Possible bad transformer.

[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?

[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.

[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.

[joshnz]

Re: Possible bad transformer.

Good work Th3_uN1Qu3

[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.

[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.

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.

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.

[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

[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.

[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.

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.

[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 .

[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)