Hi Leo
I am still searching for the component.
I got 4.7 Ohms resistor an 2n5401transistor.
Replaced 39 Ohms resistor and 2n2907.
For the time being I replaced q9070 by irf 830 which is very close to the
one you suggested in terms of switching time
and has better voltage and current performance. For cr 907 I replaced by
two 1n4936 in parallel which has switching time of 200ns which is quite
higher than switching time of Byz73g.
And put the scope on with same current limiter bulb. This time the bulb
lights on continuously bright (does not flash Ann no ticking sound) and I
could hear high pitch sound which is not very loud. But all the voltage
dropped across the limiter bulb. I could get around 7 v across c906. I
might be wrong but what I am thinking is
q907 fired on and never came to off state or there is a dead short in the
preregulator.
Of course I Wil replace those two component when I get them.
Saroj
On Mon, Jun 29, 2020, 3:25 PM Saroj Pradhan via groups.io <sarojman.Pradhan=
[email protected]> wrote:
Hi leo
Thanks once again for guiding me through repair process.
Regarding the voltage issue you are right that my power supply has poor
regulation.
As you suggested I will try my best to replace those component and get
back to you hopefully with good news.
Saroj
On Mon, Jun 29, 2020, 12:52 PM satbeginner <castellcorunas@...>
wrote:
Hello Saroj,
this is a good start! You confirmed your scope has the typical issue,
it's
almost always the primary power supply with problems.
The Power Supply are basically TWO switching inverters after each other.
By means of bypassing the first - what you now did- you narrowed your
problem down to the first one.
First about the fan:
The fan is just a simple 60mm 12VDC fan, and usually draws about
150-160mA, and in the scope its running at a slightly lower voltage.
Did you use the proper polarity when you tested the fan? The board layout
and diodes in the scope create a negative voltage in respect to ground.
But, I assume you unplugged it to test, and if it does not turn and draws
500mA, I would say it is broken.
Smaller scopes in the 22xx range do not even have this fan, so again, for
now leave it disconnected until you got a proper replacement.
The wobbling of the screen, and lower DC voltages in the secondary PS are
probably due to the fact your temporary power supply is not regulated, a
bit low (38,7V) and therefor probably has a significant ripple, I would
not
worry about these things right now.
The most important thing is: the second stage of the power supply works.
:-)
Now we concentrate on the first stage of the power supply:
I looked at all the different versions of the power supplies used in the
2213 2215 2215A 2230 etc scopes, and they are all similar, but not of
great
design (Sorry for this remark, Tek!!)
I choose to use the following approach:
I changed R909 to 4,7 Ohm, instead of 39 Ohm.
This charges and discharges the input capacity of the FET Q9070 faster,
so
it will switch ON and OF faster, therefor it will dissipate a bit less
power during switching.
You will find this lower value of R909 in some f the scopes I mentioned.
Furthermore: I replaced Q9070 by a FQPF4N90C.
This FET has a slightly lower ON-resistance (again, less dissipation
while
ON) AND it has a full plastic housing.
Because of the plastic housing I put it directly on the metal mounting
plate while I added a TO-220 heatsink as well.
I even put a TO-220 heatsink at the other side of the metal plate, to
increase the cooling surface.
Recently I used the following components to repair this power supply:
CR907 MUR460 (a bit difficult, because of the thicker wires on this
diode)
Q908 2N5401 (EBC) same pinout, so easy to replace
Q9070 FQPF4N90C Higher voltage, Lower ON resistance, plastic TO-220
housing, better mounting (IMHO)
But obviously, others will work too.
My first option would be:
Find suitable components, and leave the suspected fan disconnected for
now.
See if it works, and while working on the repair, find another 60mm fan,
maybe from an old PC or other device.
God luck, you will get it up and running.
Leo
