Viewsonic n3260w No Power, no standby

[Glitcher]

Model: n3260w
I got a Viewsonic shows no sign of power. I opened it up, checked the PSU fuse and it was bad. Mosfets checked out good, but I found that the bridge rectifier (US8K 80R) was shorted.

Bought a new Fuse and new rectifier put it in, but it still doesn't power on and doesn't have the standby light. (Assuming it does have a standby light)

Got a little buzz jolt from accidently touching the giant heatsinks on the primary side and found it measured -63V. In fact measuring a lot of the components on the primary I'm getting this -63V. (But I'm measuring with respect to ground)

Largest Capacitor measures 170V

[johnboy1313]

Re: Viewsonic n3260w No Power, no standby

I recently fixed a N3251W. My problem was with the main board, but in my search, I found this.

http://forum.eserviceinfo.com/viewtopic.php?p=161039

"I just repaired an FSP 212-3F01 power supply in a Viewsonic N3250W. I have seen the same power supply listed for the N3235W. This one was totally dead with no 5V standby present. The problem was a shorted internal voltage regulator made up of Q7 and ZD4. The Q7 is a SOT23 size device and appears to be too small and over heats in the design. In this case it shorted from the over heating and that caused ZD4 to fail and shut down a 15V bias supply on the power supply board. I replaced Q7 with a 2N4401 to give it more power capability. Any small NPN transistor with a 25V or higher Vce rating will probably work. ZD4 is a 15V zener diode. Note that there appear to be two or three power supplies used in these TVs. This fix may also apply to similar FSP power supplies such as the FSP 228 but that has not been verified."

Your PSU is different than this one, but at least it's something to check.

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

Been checking the ZDiodes they seem to be good, not sure how to test the little transistors though.

I also found a schematic for my power supply. But theres still no labels for the pinouts of the outputs...

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

Looks like I found the problem! Seems to be a shorted Diode on D12.

labeled: BYV26EGP

Anyone know the specs?

[johnboy1313]

Re: Viewsonic n3260w No Power, no standby

Awesome, make sure you let us know if the replacement diode fixes it.

[tom66]

Re: Viewsonic n3260w No Power, no standby

Possibly more detailed schematic

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

I was able to find the exact diode from a salvaged board which tested good. I replaced it on the PSU but there's still no voltages on the secondary side...

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

So the diode didn't fix it..

I noticed that there were burn marks on the frame of the panel under the PSU. I guess that kind of makes sense since the TV was said to be "dropped" and didn't turn on thereafter.

Looking under the PSU the burn marks correspond around this area in the photos. I checked around there which is how I found the shorted diode.

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

Ok so still checking around the area I think I found an cap that is bad and is reading open circuit on my LCR meter. Its the blue cap in parallel with the resistor near D12 Diode C15. Would bad cap cause the no voltages on the secondary side of the psu?'

EDIT: Strike that, the cap is good, my meter was being crazy.

[budm]

Re: Viewsonic n3260w No Power, no standby

Here is my fix:
http://s807.photobucket.com/user/bud...?sort=3&page=1
http://s807.photobucket.com/user/bud...?sort=3&page=1
By the way, when you work in the Primary side (HOT, Deadly side) of the circuit, the Negative leg of the large 450VDC cap is the Ground ref. point for voltage measurement, not the chassis ground. You should plug what ever you are working on into the GFCI outlet for protection.

Thanks for the diagram, Tom.

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

I guess I'll give it a shot. What are the specs of the transistor you used? I'll try replacing Q7.

[budm]

Re: Viewsonic n3260w No Power, no standby

2N2222, or 2N3904 NPN transistor

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

Replaced Q7 with a 2N4401 transistor but still no luck.... Made sure the emitter, base, collector were in the right spots as in Budm's photo.

So far what I've found since I've gotten this set, open main fuse, Shorted Bridge rectifier, shorted diode D12.

That shorted diode D12 and burn mark on the panel frame has me suspicious that something else in that area is shorted.

What would happen if IC2 went bad? (TEA1532A) what exactly is this IC?

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

I'm still totally stumped on this one. Still no voltages on the secondary side of the psu.

[tom66]

Re: Viewsonic n3260w No Power, no standby

Check voltage across C32 while PSU is in operation (faulty).

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

Measuring across the cap C32, I get a fluctuating voltage of 9.8V-10.1V

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

Is that a good reading? voltage seems to be going through that cap.

[Elysarian]

Re: Viewsonic n3260w No Power, no standby

My opinion?

Cascade failure of the PSU, unless you're doing the repair as a learning experience I'd look for a replacement board as it's likely that unless you replace all faulty components in one go (or have access to a variac) then any silicon you do replace is at risk of getting burned out again.

Long experience of this kind of thing from back in the days of VCRs, if we couldn't source a replacement board it was usually dry testing (no power) of all silicon, replace anything short or open then onto the variac and slowly wind up the volts while monitoring the output volts and current draw (excessive current draw and it was back to looking for burned out components).

I hated SMPS's for the longest time because of this failure mode though they generally seem to be better at shutting down safely in most cases these days.

[budm]

Re: Viewsonic n3260w No Power, no standby

Do D8 and D7 which are for the PFC Voltage booster circuit check out OK? Since the board was shorted out to the chassis and took out the bride rectifier and D12, I would think the D7 and 8 will be damaged also since they are in series with the bridge output.
By the way, that DC reading is too low, it should be around 14.4VDC range because the output is from the linear regulator transistor Q7 Emitter follower circuit, the Emitter will be 0.6V less than the 15V Zener at the Base of Q7.
Since it is pulsing, you should be able to see small pulsing at the 5V output also because the transformer is being driven on and off.
Also check the capacitance value of C33 and C32

[Glitcher]

Re: Viewsonic n3260w No Power, no standby

D8 was the few things I checked first and it was good. For some reason I can't seem to find D7 anywhere though.

I'm not sure which is the 5V output since it doesn't have any labels on the outputs, but measuring each the voltage is near zero.

Checking C32 and C33 has capacitance of about 32uf with esr less than 0.5 ohms.