CHIMEI CMV T38D - Buttons stopped working

[Agent24]

Re: CHIMEI CMV T38D - Buttons stopped working

So, on the chance that jetadm123 was right, I checked the cable for shorts - nothing wrong.

I figured this is time to look in depth at the button circuitry on the logic board. I tried to find a service manual for the monitor, but failed.

Then I remembered that the same PSU was used on some other monitors as mentioned by mockingbird.

I couldn't find one for the Viewsonic, and some photos on Google showed the logic board was different anyway

However, With the Acer AL2017 things got more interesting - one manual I got showed internal pictures pretty much identical to mine. Also, the logic board is listed as exactly the same part number! (A190A2-H-S1) ... but there are NO SCHEMATICS!!

A second manual had schematics, but they were wrong, and they actually appeared to correspond to the Viewsonic monitor (same numbers) Besides, they were not in vector format and all component values were unreadable



So, I figured I might as well try to draw one myself (for the button circuitry - it is attached below)

The schematic I have drawn is replicated for each button - each has this same circuit (but going to different pins on the OSD Controller IC, of course)

Because there are 6 Buttons, Rx, Lx, and Cx are each made up from 2x 4-way arrays.

While tracing the circuit I discovered that all resistors measured 10K Ohm (as they should have) except for the one related to the Auto Adjust button - it was a little low at 9.12k.

Aha! I thought, well - it doesn't seem low enough to cause a problem, but I desoldered it and checked it anyway - 10k, no worries

But why would it measure lower in-circuit?

Back to the diagram. If i've drawn it and understand it correctly, only 3 other things could be the problem. The inductor, the capacitor or the IC. Assuming the IC is fine (because replacement is probably going to be impossible), the problem must lie with either the inductor, or the capacitor.

The inductors all show less than 1 Ohm (taken in-circuit) (so they're probably not open-circuit? - but they may be short)

The capacitors are more interesting (taken in-circuit):
CP7 measures >2000k, 3.63k, >2000k, >2000k (relates to the Auto Adjust button!)
CP8 measures (rises to) 120K, >2000k, 1.13k, (rises to) 120k

I would have guessed all the capacitors should show infinity, so perhaps two of them are short, and two are leaky? I am just guessing there, though.

I guess if they were, I would see issues with the other buttons though. But they all function (at least in activating the Auto Adjust feature) and they all show 3.1v instead of 0.86v for the Auto Adjust button.

I know I should test them out of circuit but I don't really want to risk any damage as I don't know what I need to buy for replacements if that happened.

But of course, if they are broken already, I still don't know what to buy


Is there anything more I can do now?

[PlainBill]

Re: CHIMEI CMV T38D - Buttons stopped working

So, on the chance that jetadm123 was right, I checked the cable for shorts - nothing wrong.

I figured this is time to look in depth at the button circuitry on the logic board. I tried to find a service manual for the monitor, but failed.

Then I remembered that the same PSU was used on some other monitors as mentioned by mockingbird.

I couldn't find one for the Viewsonic, and some photos on Google showed the logic board was different anyway

However, With the Acer AL2017 things got more interesting - one manual I got showed internal pictures pretty much identical to mine. Also, the logic board is listed as exactly the same part number! (A190A2-H-S1) ... but there are NO SCHEMATICS!!

A second manual had schematics, but they were wrong, and they actually appeared to correspond to the Viewsonic monitor (same numbers) Besides, they were not in vector format and all component values were unreadable



So, I figured I might as well try to draw one myself (for the button circuitry - it is attached below)

The schematic I have drawn is replicated for each button - each has this same circuit (but going to different pins on the OSD Controller IC, of course)

Because there are 6 Buttons, Rx, Lx, and Cx are each made up from 2x 4-way arrays.

While tracing the circuit I discovered that all resistors measured 10K Ohm (as they should have) except for the one related to the Auto Adjust button - it was a little low at 9.12k.

Aha! I thought, well - it doesn't seem low enough to cause a problem, but I desoldered it and checked it anyway - 10k, no worries

But why would it measure lower in-circuit?

Back to the diagram. If i've drawn it and understand it correctly, only 3 other things could be the problem. The inductor, the capacitor or the IC. Assuming the IC is fine (because replacement is probably going to be impossible), the problem must lie with either the inductor, or the capacitor.

The inductors all show less than 1 Ohm (taken in-circuit) (so they're probably not open-circuit? - but they may be short)

The capacitors are more interesting (taken in-circuit):
CP7 measures >2000k, 3.63k, >2000k, >2000k (relates to the Auto Adjust button!)
CP8 measures (rises to) 120K, >2000k, 1.13k, (rises to) 120k

I would have guessed all the capacitors should show infinity, so perhaps two of them are short, and two are leaky? I am just guessing there, though.

I guess if they were, I would see issues with the other buttons though. But they all function (at least in activating the Auto Adjust feature) and they all show 3.1v instead of 0.86v for the Auto Adjust button.

I know I should test them out of circuit but I don't really want to risk any damage as I don't know what I need to buy for replacements if that happened.

But of course, if they are broken already, I still don't know what to buy


Is there anything more I can do now?

