Author Topic: Repairing SMPS - DELL 1800MP - Tricky question maybee??  (Read 17081 times)

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Offline Tony.TTopic starter

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Repairing SMPS - DELL 1800MP - Tricky question maybee??
« on: May 07, 2015, 01:36:57 pm »
Phuuh...tried to fix this SwitchingModePowerSupply for dell 1800MP projector.

Have to commit something...made a big mistake...sorry..stupid.....

Before the mistake.

At point A..just after the R37K resistor.  it was 10.7V. Not enough. Point B = 175V, Point C = 300VDC
 The ML4800 did not start.  Tested with a powersupply to feed in 14V...and it started to kick in. (of course the main supply was disconnected...just wanted to test the ic with low voltage..anyway..)

Thought that the resistor R37K and R27Kohm might be to high...so i put another resistor in parallell to achieve a little less ohms.  Lowered it a little to much.  (ashamed). 
Blew up the 0ohm resistor...started to sparkle ...ml4800 got shorted.


After replacement of the 0ohm resistor...and the ML4800 IC.   everything seemed ok.  All other resistors....even capacitors seemed ok..

BUT.  At point B...it is still 175V.  But at point A...it is 0.8V.  Tried to change the R37K resistor...still almost 0 volt.

Disconnected the main supply..and feed the IC with 14 volts again with powersupply directly at point A.  The IC kicked in again and works.


Soo..the questions.

1:  Why is it that there is 0 volts at point A now...after that I screwed up?  ...using the main supply as usual.

2: At the first place...can anybody see on the picture ..what dependencies there is that could have made the Point A only having 10.8 Volts? 

My guess is that the same component..that lowered the voltage to 10.8 instead of 14-15V...now is totally fucked up...and making it to drop to almost 0V.  But as i said...first i thought that there is a directly short to ground after Point A. But feeding the circuit with external powersupply with 14V...works.

I am totally lost here....

« Last Edit: May 07, 2015, 01:41:06 pm by Tony.T »
 
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Offline tautech

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #1 on: May 08, 2015, 04:45:45 am »
Common SMPS failure is the controller IC's VCC supply cap normally 22-50 uF.
Drift in value and ESR, remove and check to confirm.
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Offline Tony.TTopic starter

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #2 on: May 08, 2015, 11:55:02 am »
tautech..

Thansks for trying to help me. This one is really tricky.

I have attached a new picture.  I have desoldered the two capacitors inside the green ring. Value tested ok  The small one 100nF. the bigger one less... ESR tested it and also seemed to be ok.

Every other cap that has larger value (polarized ones) ...around 22-100uF....seems also be ok with ESR meter.   

I have painted the green line how the power comes in. At point A now it is 0.8V. before my stupid mistake..it was 10.8V.   Here i am lost.  BEcause if i put in 14V directly at point A from external PS....the power IC kicks in.  But why is the voltage dropping from 170V to almost 0 just over the 37K ohm resistor? when in main supply?...  It must depend on something?  And there is no bigger caps that is feeding the power IC what i can see .... the only ones that is around that is larger ceramic capacitors...4700nF...and another 450mF

i.ve read that there is a voltage supply directly after rectifier that kicks the IC on...and then it feeds from other side...?!!...  I have not managed to find the schematic of this board aswell..

it is "PA-4291-1D-LF" from LITEON SMPS..   

Drives my crazy....

 
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Offline wraper

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #3 on: May 08, 2015, 12:02:34 pm »
Every other cap that has larger value (polarized ones) ...around 22-100uF....seems also be ok with ESR meter.   
In this particular case more concern is about if they are leaky rather than ESR. Could be some faulty diode (low reverse resistance) too.
 
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Offline Tony.TTopic starter

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #4 on: May 08, 2015, 12:10:24 pm »
wraper

Ok...that could be after the Power IC kicking in and feeding the gate of the larger mosfet at the bottom...  but my concern now is just why i can not get 14V to power IC directly via point C-B-A.

