Maxent MX-42HPM20 Won't turn on

**el3ctroded** · 06-05-2010, 10:22 AM

Re: Maxent MX-42HPM20 Won't turn on

I forced the power supply to power up with the new Ysus board and there was some humming, but nothing lit up... All fuses are Ok.

**el3ctroded** · 06-05-2010, 04:33 PM

Re: Maxent MX-42HPM20 Won't turn on

I have now tested every cap for ESR on the board. I found 2 that had high ESR on the main logic board. Replaced them.

I also found 1 that had higher ESR than I'm used to on low voltage stuff. It's on the power supply but is a 450V 2.2uF... It read 1.8ohms. Problem is I don't know acceptable ESR range for high voltage caps?

I found 2 caps on the Zsus board that are 400V 10uF and are part of a small daughter card. They read ~2.7ohms. One of the same caps elsewhere reads 1.2ohms. But that variance could be due to the circuitry... Not sure what to do here. Should I take a couple out and measure them out of circuit?

I've also tested every diode and transistors on the power supply board, no problems.
I've tested all transistors and Diodes I can get to on the Ysus and Zsus boards, no problems. Some I can't get to because of the heatsink on that big chip that the sus boards have.

No shorts between power and ground anywhere.

Any comments? Please!
Thanks!

**PlainBill** · 06-05-2010, 04:43 PM

Re: Maxent MX-42HPM20 Won't turn on

"It's dead, Jim."

I'm going to use the terminology I learned from Philips; names can change from manufacturer to manufacturer, the concepts remain the same.

When the TV is plugged in the standby power supply comes up and supplies 5V standby to the standby processor on the small signal board. The standby processor monitors a number of things, chiefly the signals from the IR receiver and the power button (if any). When it detects a 'Turn on' signal it sends the 'Relay on' signal high, then begins monitoring the health of the power supply, etc.

Since you are not getting and signal to drive the main power supply on, the conclusion is the standby processor either is not working, or has detected a fatal fault.

PlainBill

**el3ctroded** · 06-05-2010, 05:36 PM

Re: Maxent MX-42HPM20 Won't turn on

Yes, it's dead, but can it live again?

Here's something I neglected to mention:
When I first plug it in, I do get +5v standby from the power supply, and then the logic board immediately sends +5 to the Rly_On and Vs_On signal lines. These lines go the the power supply which should cause it to power up, but before it happens, the Rly_On and Vs_On signal lines go low.

Seems sort of like an initial power-up test that is failing?

The status light is then red on the front panel. Now I press the power button and the LED goes green but nothing happens.

Fatal fault indeed! But what kind of fatal fault? The power-up test resulted in the fault being detected, right?

What I don't get is why the power supply control lines from the logic board went low before the 180V on Vs or 50V on Va ever is generated? Would that point to a control board issue? Or does it point to a feedback issue from the Ysus or Zsus or some other board?

How does this screen get it's lighting? I don't see an inverter board or any high voltage lines for CCFLs? Is it using LEDs for the backlighting? Is there something there that likely failed?

There's another board covered in a heatsink all across the bottom of this screen. I haven't taken it apart yet because it's so deep in there and I've no idea what it's for...

I've got a bunch of questions in there...

**el3ctroded** · 06-05-2010, 07:08 PM

Re: Maxent MX-42HPM20 Won't turn on

I just found a bad optoisolator in the power supply. It's marked NEC 2571 SJ605. I can't find info on it... I'd appreciate a datasheet! Otherwise think a PC817X will work?

**el3ctroded** · 06-05-2010, 07:30 PM

Re: Maxent MX-42HPM20 Won't turn on

Found it, it's pn is PS2571.

**el3ctroded** · 06-05-2010, 07:43 PM

Re: Maxent MX-42HPM20 Won't turn on

Main difference between the PC817X and the PS2571 is the current transfer ratio is a bit higher on the PS2571... I'm gonna try the PC817X and let you know how it goes. The opto was designated PC181 on the psu PCB.

**el3ctroded** · 06-05-2010, 09:17 PM

Re: Maxent MX-42HPM20 Won't turn on

Joy! Sort-of. I can't call it fixed, cause it isn't. I just know it isn't completely dead!

The power supply has a switch that says Normal and Auto. It's supposed to be at Normal. It will not work there. However I just tried moving the switch to Auto. When I do this and the press the power button the screen lights up and I get the OSD stuff, so it's working, but this isn't a permanent solution because in Auto the powersupply keeps power supplied to the Ysus board. The LCD also constantly glows dimly.

So where do I look now? What would cause the PSU to act like that?

**el3ctroded** · 06-05-2010, 09:36 PM

Re: Maxent MX-42HPM20 Won't turn on

Well, I sort of know the answer: Some feedback circuit is faulty or the controller that acts on the feedback is faulty. The question really is: How do I find the faulty feedback circuit and/or faulty control circuit?

**el3ctroded** · 06-06-2010, 08:30 PM

Re: Maxent MX-42HPM20 Won't turn on

So to reiterate, I know the screen works. It comes up and shows picture. Problem is it will only do this with the PSU switch in Auto, and when in that position the LCD always, even when it should be off, is lit up. In Normal mode, where the PSU switch should be, the LCD will not come on at all, which I think points to a faulty control circuit or faulty feedback circuit. Am I likely right?

