Re: 2445A power supply problem------Need Help!


tom jobe <tomjobe@...>
 

Hi,
I see that you have received a nice reply from Francis about your power
supply problem, and as he has said ,this is a 224X power supply you show in
your pictures, not a 24x5 power supply.
It has been a year or so since I worked with Francis on his 224x power
supply problem. He makes it sound like I helped to figure it out, but all I
did was perform some tests and modifications on a known good 224x power
supply and send him the oscilloscope screen pictures of the results. The
brain power and the good ideas all came from Francis!
As I remember, he came up with this clever fix called a "Baker Clamp", and
it saved his 2245A! The basic problem was possibly a slightly out of
specification main transformer, but in any case his fix worked very well.

Another thought on your problem would be to see if you have any of the
dreaded diodes marked ZS (or ZM?), there can be up to 17 of them in a 224x
power supply. If you do change the diodes be sure to use ultra fast diodes
such as a MUR160 or one of the many equivalent diodes.
tom jobe...

----- Original Message -----
From: "z80tw" <z80tw@yahoo.com.tw>
To: <TekScopes@yahoogroups.com>
Sent: Saturday, May 28, 2011 9:49 PM
Subject: [TekScopes] 2445A power supply problem------Need Help!


Hi,I am new to this group,this is my first post.
About one month ago I acquired a 2445A with burnt PSU,there are two black
scorch marks in the area of inverter power switching.I have found some
failed components and replaced with equivalents or substitutes.the PSU can
work now but the temperature in some components are unusually high in very
short period after power on(especially Q2214/C2210/T2205).
the temperature of heat sink of Q2214 could pick up to extreme high in less
than 30s after power on!

The 14 waveforms I measured has been uploaded into the photo album of "2445A
power supply problem"
(Measuring Condition:two loads only,-15V to Fan,+5V to a 20 ohms 10W
resistor)

I checked and compared the waveforms with service manual and found few
discrepancies:
1.The Vgs of Q2214:3.8~4V < 5V(SM)
2.The Vds of Q2214 12I:0~40Vpeak >> 3V(SM)
3.The Voltage of 12G(T2205 Pin 2 to P/GND):0~40V peak >>2~3V(SM)

I tested every parts of Inverter power switching(except T2205) they all were
fine.

My questions are:
how can I make the Vgs of Q2214 to 5V?
why the Vds of Q2214 goes up to 40V before the magnetic core of T2205
saturates?any way to eliminate this phenomenon and make the Vgs of Q2214
stays low?

The bad components have been replaced:
1.Q2214 IRF533(80V/12A/0.23 ohm) by IRF530(100V/14A/0.16 ohm)
2.C2210 4.7uf/100V by C2238(used a Nichicon 4.7uf/100V for C2238)
3.Q2209(and Q2210) by TIP41C(100V/6A)
4.R2242/R2243 2.7 ohms 1/8W by 2.7 ohms 1/4W (carbon film)

Besides above components I also replaced followings with new:
Q2201 IRF830
VR2205 6.2V zener 1/2W
Q2212/Q2213 2N5087
Q2208 2N3904
Q2202 2N3904
Q2203 2N3906
U2201 MC34060

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