Viewsonic VG150 monitor repair

[2PAC Mafia]

Hi,

I´m trying to repair this monitor. I leave the service manual which I found at Internet:

http://www.restoretronic.com/descargas/Viewsonic VG150 service manual.pdf

At the moment to follow my explanation you should check page 30 where is Power system schematic.

The problem is the monitor is not switching on. I saw a capacitor bulged so I was testing them at power supply circuit. I found C01, C12, C110, C118, C131, C132, C134, C135 damaged so I replaced them.

After that I saw 12V were not coming through the IC18. I found the ON/OFF switch damaged, I replaced it.

After that I saw how it tryes to run at pin 8 of IC18 but it stops again so I removed IC18 and I saw gate signal there is correct so ON/OFF circuit works.

I removed the IC22 and then I have 12V at pin 8 of IC8, I also have 5V at C131. The problem is I don´t have any power at C134.
I found an error at schematic at R112 which instead of that position is between D15 anode and pin 5 IC20. So if I measure at pin 5 of IC20 I have 0,6V and at pin 4 I have 0V, at pin 2 I have 0V. I guess I should measure 5V at pin 2, isn´t it?

Do you think if I remove IC22 could have any relationship? I don´t think so and I did that to isolate the problem, I expected to measure 5V at C131 and 5V at C134 but then I saw C134 is missing. Now I´m focused at this part of circuit to get the VLCD.

What do you think?

 

[2PAC Mafia]

Another fault I´ve seen at schematic, it says power to IC20 is 5V but instead of that I measure 12V...

 

[USER REDACTED]

The problem is the monitor is not switching on. I saw a capacitor bulged so I was testing them at power supply circuit. I found C01, C12, C110, C118, C131, C132, C134, C135 damaged so I replaced them.

Just a note here: do not trust the look of the capacitor. ESR meter is essential here.

It may look perfectly fine, however my be completely dry.
Or if desolder and test capacitance may be in spec., however ESR value may show it is not.

 

[2PAC Mafia]

Hi testmpc,

Just a note here: do not trust the look of the capacitor. ESR meter is essential here.

It may look perfectly fine, however my be completely dry.
Or if desolder and test capacitance may be in spec., however ESR value may show it is not.

I now, in fact, only one was bulged, the rest seemed right so I tested all of them with ESR plus others capacitors too.

I can see how at IC20 pin 1 I have a switching signal but not at pin 2...

 

[2PAC Mafia]

Hi,

I have replaced the IC20 (LM339) because it showed me fail at IC tester and now I have exactly the same gate signal at pin 2 and 4 of IC21 (IRF7304). To do that I had to remove IC18 which was damaged (240 ohm between drain and source (pin 3 and 6), so I powered directly at pin 8 with 12V.

OK, now I have 5V at C131 and only 1,5V at C134. So some problem is still there.

Also, if I measure between pin 6 of IC18 and ground at board I get only 28 ohm which I guess it´s also wrong, too low?

May be somebody is following this post...

 

[tomizett]

I've just stumbled across this - late to the party! I'm having a little trouble finding my way around that diagram at the moment... I can't see any pin numbers on IC21 for a start. I've not read the description yet either, but will have a look when I get a moment.

So, you have the same signal at p1 and p2 of IC20, and consequently the same gate drive from IC12 p2,4,0 and IC12 p10,12,15? That doesn't seem right, because if Vlcd is low (1.5v) IC20 p2 should be high to put a low at the outputs of the inverters (IC12) and turn on the IRF7304. Could there be a solder bridge or other short between pins 1 and 2? Are you sure you've got the same 5v reference voltage appearing on p5 and p7 of IC20. There's really not a lot to go wrong with it...

Are you sure there's no excessive load at the output of Vlcd (ie, accross C134), like a semi-shorted capacitor or anything? What do the voltages look like across R39 and R10? With no load, these should be about equal. If R10 has gone high-resistance that could cause problems, too.

 

[2PAC Mafia]

Thanks for your interest again Tomizett,

IC21 is marking S, D, G at schematic.

At IC20 I didn´t get the same reference voltage on pin 5 and 7, it was different because R112 was not in position as is showed in schematic, it was between pin 7 of IC20 and D16 anode.
I removed IC22 to isolate the load and work only at power supply. Later I saw damaged IC18 and IC22 too, I think it was low ohm between gate and source.
To be sure the system worked a part of these problems I decided to connect 5V at C130 for VLCD, 5V at C129 for 5Vx (I made a bridge at IC18 p3 and p6) and finally 12V at C12 (back light used 12V). Like that the monitor was black, back light hadn´t enable signal so I jumped Q15 (C-E) to force the back light and like that I had no image, only horizontal lines. So I think after power supply there are more IC damaged.

After this I decided to leave the device because it seems a chain of failures, I remember I had a 28 ohm measuring at C118 line which I would say is not normal. I think 5V line has some device in semi short...

At the moment I have more jobs to finish so when I have more time I´ll try to do it again, mainly because I would like to be sure what is happening there to learn more.

 

[tomizett]

That's understandable - Looks like it was a bit of a catastrophic failure to have taken out so much of the power supply.
Irritating that the are so many errors in the schematic though!