I really need help here. Not all the small signal lines are labeled, and I don't know where all the control circuit feedbacks are coming from/going to which lets the main logic board determine if it should power up the PSU or not. If someone could explain the typical feedbacks seen that allow the main logic board determine if a fault has occurred, it would _really_ help!

Thanks!

**el3ctroded** · 06-06-2010, 09:26 PM

Re: Maxent MX-42HPM20 Won't turn on

The powersupply is the same as this:
http://cgi.ebay.com/Maxent-POWER-SUP...QQcmdZViewItem

**crtfool** · 06-07-2010, 02:35 PM

Re: Maxent MX-42HPM20 Won't turn on

el3ctroded said:
"How does this screen get it's lighting? I don't see an inverter board or any high voltage lines for CCFLs? Is it using LEDs for the backlighting? Is there something there that likely failed?"

You are working on a PLASMA Monitor - NOT LCD. It is a totally different animal - there are no CCFL lamps or LED backlight. It uses a Plasma screen - read the following:

http://en.wikipedia.org/wiki/Plasma_display

**el3ctroded** · 06-07-2010, 03:49 PM

Re: Maxent MX-42HPM20 Won't turn on

Ok. Thanks. I feel stupid now. LOL. Also explains why I'm not getting much help: Makes me look stupid... I'd just never bothered to understand the difference b/t the two. If a transistor or cap goes bad, it doesn't care what type of panel it is, and that's how I've been dealing with a few tv's I've been messing around with. Just looking for bad parts.

In any event, this TV has a psu that won't kick in b/c of feedback or control circuitry, and I can't seem to locate the fault.

Thanks

**el3ctroded** · 06-09-2010, 08:03 AM

Re: Maxent MX-42HPM20 Won't turn on

Should I just buy the powersupply? How likely do you think this would be to fix it?

**crtfool** · 06-09-2010, 01:31 PM

Re: Maxent MX-42HPM20 Won't turn on

It definitely sounds like your monitor has / had more issues than the power supply, especially if the Y-SUS board was bad. What made it go bad? Did the power supply damage it and any other boards? If you are SURE that everything else is working, then the price for that power supply is reasonable, in my opinion.

How old is the monitor? Did you buy it new, or acquire it used? Was it working well before it died? How attached to it are you? All very logical and reasonable questions!

I myself am trying to decide if it is smart to spend approximately $200 for a replacement power supply for a 7 year old 32" Sony Plasma TV that a friend gave to me. I have never seen the TV working, and do not know what condition the Plasma screen is in - they can have burn-in and degraded brightness issues. My friend never saw it working either because someone gave him the TV - he told me that the TV works but no screen. This is a known defect caused by a defective power supply - and someone already tried to repair it, and very poorly too!! I am having great difficulty getting a schematic diagram and a parts list for the power supply - it is an OEM purchased by Sony, and they do not include any documentation for it in the service manual. I would attempt repairing it myself if I could acquire all the proper parts.

This is the first Plasma TV that I have ever worked on. It sounds like you are new to Plasma repair also. How much are you willing to spend for the learning experience?

**el3ctroded** · 06-09-2010, 02:15 PM

Re: Maxent MX-42HPM20 Won't turn on

It's not mine, it's a friends, he's paying for parts. Even if I have to replace several boards, it's still cheaper than replacing the screen / having our local repair shop do it.

Well, I've already spent $110 on the Ysus. Might as well do $120 for a used good psu.

I think the Ysus board somehow killed the psu, or vice versa. I know the panel display looks good due to the Normal/Auto switch

I actually ran the screen for several hours today with the switch in the Auto position with no problem. So I KNOW it's just a control and/or feedback circuit problem.

My main concern is that the issue is not the PSU but the circuitry on the main board that tells the psu to kick in, or the circuitry on the main board that handles the feedback from other boards, or in the feedback circuitry coming from other boards.

It it was a simply psu failure, it would be easier to know what to do... I wish I could figure out what the auto/normal switch is telling the psu to bypass.

**el3ctroded** · 06-09-2010, 03:25 PM

Re: Maxent MX-42HPM20 Won't turn on

I pulled the PSU off again. I traced and traced connections. It looks like the switch is simply bypassing the Relay_on and Vs_on inputs. Therefore, the psu seems to be good, and suggests that one of the following situations exists:
1) There is a slight problem with voltages being output from the PSU and something on a board other than the PSU is monitoring voltage output of the PSU and causing the main board to signal and not turn on the psu.
2) Some control circuit is damaged on some board other than the PSU.

Often the simplest explanation is true, so I'm looking into #2.

**el3ctroded** · 06-09-2010, 05:01 PM

Re: Maxent MX-42HPM20 Won't turn on

Traced those control lines on the main logic board to a 74hct04 inverter. Maybe it's bad...

**el3ctroded** · 06-09-2010, 06:04 PM

Re: Maxent MX-42HPM20 Won't turn on

Nope, not a bad inverter.

I'm really stumped. So I dragged out the scope. There's a bit of noise (800-900mV) on the 5V outputs, and about the same on all other outputs.

All other voltage rails are to spec. Getting clean 185V, 58V, 24V, 12V.

So do you think that's too much noise on the 5V rail? There's a filter on the main logic board that drops noise down to about 500-600mV, but that still seems high to me. What do you think?

Maxent MX-42HPM20 Won't turn on

Maxent MX-42HPM20 Won't turn on

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