Possible bad transformer.

[Behemot]

Re: Possible bad transformer.

Rod Elliott also has more than a few really well-written articles on a great number of electronics- and audio-related topics ( http://www.sound.westhost.com/articles.htm )

Indeed! I wonder if he is registrated here and what is his nick? There are few ppl here who shoed similar knowledge (inluding similar examples of things)

[momaka]

Re: Possible bad transformer.

And... we have ourselves a transformer. I documented every step with pictures for your viewing pleasure.

I always wondered how you do those traffos. Very nice pictorial documentation!
Big thank you .

[Th3_uN1Qu3]

Re: Possible bad transformer.

Running it at a more linear point perhaps?? Smooth voltage changes, rather than fully saturated/cutoff.

I don't think so. Rather, better noise immunity. I don't really dig the layout of the primary.

What a funny unit- those cheapies definately have their 'personalities' and quirks, as you've found.


"Cheers,"
-Paul

Funny it was indeed, but i'm not in a hurry to find another one like this... I brought it in the hallway and plugged the cable TV into it, and i must say, it does a pretty good job. DVD drive works fine too, tested video output via SCART and it's pristine, so my job is done here. Btw, the FM radio does NOT have RDS.

[kaboom]

Re: Possible bad transformer.

Nice job, as usual.

I like how there are TO-92 'giants' among all the SMTs.

I also stabilized the feedback in the thing, which stopped the annoying whine coming from the transformer. After trying almost every trick in the book with no success, i remembered that the datasheet of the UC3842 says you can connect a resistor from Vref to Comp to have more current flowing thru the opto's transistor. A 1k did the trick here.

Running it at a more linear point perhaps?? Smooth voltage changes, rather than fully saturated/cutoff.

What a funny unit- those cheapies definately have their 'personalities' and quirks, as you've found.


"Cheers,"
-Paul

[Th3_uN1Qu3]

Re: Possible bad transformer.

I wouldn't be surprised if more of the circuits in that thing are weird.

Sure thang they are. The 12v winding with the thicker wire is for the tuner, which uses less power than the LCD which is on the thinner one! Also, the "12v" drops to 10.8v when both LCD and tuner are on. I kinda suspected that, since 5v has 3 turns, and 12v only 6... It should have been 7. I know why they did this tho - to make it efficient and cheap, they didn't bother with a minimum load resistor on 12v, and being a flyback, the voltage goes pretty high when neither LCD and tuner are operating. I measured up to 15v as the thing starts up, and 13.4v when the lid is closed and the LCD backlight is off. I can see why that tantalum blew. I tested the one i put in up to 17v, so it should be okay, and anyway, it's all fused now.

Anyway... I stopped bothering. Both diodes on the 5v got a 2A automotive fuse each, same for each 12v rail. Job done. I'll be doing the final board cleanup today, putting it all back together, testing the DVD drive, and calling the owner to pick it up tomorrow.

I also stabilized the feedback in the thing, which stopped the annoying whine coming from the transformer. After trying almost every trick in the book with no success, i remembered that the datasheet of the UC3842 says you can connect a resistor from Vref to Comp to have more current flowing thru the opto's transistor. A 1k did the trick here.

[sam_sam_sam]

Re: Possible bad transformer.

And... we have ourselves a transformer. I documented every step with pictures for your viewing pleasure.

Dropped it in... and IT WORKS! Everything's working fine, nothing gets hot and voltages are within spec. The only one i screwed up was the one for the VFD filaments - it ended up around 17v and lit the filaments red, and some segments were showing even when they were supposed to be off.

A 100 ohm resistor in series with the diode going to the VFD dropped that voltage to 8.15 volts, and the VFD is now working great. Sure, it's a cheap hack, but as long as it avoids taking that transformer apart again... i'll take it.

I like the way you did this repair you inspire me to want to try this when I need to make a repair like this thanks for the info

[ben7]

Re: Possible bad transformer.

