Viewsonic VX724 troubleshooting

[Leopard]

Hey all,

I have a VX724 monitor which is giving me a headache. I've replaced all electrolytic caps on the power supply / inverter board (including the mains cap which had leaked and the positive lead had corroded off - good job CapXon) and 3 caps on the logic board with fresh Panasonic FR/FM/FC series caps.

Symptoms are pretty straightforward, it will not turn on (no power light) and the power supply is ticking - it is trying to start but there is a short somewhere. One day it just silently went out and didnt have any signs of life.

The caps in the Royer oscillator circuit seem to be fine (no bulges) and the transistors on the flipside of the power board measure out ok.

The thing is, I am getting a continuity reading between +12V and ground on the secondary side. My suspicions point toward the MBR10100CT schottky diode - could it produce such symptoms? How can I measure if it works? Or could there be a shorted winding on the power transformer?

I am sorry but I cannot provide any pictures right now, there are plenty of pictures of the power board on the forum though (VX924 and VX724 seem to have the same PSU)

Thanks!

[selldoor]

Re: Viewsonic VX724 troubleshooting

Really need pictures - read this thread if you have not already
https://www.badcaps.net/forum/showth...ighlight=VX724

It has values for transformer and also how to test the schottky.

[Leopard]

Re: Viewsonic VX724 troubleshooting

Back with pictures of the overall areas.

If you need pics of specific spots please tell.

I am getting stable voltage at the mains cap (around 330V which sounds correct since we have 230VAC here in Finland)

I was thinking - what would cause the Schottky to fail? Heat, high ripple current from the primary side since the mains cap was a goner?

And (in circuit - havent desoldered it yet) the Schottky measures open in all directions (from both anodes to cathode and from cathode to anodes) - that means its a goner? Or should I test it out of circuit?

[selldoor]

Re: Viewsonic VX724 troubleshooting

Shottky doesnt sound good - yes take it out and re-test. When a Big Cap goes it can take out other components, my guess that is what has happened.

[Leopard]

Re: Viewsonic VX724 troubleshooting

Removed the Schottky dual diode, both of the diodes are ka-pooff (other measures open in both directions, I cannot get a measurement from the other which indicates that it is probably shorted). The short was removed from the +12V rail. Also the base of the Schottky (where it was soldered) measures no short now - this probably indicates that the transformer is working! I'll get a replacement next week, but I have a good feeling in my gut at the moment - this will probably get the monitor up and running.

About the caps that I used, the Panasonic FR output caps were a little too high (I used 820uF instead of 470uF since these are purely for filtering - no timing element involved) so I had to cut a little piece of the metal shield. 21mm is just a little too high since there is a slope in the shield just where the caps stand. Otherwise there is clearance for 21mm caps.

[selldoor]

Re: Viewsonic VX724 troubleshooting

Ok have a good weekend hope to hear success next week

[budm]

Re: Viewsonic VX724 troubleshooting

That Diode is mounted to the heat sink with a heat transfer silicone isolation pad to separate the tab which is the Common Cathode (tab is connected is connected to the center pin of the Diode) from the heat sink, if the tab is shorted to the heat sink which is connected to circuit ground, that will damage the diode. Make sure to check the tab of the newly installed diode is not shorted to the heat sink.

[Leopard]

Re: Viewsonic VX724 troubleshooting

I went to my local electronics store this morning. They had a BYV32-150 in stock. It is a dual rectifier with the same TO-220 case and also a positive centertap like the MBR10100CT. It is rated for 18A and 150V.

Can I use that as a replacement? Or does it absolutely have to be a Schottky rectifier?

MBR10100CT pdf
BYV32-150 pdf

[jetadm123]

Re: Viewsonic VX724 troubleshooting

Due to the higher frequency, you should use Schottky. If you have an old computer power supply lying around, you might find what you need there.

[budm]

Re: Viewsonic VX724 troubleshooting

Learn more here:
http://en.wikipedia.org/wiki/Schottky_diode

[Leopard]

Re: Viewsonic VX724 troubleshooting

I decided to test the BYV32-150 in place of the Schottky. The monitor fired up and it works like new now. I'll get the (proper) replacement next week but I wanted to test it to narrow down any other possible faults.

9 caps and one rectifier later, success. Parts cost around 12€.