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 Tektronics 2445 scope PSU repair: part 2
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: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.