Bringing Mr. Dean back to life (Tek 2225 with screwed up power supply)

[The Guy]

Well here I am repairing another 222five. This one does not power up though it`s in very good cosmetic condition.

Power transformer its ok, but circuit voltages are all over the place....

Here`s the schematic attached with the actual voltage readings in red.

circuit around T902 looks highly suspicious!! Let me know your thoughts please.

Thank you.

[Toasty]

Track down the 38V line to find what's killing it to 26V.  Some of those voltages (+8.6, -8.5, +5.2)won't come up until T902 is being properly fed signal.

Also, check the main cap C900 and the 38V cap C971 for capacitance and ESR.

T

[The Guy]

Track down the 38V line to find what's killing it to 26V.  Some of those voltages (+8.6, -8.5, +5.2)won't come up until T902 is being properly fed signal.

Also, check the main cap C900 and the 38V cap C971 for capacitance and ESR.

T

Thanks! will do...

I forgot to tell that either Q950, Q980 or both get very hot! ...I`m not sure what that means.

[Toasty]

That could indicate the lack of signal (sweep/pulses/oscillations) to the transformer causing them to turn on but not do anything.  They are essentially grounding the DC from L910 through the transformer from Pin 7.

U910 and or U940 (TL594) could be faulty...

http://www.ti.com/lit/ds/symlink/tl594.pdf

T

[The Guy]

That could indicate the lack of signal (sweep/pulses/oscillations) to the transformer causing them to turn on but not do anything.  They are essentially grounding the DC from L910 through the transformer from Pin 7.

U910 and or U940 (TL594) could be faulty...

T

But then that could happen because the voltages are upset right? ...at R943 (at the upper left corner of U940 in the schematic) there are 5V where there should be 23V. With 5v there U940 cannot properly operate if operate at all i think.... am I right?

[The Guy]

Yep... in the PDF that you kindly linked it says that: "The TL594 has circuitry to provide an undervoltage-lockout functionality. A minimum recommended VCC voltage of 7 V is recommended for operation, but if the VCC voltage drops below 6 V during operation, then the device shuts off"

So that indicates that at  5V U940 is effectively shuted down. Am I getting this right?

[Toasty]

At U940 where you have 5V marked pointing to where it says +23.0V, I believe that is an error. 

There's no way for the voltage at U940 Pin 14 to be +5.0V and correct and the voltage at the top of R943 to be different.  Pins 2, 6, 13, 14 are tied together.

Where the +23.0V is referencing is the bottom of YR942 (15v Zener diode) and Pin 12 of U940:  38V - 15V = 23V 

Vcc for U940

T

BTW: Since you have several of these, you can use the others to compare voltages to this one.

[The Guy]

At U940 where you have 5V marked pointing to where it says +23.0V, I believe that is an error. 

There's no way for the voltage at U940 Pin 14 to be +5.0V and correct and the voltage at the top of R943 to be different.  Pins 2, 6, 13, 14 are tied together.

Where the +23.0V is referencing is the bottom of YR942 (15v Zener diode) and Pin 12 of U940:  38V - 15V = 23V 

Vcc for U940

T

BTW: Since you have several of these, you can use the others to compare voltages to this one.

oh wow... you are so right! Didn`t see that.

BTW, instead of those 23V there are 11V... just measured it.

I`ve been comparing measurements back and forth... still can`t figure out what is happening 

I don´t have an esr tester, so maybe I´ll just change those caps and see what happens.
Do you think measuring ripple after C900 is going to tell me if that cap is bad?

[Toasty]

It would be a good indication of -a- problem, but I believe -the- problem to be in the switching circuit that drives the transformer T902.

T

[The Guy]

So I measured ripple after C900 and it is exactly the same than one of the working 222five I have. Not sure if I measured it correctly... I grounded the probe`s ground wire to the chassis and conected the probe to the positive side of the cap. I`m not sure if that`s correct because c900 doesn`t have its negative lead connected to ground.

I also removed all the jumper connections used to apply power to the different circuit boards just to isolate the power supply from possible shorts in other sectors, but nothing changed. Power supply is currently disconnected from the rest of the circuits.

I think it is important to note that those 27 or so volts are already present instead the 38V right after Q913 (in orange in the schematic attached below). Maybe Q913, Q912 or CR912 are bad?

Thank you!

[Toasty]

Be sure to drain any charged caps safely.  Check for voltage before beginning.

Again, using another working unit for reference, start testing the transistors & diodes using your DMM in "Diode Check" mode.  Disconnect the good PSU as you did the bad one.

You can also take readings from chassis ground to the various test points.  With the DMM in Diode Check, and the -positive- lead to chassis, take comparative readings at the numbered hexagonal points on the schematic.  Make a chart with columns being the "good" machine and then the "bad" one.

Also check, in the same manner, at Pin 12 on both PWM ICs.

T

[The Guy]

Be sure to drain any charged caps safely.  Check for voltage before beginning.

Again, using another working unit for reference, start testing the transistors & diodes using your DMM in "Diode Check" mode.  Disconnect the good PSU as you did the bad one.

