usp440m-42lp issue AKAI PDP42Z5TA

[paulstef]

Hi all,

I was reluctant to asking but I already spent hours trying to find out what's wrong with this PSU.

Apparently no schematic is available so I did a little bit of reverse engineering. The problem is that the inrush current relay turned on, the PSU tried to start and then turns off.

Here is what I did:

- took the PSU board out of the TV
- connected two switches from 5Vsb to RLY-ON and VS-ON via two 1k resistors
- the 5Vsb was ok but when I started RLY-ON no output voltages 5Vsc, 9Vsc etc., PFC is ok 365VDC output

- found there was too much ripple on the capacitors of the NCP1203 flyback circuit, checked them and they where BAD indeed

- now the 5Vsc, 9Vsc, 12Vsc, 24V work (MR2920 converter)

- I flip the switch for VS-ON
- Va is ok (another MR2920 converter), about 60V
- Vs is not ok (done with a MR5060)

Here is where I'm stuck. VS workes occasionnally.

MY QUESTION:

The NCP1203 creates 2 separate voltages, about 9 to 10V for its own VCC and about 16V for the power supply of the MR2920s and the MR5060 (VS). The 16V also supply the inrush relay. This voltage drops to about 14V when supplying the MRs and the relay.

This voltage seems to be too low to me.
Relay coil voltage spec: 18V
Undervoltage lockout NCP1203: up to 8.4V
MR2920 startup voltage: >14V

IMO the NCP1203 works at the wrong output voltage and I can't see why. The voltage feedback is done with a TL431 and it gives out 5.1V on the secondary side, exactly as it should be from what I see. Everything around the NCP seems to be ok, switching frequency 100kHz, MOSFET, gate resistance, voltage clamp... apart from its output voltage of 9V and 14V.

IF ANYBODY HAS AN IDEA WHAT'S GOING ON OR HAS A SCHEMATIC IT WILL BE MUCH APPRECIATED.

Thanks for reading.

[paulstef]

partial schematic

here is a partial schematic of the NCPs feedback

and the components around MR2920 and MR5060

[paulstef]

Re: usp440m-42lp issue AKAI PDP42Z5TA

Found out that the MR5060 needs almost 17V at pin 4 to start oscillation. The MR2920 only needs 15V.

Anyone with a datasheet for the MR5060? Found one for the 2920. They have similar functions but don't know if similar specs also...

[Hoopers Cooper]

Re: partial schematic

here is a partial schematic of the NCPs feedback

and the components around MR2920 and MR5060

PaulStef,
I won't pretend to be as experienced as you or most of the techs that post on this site. I do, however, have the same PS problem with USB440M-42LP, and want to fix it. As an EE, I follow (and can appreciate) the time put into troubleshooting judging by the schematics you drafted. I guess that MoSFET 5060 is hidden behind the heatsink plate.

I don't, however, have access to a test bench anymore, so I am placing my confidence in your expertise. If I can offer any help, I'd like to try. As the original owner of this flat screen housing this PS, I can say that it always took a few seconds for the (flyback ckt)? to energize and its flat blue screen (good one) to appear. As it aged gracefully, it took ~10 seconds to come to life. Now 5-10 minutes. I was hoping a visual inspection would offer a clue, but not really. Only L105 shows heat breakdown, some resin oozing at its edges.

I guess I'll replace the two radial caps, 504-5, and see if that helps since your troubleshooting isolated them and is cost pennies.

Thanks for any further assistance.

[paulstef]

Re: usp440m-42lp issue AKAI PDP42Z5TA

There are 4 power supplies on this board. One is energized right from the beginning and supplies the 5V for the standby circuit.

My problem is different than yours, my MR5060 seems to have a lowered threshold to start oscillating. I bought a new one and am waiting for it to arrive.

Your problem really seems to have something to do with the electrolytic capacitors, so I suggest you start replacing the ones placed near the transformers, 8 in total.

[Hoopers Cooper]

Re: usp440m-42lp issue AKAI PDP42Z5TA

I think I may have misled you with my description of circuit degradation. I think it is the same problem. When switched from STANDBY, the front LED goes from red to green momentarily, then extinguishes. Sometimes, five to ten minutes later, the set will then power on, as the LED goes solid green and the screen becomes fully energized.
In my very limited troubleshooting experience, I thought caps as they store that ‘kick’ to start the HV circuit leg. But then again, I remember this from CRT flyback circuit, so its likely arcane knowledge.
I hate to throw parts at anything, I’d rather know the source of the issue. But radial caps are relatively cheap.
By the ‘8’ did you mean : C203, C210, C303, C310, C403, C428, and the two from NCP1203 (C504-5)?
And replacing the MR5060 power controller makes sense, too, since it’s the high voltage supplier, right?
Please offer an update when you can , thanks .