That's why i'm replacing it with a 0.5W one tomorrow. There's a couple more resistors that i bodged, including the startup resistor for the controller, so i have to hit the store anyway.

Now that i know all voltages, i have to say that the PSU design seems even more idiotic than it looked at first glance.

There are two separate 12v windings on the transformer, with their own diodes and everything, one feeding the LCD backlight (the one wound with only one strand of 0.3mm wire), and one feeding the tuner, which was wound with two strands. I don't yet know which one of them the DVD motors are on, we'll see.
Also there's yet another separate winding for the VFD filaments (the one i messed up and wired to the wrong place), when they could just have used a regulator from the 12v, or just a plain simple resistor.

I'll likely parallel the 12v windings - there's no real reason for them to be that way.

LOL!!!

They probably thought that would reduce noise in the tuner.

I wouldn't be surprised if more of the circuits in that thing are weird.

[Th3_uN1Qu3]

Re: Possible bad transformer.

BTW, that 100 ohm resistor will probably burn up eventually.

That's why i'm replacing it with a 0.5W one tomorrow. There's a couple more resistors that i bodged, including the startup resistor for the controller, so i have to hit the store anyway.

Now that i know all voltages, i have to say that this PSU design seems even more idiotic than it looked at first glance.

There are two separate 12v windings on the transformer, with their own diodes and caps and pi filters and everything, one feeding the LCD backlight (the one wound with only one strand of 0.3mm wire, which burned due to the shorted tantalum cap), and one feeding the tuner, which was wound with two strands. PS. I used 0.2mm wire as i have like 100 meters of it on a spool, so the number of strands is different - that's of no concern to you, i did the math. I don't yet know which one of these rails the DVD motors are on, we'll see.
Also there's yet another separate winding for the VFD filaments (the one i messed up and wired to the wrong place), when they could just have used a regulator from the 12v, or just a plain simple resistor.

I'll likely parallel the 12v windings - there's no real reason for them to be that way.

[ben7]

Re: Possible bad transformer.

And... we have ourselves a transformer. I documented every step with pictures for your viewing pleasure.

Dropped it in... and IT WORKS! Everything's working fine, nothing gets hot and voltages are within spec. The only one i screwed up was the one for the VFD filaments - it ended up around 17v and lit the filaments red, and some segments were showing even when they were supposed to be off.

A 100 ohm resistor in series with the diode going to the VFD dropped that voltage to 8.15 volts, and the VFD is now working great. Sure, it's a cheap hack, but as long as it avoids taking that transformer apart again... i'll take it.

I have some parts coming in the mail including some insulated TO-220 mosfets (if you were wondering what all the kapton tape around the primary heatsink is, it's because i use a non-isolated tab part for testing right now). I'm waiting for them to arrive, and then i'm fitting the FET, fusing all rails and wrapping this thing up.

Sick dude!

BTW, that 100 ohm resistor will probably burn up eventually.

[Th3_uN1Qu3]

Re: Possible bad transformer.

And... we have ourselves a transformer. I documented every step with pictures for your viewing pleasure.

Dropped it in... and IT WORKS! Everything's working fine, nothing gets hot and voltages are within spec. The only one i screwed up was the one for the VFD filaments - it ended up around 17v and lit the filaments red, and some segments were showing even when they were supposed to be off.

A 100 ohm resistor in series with the diode going to the VFD dropped that voltage to 8.15 volts, and the VFD is now working great. Sure, it's a cheap hack, but as long as it avoids taking that transformer apart again... i'll take it.

I have some parts coming in the mail including some insulated TO-220 mosfets (if you were wondering what all the kapton tape around the primary heatsink is, it's because i use a non-isolated tab part for testing right now). I'm waiting for them to arrive, and then i'm fitting the FET, fusing all rails and wrapping this thing up.

[joshnz]

Re: Possible bad transformer.

Good work Th3_uN1Qu3

[tom66]

Re: Possible bad transformer.

