comparison: content/TeK2445_2.rst
content/TeK2445_2.rst
- changeset 45
- 3eed12f6592b
- child 115
- 6b6e13653348
equal
deleted
inserted
replaced
| 44:96b26fb52f0b | 45:3eed12f6592b |
|---|---|
| 1 ========================================== | |
| 2 Tektronics 2445 scope PSU repair: part 2 | |
| 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: 2 | |
| 10 | |
| 11 After a `quick repair of the PSU of my Tek 2445 | |
| 12 <{filename}/TeK2445.rst>`_, I've started a more systematic replacement | |
| 13 of the capacitors. | |
| 14 | |
| 15 .. image:: {filename}/images/tek2445/psu_caps_replaced.jpg | |
| 16 :alt: Tek2445 PSU Caps being replaced. | |
| 17 | |
| 18 But I was also a bit worried because of a dirty noise coming from the | |
| 19 preregulator area. | |
| 20 | |
| 21 After having replaced most of the electrolytic capacitors, the dirty | |
| 22 noise became louder (I think. It might be unrelated, not sure | |
| 23 yet). The PSU became very untrusty: a small plume of smoke began to | |
| 24 escape from somewhere (could not identify precisely where exactly). I | |
| 25 started probing around, but at this moment, I had no idea which | |
| 26 component was getting too hot: I always shut the power off after a few | |
| 27 seconds of power, with the T1050 transformer singing as soon as the | |
| 28 preregulator control oscillator starts up (U1030). | |
| 29 | |
| 30 During this probing and checking period, I discovered that the CR1040 | |
| 31 diode from the current limitation circuit was dead | |
| 32 (short)... Unfortunately, it's a germanium diode (used for it's low | |
| 33 voltage drop, about 0.2v). Fortunately I found one of them which | |
| 34 should fit in my very old spare components casket (the one I had when | |
| 35 I was a kid). | |
| 36 | |
| 37 Then, in order to try to see things, I started to power the PSU up for | |
| 38 a few seconds (sometimes maybe tens of seconds) trying to probe | |
| 39 (without my scope, obviously, so only with my Fluke 867B which allows | |
| 40 me to see *some kind* of signals.) | |
| 41 | |
| 42 Which allowed me to finally find the culprit for the smoke: R1071 (in | |
| 43 red on the schematic below). Its value is still fine, but it's getting | |
| 44 very hot as soon as the main oscillator starts up. A bit strange, | |
| 45 since the capacitor C1071 seems fine (I do not have a ESR meter, but | |
| 46 hey, there is no way higher ESR could lead to overcurrent flowing | |
| 47 through this resistor). | |
| 48 | |
| 49 .. image:: {filename}/images/tek2445/psu_prereg.png | |
| 50 :alt: Schematic of the preregulator of teh Tek2445 | |
| 51 | |
| 52 But powering up and down the PSU again and again, with its freaking | |
| 53 and noisy switching problem, finally killed the MOSFET (IRF820) power | |
| 54 transistors (Q1050, Q1060 and Q1070) producing flames. The plastic | |
| 55 protection on Q1060 directed all the heat directly on the poor LR1060 | |
| 56 which exploded as well... Which is a problem since I do not know the | |
| 57 exact specifications of the device; the service manual do not give the | |
| 58 value of the resistor in parallel with the 2.5µH inductance. | |
| 59 | |
| 60 .. image:: {filename}/images/tek2445/dead_transistors.jpg | |
| 61 :alt: Q1050, 1060 and Q1070 are dead. | |
| 62 | |
| 63 For now, I've picked up a 3W 100Ω resistor and I've made a self by | |
| 64 hand. As I don't have a RLC meter, I don't known its exact value so | |
| 65 I've took a few measures of its impedance at several frequencies, and | |
| 66 I estimate the inductance to be around 4 µH. I have no idea whether | |
| 67 this higher value might be a problem... But I've no idea either if the | |
| 68 100Ω resistor I've used as core for the coil is fine or not. | |
| 69 | |
| 70 And I have other problems to fix. First, the 3 MOSFETs have to be | |
| 71 replaced... As I don't have anything near IRP820 in my hood, I've | |
| 72 picked up some IRFP450 I extracted from computer PSUs. These are much | |
| 73 bigger than the IRF820, but having beafier models (with higher max | |
| 74 current and voltage) should not be a problem there. The only thing | |
| 75 that worries me a bit is the much lower Rds value (0.4Ω versus 3Ω for | |
| 76 the IRF820). Means higher peak currents. Q1030 and R1052 also | |
| 77 died. The transistor is a 2N3905 (which I don't have around either) so | |
| 78 I've replaced it with a bigger TIP32C. | |
| 79 | |
| 80 .. image:: {filename}/images/tek2445/beafier_transistors.jpg | |