[paulstef]

Re: usp440m-42lp issue AKAI PDP42Z5TA

Yes, all the small caps in the area where the 4 transformers are.

Don't replace the MR yet. Start with the caps and see if this solves your problem.

[Hoopers Cooper]

Re: usp440m-42lp issue AKAI PDP42Z5TA

More basic questions.
I know I need to get replacement caps with matched capacitance, and here with correct max temp (shown as 105C).

I think I can use ones with higher voltage rating for same capacitance?

Also, I found there are many subproduct families, I guess with circuit targeted properties. For instance, Nichicon has VZ, PS, PW etc families.
Are there other properties I should consider besides the max voltage and dimensions?

Thanks.

[paulstef]

Re: usp440m-42lp issue AKAI PDP42Z5TA

More basic questions.
I know I need to get replacement caps with matched capacitance, and here with correct max temp (shown as 105C).

I think I can use ones with higher voltage rating for same capacitance?

Also, I found there are many subproduct families, I guess with circuit targeted properties. For instance, Nichicon has VZ, PS, PW etc families.
Are there other properties I should consider besides the max voltage and dimensions?

Thanks.

In general for SMPS low ESR are good. Actually any type will work, but some won't last very long. Take what's recommended on BadCaps.

[paulstef]

Re: usp440m-42lp issue AKAI PDP42Z5TA

Update:

The MR5060 was the culprit. The working one has a lower threshold voltage to start oscillation. VS now starts as well.

All good. At least on this board.

[Hoopers Cooper]

Re: usp440m-42lp issue AKAI PDP42Z5TA

My apologies for such a long time away. We were out of town for a couple weeks, and a few other home projects required immediate attention, such as a leaking toilet ...

I replaced all these caps, still won't turn on. I can hear a relay or something else clicking on and then off. Also, it is stuck between Standby and On. The indicator LED goes from red to off, but does not turn green. I have to remove it from AC power to retry.

Any other suggestions?

[Hoopers Cooper]

Re: usp440m-42lp issue AKAI PDP42Z5TA

Update:

The MR5060 was the culprit. The working one has a lower threshold voltage to start oscillation. VS now starts as well.

All good. At least on this board.

Paulstef,
I tried your suggestions and still no luck.
I don't have comprehensive bench test equipment, so I'm stuck.

Could I hire you to repair this board? You had the learning curve on this one, so might easier a second go aroound?

Pls let me know if you can and rates. I'll ship to you if terms are agreeable.

Thanks,
Harrison Smith

[paulstef]

Re: usp440m-42lp issue AKAI PDP42Z5TA

I sent you a PM

Did you replace the MR5060? Did you replace all the small caps on the primary side?
What is the voltage on the 2 zener diodes (my first post) when in standby and shortly after you try to turn it on?

[mark_maher]

Re: usp440m-42lp issue AKAI PDP42Z5TA

Hi, I just got involved with a USP440M-42LP repair, which has similar symptoms. (Relay closes, nothing comes out of the secondary side outputs, then it shuts down again). My thanks to paulstef for his drawings - very helpful!

In my case, I found the supply for the MR2920's & MR5060 to drop to 12.8V when the RLY-ON switch was closed. All caps measure okay with the ESR meter. To prove the low voltage issue, I modified the voltage divider resistors for the TL431, to make 5Vctrl = 5.7V, which now gives me 14.6V at the MR2920's. Now the PSU board powers up (Va, Vs,and the 5Vsc, 9Vsc, 12Vsc and 24V) . Great! Only 5Vctrl is too high (or is it?)...
I even swapped the two MR2920's over - it made no difference.

Any ideas? Has T501 gone bad?

[paulstef]

Re: usp440m-42lp issue AKAI PDP42Z5TA

How does the voltage on C505 look like (ripple)?

As far as I remember the NCP is 100kHz fixed frequency. Can you verify if it's really 100kHz?
In that circuit it would seem the only way to have lower voltage on the MR supply winding while the feedback voltage is ok (5V) would be too much load on the MR winding...