Before it was 10.8V.

But if i should comment your thing around zener diodes.  See the picture. marked them with a green ring...

Those 2 zenerdiodes i measured 0.0045V voltage drop...on both side.  Unsoldered them....and they were ok...0.5V voltage drop..  Measured without them inserted...and there is something else shortening the tracks.  This zenerdiodes is connected to the PFC trace from the power IC.  But as i said..this seems to be another issue.  The first one is that the PowerIC does not get 14 V directly to its Vcc pin.....  the green painted line i have inserted...through 27K and 37k resistor...point c-b to a...
 
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Offline wraper

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #5 on: May 08, 2015, 12:54:39 pm »
PowerIC does not get 14 V directly to its Vcc pin.....
As the current before PSU is started is pretty low, it won't ever reach 14V if something have increased current flow. What do you mean by power IC? There are mosfets only.
Comparing with this schematic, maybe the mosfets have blown gates?
 
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Offline Tony.TTopic starter

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #6 on: May 08, 2015, 01:06:15 pm »
I meant the ML4800...to its Vcc pin.   This is here where i used a external PSU and feed in 14V....to pin 13.

Then i could read out ramp signal at pin 7 with my oscilloscope.  When the voltage came to 14V.   

But when i power up the DUT...this SMPS via its main supply...the voltage drops to 0V at point A...from 170V at point B.

Before my mistake..it was at 10.8V at point A....not enough for ml4800 to start...as it needs 14V.

I have tested the mosfets...and all the pins seems ok....the mosfets are feed via PFC...and PWM pins from ml4800.

But it could be the Q4 transistor that is shorted?...  but does this affect the VCC pin..the voltage input there??




Edit:

Well...there was a little fellow transistor..that was totally shorted...  green ring....
Must change it.....  i´ll be bbbbbbbbbaaaaaaackkkkkk!!    ;-) O0


Im back....the voltage at pin 13 is now 9V..  :-).  So back to square 1.  Good.   Now hunting the reason why it is only 9 and not 14.....

Must be some cap where the capacitans is drifting.....
« Last Edit: May 08, 2015, 02:08:11 pm by Tony.T »
 
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Offline wraper

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #7 on: May 08, 2015, 03:26:11 pm »
Are zeners no longer shorted?
 
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Offline SeanB

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #8 on: May 08, 2015, 03:32:42 pm »
Change that 220uF cap and the one next to it. TRhose are the startup caps, and are probably the leaky ones. You will find a trace leading to them from the top side of the board, connected to the power pin.
 
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Offline Tony.TTopic starter

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #9 on: May 08, 2015, 04:12:23 pm »
hmm.....zenerdiodes still shorted somehow...from pin 14 ...PFC and through 2 zenerdiodes...

Replaced 2 caps (220uF 50v...and 100uF 25V)...and one zenerdiode....and another cap at top right hand corner...

Still 9 V ...

BUT....i also read 9V at the emitter of the NPN transistor....at top left corner...marked with red circle.  One can see that there is a trace going to the right and to a via hole.

Need to trace it to see where it goes.....  but need to eat first...have not eaten anything the whole day ..  :-).

Have attached pictures to show that components have been replaced (red  circle in pic 2015-bla--bla--)....  Thinking of replacing a ceramic cap and another zenerdiode..marked with green circle in same pic...

 
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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #10 on: May 08, 2015, 06:13:37 pm »
Found a shorted smd cap.

This fellow is connected through a 1AW transistor directly to the gate of the shorted transistor that i have replaced....where it is 9V at the emitter...

Thought that the gate is not high enough...and traced the gate to the other side of the board and found this cap shorted.

Now next tricky question....for ones that are not experienced...like me :-)

What size of capacitor ???..no marking at all at this shorted fellow.   See picture....red circle...

Hate those smd caps without markings...

Second picture....followed the gate trace all the way to the bad smd cap..