Congratulations on a good job of analzsing the problem. It's a welcome change from the "my neighbor replaced a capacitor on his monitor and fixed it, what capacitor should I replace on my monitor" question we deal with all the time.

Out of respect for the effort YOU put in, I did a little research and checked the schematics for several monitors. I hit paydirt on the schematic for a Belinea 101910 monitor. It has the identical circuit, and calls for 0.1uF, 25V chip capacitors in the 0603 package. As an example, they are available from Digikey as catalog number 490-1575-1-ND, part number GRM188F51E104ZA01D. However, you can't order just 1, you have to buy 10 at $0.018 each.

PlainBill

[jetadm123]

Re: CHIMEI CMV T38D - Buttons stopped working

Great work by PlainBill for finding the schematic of an obscur monitor with one from another obscur monitor!

Agent24, I take my beanie cap off to you for your persistence in troubleshooting the problem. Since you're in New Zealand, ordering from Digikey might be a bit pricey shipping wise. I think Farnell might be a better fit. If you find a good/cheap place to order from, please share as it will be a good future reference.

[Agent24]

Re: CHIMEI CMV T38D - Buttons stopped working

Wow! Thank you very much for that, PlainBill

Now at least I know what I need to get - but it doesn't look like it will be easy

The Belinea monitor uses a separate capacitor for each button, whereas my Chimei uses 2x 4-way capacitor arrays in a 1206 size package.

Neither Farnell or RS-Components appear to sell these (not in 1206 size, anyway)

I suppose I could use four single capacitors, but in that case I think I'll need a bit more practice before I attempt it...

[PlainBill]

Re: CHIMEI CMV T38D - Buttons stopped working

Wow! Thank you very much for that, PlainBill

Now at least I know what I need to get - but it doesn't look like it will be easy

The Belinea monitor uses a separate capacitor for each button, whereas my Chimei uses 2x 4-way capacitor arrays in a 1206 size package.

Neither Farnell or RS-Components appear to sell these (not in 1206 size, anyway)

I suppose I could use four single capacitors, but in that case I think I'll need a bit more practice before I attempt it...

Darn!!! In the USA Mouser has the array at $.62 each.

PlainBill

[jetadm123]

Re: CHIMEI CMV T38D - Buttons stopped working

Wow! Thank you very much for that, PlainBill

Now at least I know what I need to get - but it doesn't look like it will be easy

The Belinea monitor uses a separate capacitor for each button, whereas my Chimei uses 2x 4-way capacitor arrays in a 1206 size package.

Neither Farnell or RS-Components appear to sell these (not in 1206 size, anyway)

I suppose I could use four single capacitors, but in that case I think I'll need a bit more practice before I attempt it...

How about a photo of the component side of the pushbutton board. Maybe someone can suggest a workaround to your cap problem.

[Toasty]

Re: CHIMEI CMV T38D - Buttons stopped working

<snip>
UPDATE: Removed the button, still the same problem. (All buttons only perform Auto Adjust feature)
Still reading 0.86v on the pads where the button was, and 3.1v on the other buttons.

What is voltage on (-) button?

If 3.1v there and low on (Auto), then either the Auto button trace is bad (always on) or the resistor that feeds (Auto) pad has gone high or open.

Toast

FYI:
(Auto) and (-) are same circuit, resistor makes them logic high or low.
(Menu) and (+) are same circuit, resistor makes them logic high or low.
(Power) has no resistor in line.

[Agent24]

Re: CHIMEI CMV T38D - Buttons stopped working

How about a photo of the component side of the pushbutton board. Maybe someone can suggest a workaround to your cap problem.

On the button board, there are pads for diodes across the buttons, which aren't used.
Putting the capacitor there instead would fit the schematic, but isn't the point of them being on the logic board that they're close to the ICs? (For decoupling or whatever they do to work effectively)

What is voltage on (-) button?

If 3.1v there and low on (Auto), then either the Auto button trace is bad (always on) or the resistor that feeds (Auto) pad has gone high or open.

Toast

FYI:
(Auto) and (-) are same circuit, resistor makes them logic high or low.
(Menu) and (+) are same circuit, resistor makes them logic high or low.
(Power) has no resistor in line.

The resistor was OK when checked out of circuit. It seems the capacitor is shorted and appears to the IC as if the button is held down.

[jetadm123]

Re: CHIMEI CMV T38D - Buttons stopped working

[QUOTE=Agent24;138201]On the button board, there are pads for diodes across the buttons, which aren't used.
Putting the capacitor there instead would fit the schematic, but isn't the point of them being on the logic board that they're close to the ICs? (For decoupling or whatever they do to work effectively)

QUOTE]

Oops! My mistake, I thought the cap array was on the back of the button board.

[Agent24]

Re: CHIMEI CMV T38D - Buttons stopped working

Good news! It would appear that I was wrong about not being able to buy capacitor arrays

After buying 50 capacitors with the idea of soldering in 4 single ones, I found out that they were actually too wide (good thing they only cost me $1.15)

I then had another search on RS and Farnell, hoping to find some thinner capacitors, when I noticed that there were some arrays marked with 0612 size....Yeah, that's right, 1206/0612 - same thing! My brain mustn't have been working properly the first time!