| 81 :alt: Beafier replacement rtansistors, and a handmade coil. | |
| 82 | |
| 83 From there, I began to check most of the components in this | |
| 84 preregulator part of the PSU before even attempting to apply power. | |
| 85 | |
| 86 I've checked the behavior of the main control oscillator (made | |
| 87 of U1030), by powering it with my new old Lambda linear power | |
| 88 supply. Applying power at the leads of C1025, I could also check the | |
| 89 behavior of the Q1020/Q1021 stage. Their purpose is to regulate the | |
| 90 supply voltage for the U1030 main oscillator. | |
| 91 | |
| 92 .. image:: {filename}/images/tek2445/psu_prereg_test.jpg | |
| 93 :alt: Testing the main control oscillator. | |
| 94 | |
| 95 At power-up time, when the oscillator has not yet started, R1020 fill | |
| 96 C1025 from the input high voltage DC. When the voltage on C1025 | |
| 97 reaches about 21V, Q1021 is activated, powering U1030 (from C1025), | |
| 98 and doing so, putting R1024 in parallel with R1020, making the voltage | |
| 99 required to keep Q1021 saturated much lower (around 8V theorically). | |
| 100 | |
| 101 With the energy stored in C1025, U1030 can start oscillating, starting | |
| 102 effecively the preregulated power supply, which make switching current | |
| 103 flow through T1050, from which the coil at leads 6 and 7 should now | |
| 104 provide something like the 15V used to power the main preregulator | |
| 105 oscillator. So far so good. On my PSU, this main oscillator seems to | |
| 106 work fine even if I've found that the voltages at which thing happen | |
| 107 are significantly smaller than the values above (which come from the | |
| 108 service manual). It's more like: | |
| 109 | |
| 110 - 18V for Q1021 to be saturated (instead of 21V), | |
| 111 - 7V for Q1021 to be blocked again (instead of 8V). | |
| 112 | |
| 113 Not sure whether I should care about this. Probably not. | |
| 114 | |
| 115 The other part of the preregulator is the inverter drive. Its purpose | |
| 116 is to switch the current flowing in the T1060 transformer (the output | |
| 117 of the preregulator part) from one direction to the other in its | |
| 118 primary circuit at each pulse of the switching in T1050. | |
| 119 | |
| 120 With Q1060 and Q1070 being dead, I was expecting a few more components | |
| 121 around to have suffer. And some more dead puppies there were: R1060 | |
| 122 and R1070 have died also (open) and, more annoying, U1066. It's a bit | |
| 123 more annoying since this chip is a DS0026CN, which is obsolete and | |
| 124 cannot be found at DigiKey or Mouser any more. | |
| 125 | |
| 126 Thanksfully, it's still available on ebay. So I've bough a pair of | |
| 127 them, and that's where I am now in this epic attempt to repair this | |
| 128 poor PSU. | |
| 129 | |
| 130 I've also bought a Rigol DS1054Z (waiting for it to be delivered | |
| 131 also), I hope it will help me figure what's really going wrong in this | |
| 132 preregulator stage. Obviously there is something not working properly, | |
| 133 probably a high frequency signal in the switching. The creepy noise in | |
| 134 T1050 and R1071 beginning to burn are clear signs of this high | |
| 135 frequency noise, probably inducing high voltage spikes or some stuff | |
| 136 like that. | |
| 137 | |
| 138 | |
| 139 A few things I've noticed that seems rather strange to me: | |
| 140 | |
| 141 - R1069 is marked as 100kΩ on the service manual, it was 33kΩ on mine, | |
| 142 | |
| 143 - the schematic indicate a +264V at the output of the T1020 choke, but | |
| 144 I don't understand how such a low value is possible (with a 240VAC | |
| 145 input voltage, I expect this value to be around 330V, which is the | |
| 146 value I measured IIRC), | |
| 147 | |
| 148 - voltage values indicated on the schematic in the service manual with | |
| 149 a * sign should be with respect to REF signal. But I don't see how | |
| 150 the voltage at the terminals of U1030 can be given relative to | |
| 151 REF. I read (and measured) these values realted to the GND pins of | |
| 152 U1030, which is the switched +264V (or +330V in my case) high | |
| 153 voltage at the middle terminal of the primary coil of the output | |
| 154 transformer. | |
| 155 | |
| 156 While waiting for my DS0026 spare ICs, I'm trying to figure how I can | |
| 157 elaborate a setup in which I am able to test the PSU without risking | |
| 158 its life again. I'll have to find my variac in my cellar, I guess. | |
| 159 |