As i said....i only get 9 Volts into the ML4800...and it could be that the transistor is not letting through enough....and this cap seems to shorten the circuit..just a little...enough for dropping it from 15V to 9V.   The ml4800 needs 14 volts at minimum...15V is ok.
« Last Edit: May 08, 2015, 06:21:35 pm by Tony.T »
 
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Offline tautech

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #11 on: May 08, 2015, 06:29:25 pm »
Most unusual.
Remove and recheck.

In-circuit tests are not always reliable. I often swap leads to eliminate a pn junction path giving a wrong result.

IMHO controller datasheets, that is from as many manufacturers as you can find, are invaluable for "typical applications", often your circuit will match one exactly. Man, can that help fault finding.  :clap:
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Offline Tony.TTopic starter

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #12 on: May 08, 2015, 06:32:05 pm »
still the small smd cap is shorted.  If i just could replace it with a similar....i could continue searching.....but what size???

Just measured the gate voltage.  0.8V.   At the collector...it was 0.

strange...


but something else...i removed the original shorted transistor  ..MPS 651....with another that I had.... BC237.  Could this be the reason?  Did not have the same.  I also have a bc547.  The difference was the pinout.  pin 1 collector....pin 2 base...pin 3 emitter.   The original had it opposite....pin 3 collector.  So i had to turn the transistor 180 degrees...compared to the original.  But anyway...is this different NPN transistor the reason for having only 9V?

But again...the smd cap is shorted..and i must find a similar value...somehow...

SOrry for stupid questions....and maybee stupid mistakes...but hey...im trying  :-)
« Last Edit: May 08, 2015, 06:39:00 pm by Tony.T »
 
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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #13 on: May 08, 2015, 06:42:26 pm »
still the small smd cap is shorted.  If i just could replace it with a similar....i could continue searching.....but what size???

Just measured the gate voltage.  0.8V.   At the collector...it was 0.

strange...


but something else...i removed the original shorted transistor  ..MPS 651....with another that I had.... BC237.  Could this be the reason?  Did not have the same.  I also have a bc547.  The difference was the pinout.  pin 1 collector....pin 2 base...pin 3 emitter.   The original had it opposite....pin 3 collector.  So i had to turn the transistor 180 degrees...compared to the original.  But anyway...is this different NPN transistor the reason for having only 9V?

But again...the smd cap is shorted..and i must find a similar value...somehow...

SOrry for stupid questions....and maybee stupid mistakes...but hey...im trying  :-)
Try a value of 10 nF, 0.01 uF.  ;)
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Offline Tony.TTopic starter

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #14 on: May 08, 2015, 07:23:05 pm »
damn...

it was not shorted...removed the cap...and measured shortage over the pads where the cap was.  So...it is something else shorted nearby....

pjuuh....
 
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Offline ljcarballo

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #15 on: April 26, 2017, 07:26:49 pm »
Did you manage to repair the source?
I have the same problem, in the leg 13 I have 10.9V and I think they are insufficient to power the integrated, I checked the resistors and everything is OK, but in the IN5406 I fall 27V, would it have to have 1.2V? .
a greeting

¿Lograste reparar la fuente?
Yo tengo el mismo problema, en la pata 13 tengo 10,9V y pienso que son insuficientes para alimentar el integrado, he comprobado las resistencias y todo esta OK, pero en el diodo IN5406 me caen 27V, ¿Tendria que tener 1,2V?.
Un saludo
 

Offline dicky96

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #16 on: April 27, 2017, 05:25:43 pm »
This is a very old thread so I am not sure he found the fault.

From my somewhat limited experience of SMPS faults it sounds like he was looking for this fault in the wrong area of the circuit, though I have no knowledge of this actual psu, but I do know what is commonly found in these designs.

As he points out the ML4800CP will power up when he applies 14V to the Vcc input, but he only has 10V or so on that pin so it can not start up.

He does mention at one point...  'i.ve read that there is a voltage supply directly after rectifier that kicks the IC on...and then it feeds from other side...?!!' and that is the thing I think everyone was missing.