Farnell is a lot cheaper with a MOQ of 5 rather than 25 from RS, but since they don't offer free shipping I'll have to put them in with my next order

So this story will continue, just not right away... we shall see.

[Agent24]

Re: CHIMEI CMV T38D - Buttons stopped working

Looks like another thread I never finished... well, for anyone still tuned in, here's the end of the story – be prepared for some reading!:


Before I could get the new capacitor array, the thing died completely. I was using it and it just shut off, and wouldn't come back on again. Doubting the repair to be cost-effective and my own skills at SMD soldering at the time (It had been hard enough putting the resistor array back with the iron I had then) I put it aside and forgot about it. Then recently, I decided to tackle it again.

Firstly, I wasn't convinced that the capacitor array I had ordered originally was correct, as I had never had a schematic. I searched the logic board model number and came up with a Viewsonic service manual that had the exact same board. The capacitor arrays were specified as 100pF, not 0.1uF as was originally guessed.

I ordered the correct ones (third time's the charm!) from Farnell with my next order, figuring they were cheap so no loss if I couldn't repair the no-power fault, and then took a look at what had caused the total shutdown.


I started by measuring the voltages from the PSU, which is supposed to supply 12v and 5v to the logic board. The 12v rail was there, but to my surprise, the 5v rail was bouncing up and down around 2.4v!

I tried with the panel disconnected “just in case” but the symptoms did not change. At least I knew the panel was probably OK.

I thought something might have shorted on the logic board, but nothing was hot, and a resistance test across the 5v rail on the logic board showed nothing untoward, so I decided to take a look at the PSU instead.

I tried it without a load, and got fluctuating voltages on both rails and ticking, but I didn't much notice of that since a previously repaired LCD monitor PSU also power-cycled without a load. I then cobbled together some power resistors and hooked them up for a dummy load, and got even less power out of the thing than I had before.

As a last test, I hooked the logic board up to an ATX PSU and fired it up – and the power LED shone bright green!


So, the PSU was looking like the problem. I wasn't sure where to look first, but I figured that to get anything out of it, the primary side had to be OK, so I looked on the secondary first. There were a couple of fuses and both tested fine. Then I went for the rectifier diodes as they were the next easiest thing to check and struck gold – the rectifier for the inverter's power rail had one side dead shorted!

The rectifier was an SB20200FCT; a 20A, 200v unit with plastic body. I found Jaycar sold MBR20100CTs for $2.80, which seemed like a good choice, aside from the non-insulated body, but that was OK as I have enough insulating pads and washers pinched from old crappy PC PSUs

The 100v rating was questionable at half that of the original rectifier but I figure there is no way I was going to need more than, say, 50v for an inverter so it's probably OK. In any case, they probably used that part for lower power dissipation with the plastic body. If anyone knows different though, let me know and I'll change it!

I installed the new rectifier, and initially powered up the PSU by itself in case there was some other fault, to avoid blowing up anything else. All rails came up, 5v steady this time, and the new rectifier supplying 20v to the inverter section.


With the PSU issue out of the way, next up was the capacitor array. As previously found, one of the capacitors in this array was shorted or leaky or breaking down under load.

At the time, there was also the problem of my bad SMT soldering skills and equipment or lack thereof. However, now I have a hot air rework station plus a decent set of tips on a temperature-controlled soldering station and have had practice since then. At least with single components and the soldering iron.

But, I had tried arrays with the iron before and still had hard-to-deal-with bridges, so after watching this video: (https://www.youtube.com/watch?v=IgrEbJN8dcQ) I pulled out an old junk motherboard and had some more practice by removing resistor arrays and other SMDs and reinstalling them with the hot air and solder paste.

I still need a better solution to dispensing the paste, but I managed to get by with tweezers and a bit of dicking around... When I was convinced that I could do it, it was time for the real thing. The old capacitor array came off cleanly with the hot air and then I cleaned up the pads with the iron and solder wick.

After taking care to put a uniform but thin amount of paste on the pads (you really need a lot less solder on these arrays than with bigger parts) I brought the hot air in on slow speed, and reflowed the solder perfectly See the attached photo. (The new array is the white one) Also note the quality of the solder compared to my original job with a too-big iron!

But, “does it work?”, I hear you say – well, let the accompanying photos speak for themselves.
(And in case you were wondering, yes, the capacitor array was indeed 100pF – also see my method for dealing with measuring the damn thing!)


Thanks again to everyone who helped on this one. Sorry it took so long to be done with!

[ek2oo2]

Re: CHIMEI CMV T38D - Buttons stopped working

this thread gave me a hint. having random auto adjust at this client monitor at the moment.. thanks and congrats..

[Agent24]

Re: CHIMEI CMV T38D - Buttons stopped working

this thread gave me a hint. having random auto adjust at this client monitor at the moment.. thanks and congrats..

If it's intermittent I would suspect the button itself being faulty - leaky or shorted. I've seen bad buttons (leaky or high resistance eg: ~200 ohm when pushed) show up on several monitors\TVs now.