I have a Creative sound card with some WINCAPS. They are just used for bypassing and coupling, nothing much demanding.

[Th3_uN1Qu3]

Re: Possible bad transformer.

Have Wincap capacitors on a soundcard that's almost as old as i am. They're still in good shape. But "Maxcap"... sounds like "Maxpower" PSUs.

[ben7]

Re: Possible bad transformer.

Someone brought a car DVD player at my old work last week and I took the task of opening it. My friends there were wondering why I was giggling the whole time. Well, if I had told you the cap brands I saw in it, you'd smile a little as well. I remember two of the brands... one was "Wincap", the other "Maxcap". I'm still chuckling a bit when I say those names. Helps if you say it in a funny voice with the right intonation too .

LOL, were they all bulged?

[momaka]

Re: Possible bad transformer.

On this forum, i don't think so. When nobody replies to my threads, i take it that maybe it's a new situation for you as well, and you don't want to say anything stupid.

That would be people like me .
I do often bookmark and refer back to your threads for knowledge, though .

It's certainly better than having all different noname brands scattered throughout the whole board.

True.
Someone brought a car DVD player at my old work last week and I took the task of opening it. My friends there were wondering why I was giggling the whole time. Well, if I had told you the cap brands I saw in it, you'd smile a little as well. I remember two of the brands... one was "Wincap", the other "Maxcap". I'm still chuckling a bit when I say those names. Helps if you say it in a funny voice with the right intonation too .

[Th3_uN1Qu3]

Re: Possible bad transformer.

LOL! "Cheertek" !!!

Apparently they bought Winbond's flat panel division in 2006 (never knew Winbond made displays), but also they don't apparently, you know, uh, exist. At least i can be "cheer"ful that it's still alive. Hmm. They appear to have been bought out by Novatek in 2007. That explains why i couldn't find much about them or their products.

And its amazing how many people buy these super cheap things, and yet still think that they should last a long time!

Don't think it was exactly cheap. Also, the menu is in French, so i assume this has been bought or imported from France. It likely cost a pretty penny.

Tbh, if i saw this in a thrift store, i too would think it's just a cheap gimmick, but actually, i'm pretty amazed at the engineering that went into this thing (apart from the stupid PSU of course). This is all on one huge board, but everything's divided nicely into sections, clearly marked on the silkscreen. The board is all double sided, and has plated thru-hole. Too bad it doesn't give any clue to who actually made it.

As for the power supply, it ain't THAT bad actually. Apart from the obvious omissions which brought its demise, there are pi filters on all outputs, and most caps are Aishi, with a few by Cheng. Aishi aren't bad, i'd compare them with the likes of Samxon. It's certainly better than having all different noname brands scattered throughout the whole board.

[ben7]

Re: Possible bad transformer.

LOL! "Cheertek" !!!

We should start a thread about dangerous chinese products

And its amazing how many people buy these super cheap things, and yet still think that they should last a long time!

[Th3_uN1Qu3]

Re: Possible bad transformer.

It's kind of incredible how bodged together these multi-function devices are. For example, why have both a VFD, and an LCD? Why not simply use the LCD and embed an OSD for control?

It *does* have an OSD. Actually, the VFD is pretty useless - the source selection for example, only appears on the LCD! The VFD just sits there showing "MENU" at that time. It only shows progress in the current track/video, channel and frequency for TV and FM (possibly RDS as well, but the FM tuner is screwed into the case, so i disconnected the cable and left it there, will test once it's all back together again), whether a disc is inserted in the DVD drive, "NO USB" if you attempt to select USB without a device inserted... and that's about it. Everything else shows on the LCD screen only.

Many CE certifications require the power supply to remain safe when rail shorts occur so I wouldn't be surprised if that was the China Export edition of the CE certification.

You can bet on it. This is the worst power supply failure i have seen so far.

is there a way to make a continuous flyback?

