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improve hgignore

45
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1 ==========================================
3eed12f6592b [tek] part 2 of the PSU repair
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2 Tektronics 2445 scope PSU repair: part 2
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3 ==========================================
3eed12f6592b [tek] part 2 of the PSU repair
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4
3eed12f6592b [tek] part 2 of the PSU repair
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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: 2
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10
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11 After a `quick repair of the PSU of my Tek 2445
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12 <{filename}/TeK2445.rst>`_, I've started a more systematic replacement
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13 of the capacitors.
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14
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15 .. image:: {filename}/images/tek2445/psu_caps_replaced.jpg
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16 :alt: Tek2445 PSU Caps being replaced.
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17
3eed12f6592b [tek] part 2 of the PSU repair
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18 But I was also a bit worried because of a dirty noise coming from the
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19 preregulator area.
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20
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21 After having replaced most of the electrolytic capacitors, the dirty
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22 noise became louder (I think. It might be unrelated, not sure
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23 yet). The PSU became very untrusty: a small plume of smoke began to
3eed12f6592b [tek] part 2 of the PSU repair
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24 escape from somewhere (could not identify precisely where exactly). I
3eed12f6592b [tek] part 2 of the PSU repair
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25 started probing around, but at this moment, I had no idea which
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26 component was getting too hot: I always shut the power off after a few
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27 seconds of power, with the T1050 transformer singing as soon as the
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28 preregulator control oscillator starts up (U1030).
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29
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30 During this probing and checking period, I discovered that the CR1040
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31 diode from the current limitation circuit was dead
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32 (short)... Unfortunately, it's a germanium diode (used for it's low
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33 voltage drop, about 0.2v). Fortunately I found one of them which
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34 should fit in my very old spare components casket (the one I had when
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35 I was a kid).
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36
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37 Then, in order to try to see things, I started to power the PSU up for
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38 a few seconds (sometimes maybe tens of seconds) trying to probe
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39 (without my scope, obviously, so only with my Fluke 867B which allows
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40 me to see *some kind* of signals.)
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41
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42 Which allowed me to finally find the culprit for the smoke: R1071 (in
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43 red on the schematic below). Its value is still fine, but it's getting
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44 very hot as soon as the main oscillator starts up. A bit strange,
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45 since the capacitor C1071 seems fine (I do not have a ESR meter, but
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46 hey, there is no way higher ESR could lead to overcurrent flowing
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47 through this resistor).
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48
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49 .. image:: {filename}/images/tek2445/psu_prereg.png
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50 :alt: Schematic of the preregulator of teh Tek2445
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51
3eed12f6592b [tek] part 2 of the PSU repair
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52 But powering up and down the PSU again and again, with its freaking
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53 and noisy switching problem, finally killed the MOSFET (IRF820) power
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54 transistors (Q1050, Q1060 and Q1070) producing flames. The plastic
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55 protection on Q1060 directed all the heat directly on the poor LR1060
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56 which exploded as well... Which is a problem since I do not know the
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57 exact specifications of the device; the service manual do not give the
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58 value of the resistor in parallel with the 2.5µH inductance.
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59
3eed12f6592b [tek] part 2 of the PSU repair
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60 .. image:: {filename}/images/tek2445/dead_transistors.jpg
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61 :alt: Q1050, 1060 and Q1070 are dead.
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62
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63 For now, I've picked up a 3W 100Ω resistor and I've made a self by
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64 hand. As I don't have a RLC meter, I don't known its exact value so
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65 I've took a few measures of its impedance at several frequencies, and
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66 I estimate the inductance to be around 4 µH. I have no idea whether
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67 this higher value might be a problem... But I've no idea either if the
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68 100Ω resistor I've used as core for the coil is fine or not.
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69
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70 And I have other problems to fix. First, the 3 MOSFETs have to be
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71 replaced... As I don't have anything near IRP820 in my hood, I've
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72 picked up some IRFP450 I extracted from computer PSUs. These are much
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73 bigger than the IRF820, but having beafier models (with higher max
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74 current and voltage) should not be a problem there. The only thing
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75 that worries me a bit is the much lower Rds value (0.4Ω versus 3Ω for
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76 the IRF820). Means higher peak currents. Q1030 and R1052 also
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77 died. The transistor is a 2N3905 (which I don't have around either) so
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78 I've replaced it with a bigger TIP32C.
