# HG changeset patch # User David Douard # Date 1459114590 -7200 # Node ID 3eed12f6592b200b9090a9766f57c4a2f51034a9 # Parent 96b26fb52f0b6b6d156a0d928914173b7bf2b266 [tek] part 2 of the PSU repair diff -r 96b26fb52f0b -r 3eed12f6592b content/TeK2445.rst --- a/content/TeK2445.rst Sat Mar 19 00:55:18 2016 +0100 +++ b/content/TeK2445.rst Sun Mar 27 23:36:30 2016 +0200 @@ -1,10 +1,12 @@ -================================== - Tektronics 2445 scope PSU repair -================================== +========================================== + Tektronics 2445 scope PSU repair: part 1 +========================================== :Author: David Douard :Category: Electronics :Tags: Tektronics, 2445, analog scope, repair, test equipment +:series: Tek2445 PSU repair +:series_index: 1 I've recently took my good old Tek 2445 scope from the cellar (along with my {tag}HP8904A signal generator), and it showed signs of diff -r 96b26fb52f0b -r 3eed12f6592b content/TeK2445_2.rst --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/content/TeK2445_2.rst Sun Mar 27 23:36:30 2016 +0200 @@ -0,0 +1,159 @@ +========================================== + Tektronics 2445 scope PSU repair: part 2 +========================================== + +:Author: David Douard +:Category: Electronics +:Tags: Tektronics, 2445, analog scope, repair, test equipment +:series: Tek2445 PSU repair +:series_index: 2 + +After a `quick repair of the PSU of my Tek 2445 +<{filename}/TeK2445.rst>`_, I've started a more systematic replacement +of the capacitors. + +.. image:: {filename}/images/tek2445/psu_caps_replaced.jpg + :alt: Tek2445 PSU Caps being replaced. + +But I was also a bit worried because of a dirty noise coming from the +preregulator area. + +After having replaced most of the electrolytic capacitors, the dirty +noise became louder (I think. It might be unrelated, not sure +yet). The PSU became very untrusty: a small plume of smoke began to +escape from somewhere (could not identify precisely where exactly). I +started probing around, but at this moment, I had no idea which +component was getting too hot: I always shut the power off after a few +seconds of power, with the T1050 transformer singing as soon as the +preregulator control oscillator starts up (U1030). + +During this probing and checking period, I discovered that the CR1040 +diode from the current limitation circuit was dead +(short)... Unfortunately, it's a germanium diode (used for it's low +voltage drop, about 0.2v). Fortunately I found one of them which +should fit in my very old spare components casket (the one I had when +I was a kid). + +Then, in order to try to see things, I started to power the PSU up for +a few seconds (sometimes maybe tens of seconds) trying to probe +(without my scope, obviously, so only with my Fluke 867B which allows +me to see *some kind* of signals.) + +Which allowed me to finally find the culprit for the smoke: R1071 (in +red on the schematic below). Its value is still fine, but it's getting +very hot as soon as the main oscillator starts up. A bit strange, +since the capacitor C1071 seems fine (I do not have a ESR meter, but +hey, there is no way higher ESR could lead to overcurrent flowing +through this resistor). + +.. image:: {filename}/images/tek2445/psu_prereg.png + :alt: Schematic of the preregulator of teh Tek2445 + +But powering up and down the PSU again and again, with its freaking +and noisy switching problem, finally killed the MOSFET (IRF820) power +transistors (Q1050, Q1060 and Q1070) producing flames. The plastic +protection on Q1060 directed all the heat directly on the poor LR1060 +which exploded as well... Which is a problem since I do not know the +exact specifications of the device; the service manual do not give the +value of the resistor in parallel with the 2.5µH inductance. + +.. image:: {filename}/images/tek2445/dead_transistors.jpg + :alt: Q1050, 1060 and Q1070 are dead. + +For now, I've picked up a 3W 100Ω resistor and I've made a self by +hand. As I don't have a RLC meter, I don't known its exact value so +I've took a few measures of its impedance at several frequencies, and +I estimate the inductance to be around 4 µH. I have no idea whether +this higher value might be a problem... But I've no idea either if the +100Ω resistor I've used as core for the coil is fine or not. + +And I have other problems to fix. First, the 3 MOSFETs have to be +replaced... As I don't have anything near IRP820 in my hood, I've +picked up some IRFP450 I extracted from computer PSUs. These are much +bigger than the IRF820, but having beafier models (with higher max +current and voltage) should not be a problem there. The only thing +that worries me a bit is the much lower Rds value (0.4Ω versus 3Ω for +the IRF820). Means higher peak