Yes. But it complicates control loop compensation, hence why they are generally avoided. Recently there have been special ICs developed that work at the transition between continuous and discontinuous at all times, thus getting the best of both worlds. But y'know... they cost money.

[tom66]

Re: Possible bad transformer.

It's kind of incredible how bodged together these multi-function devices are. For example, why have both a VFD, and an LCD? Why not simply use the LCD and embed an OSD for control? It's just like that DVD player I repaired which used a massive transformer for the audio and then a switching supply for the rest, why they didn't just buck down the 30V from the transfomer I don't know.

Interesting failure mode. Many CE certifications require the power supply to remain safe when rail shorts occur so I wouldn't be surprised if that was the China Export edition of the CE certification.

Also one thing I've noticed with flybacks is the capacitor ripple current is often substantially high due to them operating discontinuously - is there a way to make a continuous flyback?

[Th3_uN1Qu3]

Re: Possible bad transformer.

Unwound the transformer, and it confirms my findings. Tantalum cap on 12v rail went short when no one was watching. 12v rail had a single 0.3mm wire - too thin to trigger the primary overcurrent protection. Insulation melted and it shorted with the 5v rail. Switching transistor keeps pumping more current into the primary in an attempt to keep the 5v regulated, auxiliary voltage rises, and the whole thing goes boom.

No visible damage to the primary tho, but judging by the inductance it was definitely shorted as well. The way it was wound was atrocious - each half primary has two layers (it's just 40 turns in total, 20 on the inside, 20 on the ouside, but for some reason they only wound on the middle of the bobbin and left a lot of space on the top and bottom), and there was no insulating tape between the two layers of each half primary. That's a sure recipe for disaster. Worse, the melted windings went thru the tape and were less than 1mm apart from shorting to the primary. Imagine the destruction and the safety risk that would have been.

Now let's mention what could have been done to avoid this disaster. If a zener would have been used on the aux winding, as soon as the aux rose beyond limits it would have shorted, brought the controller Vcc down, and a lot of the damage would have been avoided. But simpler than that: A plain, simple FUSE on the 12v rail would have made this repair a breeze. Tantalum shorts, fuse blows, 12v goes out, everything else keeps working, easy repair. But no. They were too inept to do that, and their idiocy resulted in this catastrophic failure. I will be winding the 12v with two strands of wire instead of just one, and also fusing all rails and adding a zener to the PWM controller supply.

Sorry, no pics, i was too busy noting down the starts and ends and number of turns on each winding. At least they used 0.3mm wire throughout, with several in parallel when higher current was required, so it's gonna be cheap to rewind.

Now let's talk a bit about the hardware used in this thing.

Main processor is a Cheertek CT219RZ-LF, which i didn't find any data on. It's in a rectangular TQFP package either 100 or 144 pins, can't be bothered to count them and they aren't numbered on the PCB.
Next to it is a CT901A-LF which could be the DSP/Dolby Digital decoder (thanks Chris for the suggestion), again no data on it.
In the same area there's a Winbond W9864G6GH-6 which is a 8Mbyte SDRAM chip, and a Eon EN25F80-100HCP 1Mbyte Flash memory, and a AT5669H which is the motor controller for the DVD drive.
There's also a bunch of vregs, some digital logic chips and stuff like that.

Next up we have a Princeton PT2314 4-channel input audio processor, a couple more logic ICs, and an Afa AF9013AS DVB-T demodulator, in charge of the TV tuner.
VFD display driver is again by Princeton, a PT6311.
Audio amp is a class D, 25W/ch TI TPA3123D2 which is a pretty amazing chip - the supporting caps and inductors take like 10 times the space of the IC itself!
LCD controller is a MStar MST720A-LF which is a component/s-video input chip designed specifically for car LCD applications. An analog input LCD, just as i suspected. That explains the interference i'm getting on the screen. It's gotten better since i put a proper tantalum instead of the lytic i initially tried in place of the blown tant, but the other tantalums may need replacement as well.