Mercurial > whatever / changeset

--- 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

--- /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.
+

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