currents. Q1030 and R1052 also +died. The transistor is a 2N3905 (which I don't have around either) so +I've replaced it with a bigger TIP32C. + +.. image:: {filename}/images/tek2445/beafier_transistors.jpg + :alt: Beafier replacement rtansistors, and a handmade coil. + +From there, I began to check most of the components in this +preregulator part of the PSU before even attempting to apply power. + +I've checked the behavior of the main control oscillator (made +of U1030), by powering it with my new old Lambda linear power +supply. Applying power at the leads of C1025, I could also check the +behavior of the Q1020/Q1021 stage. Their purpose is to regulate the +supply voltage for the U1030 main oscillator. + +.. image:: {filename}/images/tek2445/psu_prereg_test.jpg + :alt: Testing the main control oscillator. + +At power-up time, when the oscillator has not yet started, R1020 fill +C1025 from the input high voltage DC. When the voltage on C1025 +reaches about 21V, Q1021 is activated, powering U1030 (from C1025), +and doing so, putting R1024 in parallel with R1020, making the voltage +required to keep Q1021 saturated much lower (around 8V theorically). + +With the energy stored in C1025, U1030 can start oscillating, starting +effecively the preregulated power supply, which make switching current +flow through T1050, from which the coil at leads 6 and 7 should now +provide something like the 15V used to power the main preregulator +oscillator. So far so good. On my PSU, this main oscillator seems to +work fine even if I've found that the voltages at which thing happen +are significantly smaller than the values above (which come from the +service manual). It's more like: + +- 18V for Q1021 to be saturated (instead of 21V), +- 7V for Q1021 to be blocked again (instead of 8V). + +Not sure whether I should care about this. Probably not. + +The other part of the preregulator is the inverter drive. Its purpose +is to switch the current flowing in the T1060 transformer (the output +of the preregulator part) from one direction to the other in its +primary circuit at each pulse of the switching in T1050. + +With Q1060 and Q1070 being dead, I was expecting a few more components +around to have suffer. And some more dead puppies there were: R1060 +and R1070 have died also (open) and, more annoying, U1066. It's a bit +more annoying since this chip is a DS0026CN, which is obsolete and +cannot be found at DigiKey or Mouser any more. + +Thanksfully, it's still available on ebay. So I've bough a pair of +them, and that's where I am now in this epic attempt to repair this +poor PSU. + +I've also bought a Rigol DS1054Z (waiting for it to be delivered +also), I hope it will help me figure what's really going wrong in this +preregulator stage. Obviously there is something not working properly, +probably a high frequency signal in the switching. The creepy noise in +T1050 and R1071 beginning to burn are clear signs of this high +frequency noise, probably inducing high voltage spikes or some stuff +like that. + + +A few things I've noticed that seems rather strange to me: + +- R1069 is marked as 100kΩ on the service manual, it was 33kΩ on mine, + +- the schematic indicate a +264V at the output of the T1020 choke, but + I don't understand how such a low value is possible (with a 240VAC + input voltage, I expect this value to be around 330V, which is the + value I measured IIRC), + +- voltage values indicated on the schematic in the service manual with + a * sign should be with respect to REF signal. But I don't see how + the voltage at the terminals of U1030 can be given relative to + REF. I read (and measured) these values realted to the GND pins of + U1030, which is the switched +264V (or +330V in my case) high + voltage at the middle terminal of the primary coil of the output + transformer. + +While waiting for my DS0026 spare ICs, I'm trying to figure how I can +elaborate a setup in which I am able to test the PSU without risking +its life again. I'll have to find my variac in my cellar, I guess. + diff -r 96b26fb52f0b -r 3eed12f6592b content/images/tek2445/beafier_transistors.jpg Binary file content/images/tek2445/beafier_transistors.jpg has changed diff -r 96b26fb52f0b -r 3eed12f6592b content/images/tek2445/dead_transistors.jpg Binary file content/images/tek2445/dead_transistors.jpg has changed diff -r 96b26fb52f0b -r 3eed12f6592b content/images/tek2445/psu_caps_replaced.jpg Binary file content/images/tek2445/psu_caps_replaced.jpg has changed diff -r 96b26fb52f0b -r 3eed12f6592b content/images/tek2445/psu_prereg.png Binary file content/images/tek2445/psu_prereg.png has changed diff -r 96b26fb52f0b -r 3eed12f6592b content/images/tek2445/psu_prereg_test.jpg Binary file content/images/tek2445/psu_prereg_test.jpg has changed