You can also take readings from chassis ground to the various test points.  With the DMM in Diode Check, and the -positive- lead to chassis, take comparative readings at the numbered hexagonal points on the schematic.  Make a chart with columns being the "good" machine and then the "bad" one.

Also check, in the same manner, at Pin 12 on both PWM ICs.

T

okey dokey....I took all the other measurements you suggested and they read as follow:

point number:

30 reads 35.5V good scope  and 26.7V bad scope
31 reads 35.5V good scope and 26.8V bad scope
32 reads 2V good scope and 2.3V bad scope
33 reads 36V good scope and 27.5V bad scope
34 reads 18V good scope and 10.5V bad scope (referenced to pin six)
35 reads -0.2V good scope and -3.6V bad scope (referenced to pin five)

pin 12 at U910 read 29V good unit 20V bad unit
pin 12 at U940 read 20V god unit 12V bad unit

[Toasty]

Ummm, no.

DMM in Diode Check mode, Not voltage - no power applied, not plugged in.

T

[The Guy]

Ummm, no.

DMM in Diode Check mode, Not voltage - no power applied, not plugged in.

T

Dang! What was I thinking.....  Alright here are the measurements in diode mode, no power applied:

Point
30 reads 3.2V good scope and 3.2V bad scope
31 reads 3.2V good scope and 3.2V bad scope
32 reads 0V good scope and 0V bad scope
33 reads 3.2V good scope and 3.2V bad scope
34 reads 0V good scope and 0V bad scope (referenced to pin six)
35 reads 0V good scope and 0V bad scope (referenced to pin five)

pin 12 at U910 read 0V good unit 0V bad unit
pin 12 at U940 read 1.1V god unit 1.1V bad unit

Also replaced C971 and C915... but nothing changed. Still need to try replacing c927, c972, c953 and c952 to see if some of them are leaking.

Thank you!

[Toasty]

?? 34 reads --- (referenced to pin six)
?? 35 reads --- (referenced to pin five)

Why the "referenced to" instead of chassis ground?

The readings can be taken from any of the points highlighted in the attached image.

Please check resistor R907 for value & compare to good unit.

T

[The Guy]

?? 34 reads --- (referenced to pin six)
?? 35 reads --- (referenced to pin five)

Why the "referenced to" instead of chassis ground?

The readings can be taken from any of the points highlighted in the attached image.

Please check resistor R907 for value & compare to good unit.

T

Sorry bad use of language... i meant that points 34 and 35 were measured at said pins.

Seems that resistor R907 has a problem because the measurement jumps around... no stable at all... in the working scope it´s dead stable at 0.4ohm

[The Guy]

hmmmm.... took the resistor out and it reads stable at 0.4ohm now... weird. Will still replace it.

[The Guy]

Changed R907 and reading is stable now but the voltages are still off... nothing changed really.

[Toasty]

Okay.  Dang it.

DMM in Diode mode

+ lead to any of these pins on U910: 4, 7, 9, 10, 13, 16

- lead to U910 pin 12

Good vs Bad units

T

[The Guy]

Okay.  Dang it.

DMM in Diode mode

+ lead to any of these pins on U910: 4, 7, 9, 10, 13, 16

- lead to U910 pin 12

Good vs Bad units

T

Alrighty.... will do that tomorrow... it´s late here.... 

I just finished replacing caps c927, c972, c953 and c952 to rule out any leaking path, but nothing. Mr. Dean still´s deader than a plank of wood... 

[Toasty]

Replacing parts in a shotgun fashion doesn't solve anything.

Where is "here"?  Your profile is missing a flag/location.

Good night!

T

[The Guy]

Replacing parts in a shotgun fashion doesn't solve anything.

Where is "here"?  Your profile is missing a flag/location.

Good night!

T

I know it´s just that I thought that if there was any cap leaking along that 38V line (26V currently) i might fix the problem replacing them one by one. Had I a cap tester I would´ve checked´em out

I´m a little weary about giving my location on the internet... dunno.... I´m just sort of a private person I think

I really appreciate your time and help.

Thanks a bunch and talk later!

Good night

 

[Toasty]

USA, China, Outer Mongolia, North Pole?  Jeeze!

[oldway]

I have not read all the posts but I give you my first impression: I believe that + 38V consumption is excessive (to check).
This would come from the inverter, as seems to be demonstrated by the fact that Q950 and Q980 heat up too much.
Possible reasons:
U975 defective (try to disconnect it from T902)
T902 defective
Secondary overload of T902.

[The Guy]

I have not read all the posts but I give you my first impression: I believe that + 38V consumption is excessive (to check).
This would come from the inverter, as seems to be demonstrated by the fact that Q950 and Q980 heat up too much.
Possible reasons:
U975 defective (try to disconnect it from T902)
T902 defective
Secondary overload of T902.

Soooooo......    ..........   

Disconnected U975 and nothing changed... then swapped T902 for the one in the 222five I have for parts and BoooM! Got a working scope!!!

Though now i need to get those sweeps on the screen cuz they´re completely deflected to the left side.

Huge thanks oldway and Toasty! 

you guys rule... Mr. Dean sends his greetings