Chimei 19" Monitor CMV938D

[ijwcomp]

Hi everyone,

I've got a Chimei 19" Monitor (CMV938D) which has a backlight problem, under no light, you can still barely see the display.

Symptoms:
Turns on as normal. Detects input fine (as evident from green led light), goes to standby if no input detected (as par normal). However, the backlight seems to be dead.

Cap info:
Big black cap: 120uF, 400V 0615(M) 105C(AQ)P
Brown cap (bottom-mid): 22uF 50V, P61?, CapXon
Black cap (top-right, lying down): ?uF, ?V, P614 VENT KF 105C, unknown brand
Black cap (left of the cap above): CapXon, 220uF, 25V, P616 VENT, KF 105C
Black cap (right of the lying down cap): CapXon, 220uF, 25V, P616 VENT, KF 105C
Biggest Green cap: Taicon, 10V 2200uF
Medium green cap: Taicon, 1000uF 10V
Smallest green cap: Taicon, 25V 330uF
Black cap between green caps: CapXon, 220uF, 25V, P616 VENT, KF 105C

The green caps and the biggest black cap looks to be fine with flat tops.
However, the other black caps do have a slight bulge at the top, does this mean they are bad?

What can I do next to try fix this problem?
Thanks.

[japlytic]

Re: Chimei 19" Monitor CMV938D

Welcome, ijwcomp!
Try replacing the caps with a bulge first with low-ESR units, and see if that works.

[goontron]

Re: Chimei 19" Monitor CMV938D

those 2 IC look cooked

[lexwalker]

Re: Chimei 19" Monitor CMV938D

I've got a Chimei 19" Monitor (CMV938D) which has a backlight problem, under no light, you can still barely see the display.

Symptoms:
Turns on as normal. Detects input fine (as evident from green led light), goes to standby if no input detected (as par normal). However, the backlight seems to be dead.

Cap info:
Big black cap: 120uF, 400V 0615(M) 105C(AQ)P
Brown cap (bottom-mid): 22uF 50V, P61?, CapXon
Black cap (top-right, lying down): ?uF, ?V, P614 VENT KF 105C, unknown brand
Black cap (left of the cap above): CapXon, 220uF, 25V, P616 VENT, KF 105C
Black cap (right of the lying down cap): CapXon, 220uF, 25V, P616 VENT, KF 105C
Biggest Green cap: Taicon, 10V 2200uF
Medium green cap: Taicon, 1000uF 10V
Smallest green cap: Taicon, 25V 330uF
Black cap between green caps: CapXon, 220uF, 25V, P616 VENT, KF 105C

The green caps and the biggest black cap looks to be fine with flat tops.
However, the other black caps do have a slight bulge at the top, does this mean they are bad?

What can I do next to try fix this problem?

First off, try replacing all those capacitors on the secondary side, especially all those CapXons. Those CapXon KF series capacitors are notorious for looking good today and then goes bulging the very next day.

those 2 IC look cooked

I think they are MOSFETs in I.C package...

[kaboom]

Re: Chimei 19" Monitor CMV938D

those 2 IC look cooked

Those are MOSFET arrays, a P-channel and an N-channel , in those packages, driving the transformer in full-bridge.

When (not if )those lousy Crapxon KFs go bad, the resulting ripple from the power supply intermods/mixes with the switching frequency of the backlight inverter. The green samxons aren't much better.

Discolored areas around the power traces to the inverter are common when this happens. There's quite a bit of circulating current as a result. Now, there is usually a small fuse in the DC supply to the inverter. Those circulating currents will blow it. This is an SMT device, "F-200," a 3A fuse.

Replace all the caps (I do mean all) and check those MOSFET packs for shorts. You can "cheat" and measure from the inverter side of the fuse to GND, and also from the primary winding of the inverter TX to both GND and inverter "VIN."


I'd use a little bit lower ESR cap for C200. It decouples the ripple from the inverter from the rest of the supply. A Nichicon HZ, HD or even a poly would be good here. The 120u/400V cap needs an insulator between its leads and the heatsink- it's pretty close there.

[budm]

Re: Chimei 19" Monitor CMV938D

Check the fuse, if it is blown, then those two MOSFETs may be shorted out also.

[goontron]

Re: Chimei 19" Monitor CMV938D

Those are MOSFET arrays, a P-channel and an N-channel , in those packages, driving the transformer in full-bridge.

When (not if )those lousy Crapxon KFs go bad, the resulting ripple from the power supply intermods/mixes with the switching frequency of the backlight inverter. The green samxons aren't much better.

Discolored areas around the power traces to the inverter are common when this happens. There's quite a bit of circulating current as a result. Now, there is usually a small fuse in the DC supply to the inverter. Those circulating currents will blow it. This is an SMT device, "F-200," a 3A fuse.

Replace all the caps (I do mean all) and check those MOSFET packs for shorts. You can "cheat" and measure from the inverter side of the fuse to GND, and also from the primary winding of the inverter TX to both GND and inverter "VIN."


I'd use a little bit lower ESR cap for C200. It decouples the ripple from the inverter from the rest of the supply. A Nichicon HZ, HD or even a poly would be good here. The 120u/400V cap needs an insulator between its leads and the heatsink- it's pretty close there.

i couldn't tell, i need a magnifying glass to read ram labels

[ijwcomp]

Re: Chimei 19" Monitor CMV938D

I've tested the fuse with a multimeter (resistance), and it comes up with 0, meaning the fuse should be working.

I've done a bit of research on which capacitors to get.... is the below correct?

Capacitors to change:

Note: Impedance ≈ ESR (Equivalent Series Resistance)?
Capacitance same?
Voltage same or higher?
Ripple same or higher?
Impedance same or lower (not too much lower)?

Replace 400V 120uF (at location CE106)
Aluminium cap, M, 18*31.5

Replace 3x Capxon 220uF 25V P616 KF Series (at location C200, CE101, CE109)
Aluminium cap, M, 8*11.5, 550 ripple, 0.15 impedance

Replace 2x Capxon 1000uF 25V P614 KF Series (at location CE102, CE103)
Aluminium cap, M, 13*20, TP5, 1650 ripple, 0.045 impedance

Replace 1x Capxon 22uF 50V (at location CE107)
Aluminium cap, M, 5*11, 125 ripple, 1.95 impedance

Replace 1x Taicon 2200uF 10V M 13*20 (at location CE105)
180 ripple, 0.70 impedance

Replace 1x Taicon 1000uF 10V M 10*16 (at location CE104)
1210 ripple, 0.060 impedance

Replace 1x Taicon 330uF 25V M 8*16 (at location CE108)
840 ripple, 0.087 impedance

Brands wanted:
Nichicon PM, PW, HV, HD, HE
Panasonic FC, FK, FM, FR
Rubycon ZL, ZLG, ZLH
Nippon Chemi-con GPA, GXE, KZA

Not sure how to check if the MOSFETs are shorted or not.

[jetadm123]

Re: Chimei 19" Monitor CMV938D

You can probably get by without replacing the 120uf 450v cap as it rarely fails. If you go with Chemi-con, use KY or KZE. For the Panasonic, just stick with FC, FM or FR.