If you look at the datasheet for ML4800CP you will see that is has a startup current of 200uA and a running current of 5.5mA

Quite often what you will find is a high value resistor connected from the +ve of the main smoothing capacitor after the bridge rectifier (approx 350V) - the high value resistor then charges an electrolytic and this is connected to the Vcc pin of the driver IC.   When you turn the PSU on, this high value resistor + capacitor give enough voltage to start  the driver IC oscillating.  However this can only supply the start up current not the running current.   So there is usually a diode connected from a sparate secondary on the SMPS transformer which takes over and supplies power to the driver IC once it starts up.

Now the fault he was describing is typical of a situation where you get the startup current, but the secondary Vcc supply can not take over due to a fault in the SMPS.  This explains why it will poer up if you use a bench PSU to power the driver chip.   

One of the most common faults that stops the PSU firing up in this situation is a short circuit on one of the secondaries.  This can be caused by a short circuit load (try disconnecting the load and see if it fires up) but more often is a short circuit Shottky Diode on one of the secondaries.

I would be looking for short circuits on the SMPS outputs.   

If I didn't find any shorts on the outputs then my next step would be to power the supply up with a 60W 240V light bulb in series with mains live.  Then I would use the bench PSU to start the driver ship with the mains applied (via the light bulb) and using my scope I hopefully would be able to fault find as to why it is not running.   I would do tis via a mains isolation transformer but if you don't have one you would still be OK as long as your bench power supply has an isolated (floating) output

The light bulb is there so that if something goes short when you force the driver chip to run (or for example due to a fault the driver IC turns the power Fet ON rather than driving it with PWM) then the worse thing that will happen is the bulb will light up and not something going bang.

Hope that helps
Rich





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Offline dicky96

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #17 on: April 27, 2017, 05:29:53 pm »
Oh and forgot to mention don't connect a scope ground to anywhere on the HT side of the PSU unless you have an isolation transformer!!!
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Offline gbrettell

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #18 on: April 27, 2017, 08:03:43 pm »
In my experience with repairing SMPS ... it is typically the capacitors on the secondary @ the output.  On a rare occasion it was the small electrolytic cap on the PWM controller feeding the primary.  Think of which parts are doing the most 'work' and that is typically where the failure will occur.  Of course there are numerous other possible scenarios ... have fun!
 
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Offline dicky96

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #19 on: April 27, 2017, 10:16:18 pm »
Yes that could cause this sort of problme... and if you are really unlucky, short circuit turns in the SPMS transformer  :-\
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Offline dicky96

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #20 on: April 28, 2017, 10:30:25 am »
Take a look at the circuit diagram on post #5.  The following is the correct operation of the PSU.

When the PSU is first powered up, the Vcc supply for the ML4800 is as follows:

Current flows from the positive end of the bridge rectifier BR1 via R27 (82k) to charge C26 (47uF). 

When the voltage across  C26 reaches 15V the Zener diode D2 clamps the voltage to 15V so it can not rise any further.

This voltage is connected to pin 13 Vcc of the ML4800 and is sufficient to start the chip oscillating.  The startup current is 200uA but once the chip starts to oscillate the running current is 5.5mA and this starts to discharge C26.

With ML4800 running should now produce PWM output from pin 11 of ML4800

The PWM output drives the FETs and the power supply starts to run

Referring back to the circuit diagram there is a winding on the SMPS transformer marked T2C. The voltage generated in this winding flows via the rectifier D12 to charge C12 (10uF).  This voltage feeds the 15V zener D2 via R28 (240R) And again the Zener clamps this to 15v and charges C26 47uF.

THIS is the circuit path that keeps the ML4800 running.
Startup path R27 -> C26 -> ML4800 pin 13
Running path D12 -> C12 -> R28 -> C26 -> ML4800 pin 13

Once you understand that (and this is where Tony.T failed) you will realise that R27 can only supply the 15V long enough to get the ML4800 running, then the capacitor C26 will start to discharge, and if D12/R28 don’t take over supplying the running current for ML4800 the Vcc will drop to about 10V and it will stop.