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79
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80 .. image:: {filename}/images/tek2445/beafier_transistors.jpg
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81 :alt: Beafier replacement rtansistors, and a handmade coil.
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82
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83 From there, I began to check most of the components in this
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84 preregulator part of the PSU before even attempting to apply power.
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85
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86 I've checked the behavior of the main control oscillator (made
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87 of U1030), by powering it with my new old Lambda linear power
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88 supply. Applying power at the leads of C1025, I could also check the
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89 behavior of the Q1020/Q1021 stage. Their purpose is to regulate the
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90 supply voltage for the U1030 main oscillator.
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91
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92 .. image:: {filename}/images/tek2445/psu_prereg_test.jpg
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93 :alt: Testing the main control oscillator.
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94
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95 At power-up time, when the oscillator has not yet started, R1020 fill
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96 C1025 from the input high voltage DC. When the voltage on C1025
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97 reaches about 21V, Q1021 is activated, powering U1030 (from C1025),
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98 and doing so, putting R1024 in parallel with R1020, making the voltage
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99 required to keep Q1021 saturated much lower (around 8V theorically).
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100
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101 With the energy stored in C1025, U1030 can start oscillating, starting
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102 effecively the preregulated power supply, which make switching current
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103 flow through T1050, from which the coil at leads 6 and 7 should now
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104 provide something like the 15V used to power the main preregulator
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105 oscillator. So far so good. On my PSU, this main oscillator seems to
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106 work fine even if I've found that the voltages at which thing happen
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107 are significantly smaller than the values above (which come from the
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108 service manual). It's more like:
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109
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110 - 18V for Q1021 to be saturated (instead of 21V),
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111 - 7V for Q1021 to be blocked again (instead of 8V).
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112
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113 Not sure whether I should care about this. Probably not.
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114
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115 The other part of the preregulator is the inverter drive. Its purpose
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116 is to switch the current flowing in the T1060 transformer (the output
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117 of the preregulator part) from one direction to the other in its
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118 primary circuit at each pulse of the switching in T1050.
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119
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120 With Q1060 and Q1070 being dead, I was expecting a few more components
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121 around to have suffer. And some more dead puppies there were: R1060
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122 and R1070 have died also (open) and, more annoying, U1066. It's a bit
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123 more annoying since this chip is a DS0026CN, which is obsolete and
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124 cannot be found at DigiKey or Mouser any more.
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125
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126 Thanksfully, it's still available on ebay. So I've bough a pair of
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127 them, and that's where I am now in this epic attempt to repair this
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128 poor PSU.
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129
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130 I've also bought a Rigol DS1054Z (waiting for it to be delivered
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131 also), I hope it will help me figure what's really going wrong in this
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132 preregulator stage. Obviously there is something not working properly,
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133 probably a high frequency signal in the switching. The creepy noise in
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134 T1050 and R1071 beginning to burn are clear signs of this high
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135 frequency noise, probably inducing high voltage spikes or some stuff
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136 like that.
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137
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138
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139 A few things I've noticed that seems rather strange to me:
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140
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141 - R1069 is marked as 100kΩ on the service manual, it was 33kΩ on mine,
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142
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143 - the schematic indicate a +264V at the output of the T1020 choke, but
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144 I don't understand how such a low value is possible (with a 240VAC
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145 input voltage, I expect this value to be around 330V, which is the
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146 value I measured IIRC),
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147
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148 - voltage values indicated on the schematic in the service manual with
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149 a * sign should be with respect to REF signal. But I don't see how
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150 the voltage at the terminals of U1030 can be given relative to
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151 REF. I read (and measured) these values realted to the GND pins of
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152 U1030, which is the switched +264V (or +330V in my case) high
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153 voltage at the middle terminal of the primary coil of the output
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154 transformer.
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155
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156 While waiting for my DS0026 spare ICs, I'm trying to figure how I can
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157 elaborate a setup in which I am able to test the PSU without risking
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158 its life again. I'll have to find my variac in my cellar, I guess.
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159

mercurial