So your problem is either
1.   The ML4800 is faulty and can not drive the power FETs (you pretty much eliminated that by using a bench supply)

2.   There is a fault in the power FETS themselves – I doubt this as they normally go short circuit when they fail.  If you notice from the diagram on post #5 the power FET Q3 is driven from the pwm output on ML4800 pin 11 and power FET Q2 is driven from winding T1B on the transformer.  Any problems in this area could stop the power supply from firing up.

3.   There is some load (probably a short circuit) on the power supply output that prevents it from running. This is very  common and what I would check first – check D11A and D11B – these shottky diodes will most likely be in a TO220 style package and look like a power transistor.  It is very likely you have a short circuit here.

4. L1 & D1 is probably the inductor and 1N5406 you see on your board.  In which direction are you reading 27V across the 1N5406 – forward biased or reverse biased?  How does the 1N5406 check on a multimeter diode test?   L1 & D1 form part of the circuit known as power factor correction. A problem here such as open circuit D1 would also cause the power supply to fail to start in the way you are seeing

5. There is another scenario which would stop the psu from starting up.  If R27 goes high resistance or open circuit, or alternatively C26, D2, C12, C15 or D12 are leaky or short circuit you will never get the 15V to start ML4800 in the first place.  Of these possibilities I would say R27 high value or open circuit is the most likely but you should check them all.

Rich

« Last Edit: April 28, 2017, 01:04:28 pm by dicky96 »
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Offline dicky96

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #21 on: April 28, 2017, 10:40:33 am »
Correction

That should read:

Referring back to the circuit diagram there is a winding on the SMPS transformer marked T2C. The voltage generated in this winding flows via the rectifier D12 to charge C12 (10uF).  This voltage feeds the 15V zener D2 via R28 (240R) And again the Zener clamps this to 15v and charges C26 47uF.
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Offline tautech

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #22 on: April 28, 2017, 11:28:38 am »
@ dicky96
Thanks for your clear and concise write up, this is just as most of us experienced with these SMPS see it too.
The basic common layouts are quite similar with the IC and its individual peculiarities being the greatest variable in the design.

You should properly edit your post, use either Modify at top right or the little box at bottom right of your post to enter the edit window. Then you can delete, copy and paste from your last post to where you want to correct.
Try it.  ;)
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Offline dicky96

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #23 on: April 28, 2017, 12:48:24 pm »
Thanks tautech -  I didn't see the 'edit post' button.  I 've now corrected the original post and fixed a couple speeling errors as well  ;)

I also added scenario 5.  :D

« Last Edit: April 28, 2017, 12:55:11 pm by dicky96 »
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Offline ljcarballo

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Re: Repairing SMPS - DELL 1800MP - Tricky question maybee??
« Reply #24 on: April 28, 2017, 09:02:16 pm »
We continue to try to repair the source, thank you very much for all the comments that you have put, I have reviewed all the components that reach the leg 13 of the integrated and everything seems correct, my next test is going to be to change the resistance of 37K (when I measured it calculate 5% and was in the range), but now writing this comment, I think that the colors marked 39K and what I measured were 36K2, tomorrow I withdraw and I measure it again, because the lower that value the less tension I go To have in leg 13. The current that is passing is about 4mA. Tomorrow after the tests I say something.

Seguimos intentando reparar la fuente, agradezco mucho todos los comentarios que habéis puesto, he revisado todos los componentes que llegan a la pata 13 del integrado y todo parece estar correcto, mi próxima prueba va a ser cambiar la resistencia de 37K (cuando la medí calcule el 5% y estaba en el rango), pero ahora escribiendo este comentario, creo recordar que los colores marcaban 39K y lo que yo medí fueron 36K2, mañana la retiro y la vuelvo a medir fuera , porque cuanto mas baje ese valor menos tensión voy a tener en la pata 13. La corriente que esta pasando son unos 4mA. Mañana después de las pruebas os digo algo.
 


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