Electronics / Circuit Board Wizards?

[LIL EVO]

I'm working on a circuit board

http://lilevo.com/elec/board.jpg

What normally happens is two 220uf 25v capacitors leak (C14, C15), upper right, they are missing in the picture. They take out the diodes below them (D6, D7) and then a small 1ohm, 1watt resistor at location R38 burns up (left side).

I have replaced both capacitors and both diodes with IN4004's. The problem is that the R38 resistor still smokes and gets super hot.

The unit works fine other than the resistor issue. I threw on a big 3W, 1ohm resistor to make sure the display does work, which it does, but the resistor gets insanely hot after 30 seconds or so.

There has to be something else that I need to replace. I'm just at a loss as to what. Nothing else looks visibly damaged and I can't figure out what "feeds" this resistor.

Any ideas?

[SSLEVO]

What is the application?

[LIL EVO]

It drives a VFD.

It should be noted that if I install a 2ohm - 10ohm resistor it does not smoke, however the display is very dim.

The brightness of the VFD has something to do with the draw

One lead of the resistor goes to R37 which is three resistors to the right and down one, the furthest right lead on the green transistor above it, and the copper wound thing to the left of the transistor

The other lead also goes to the furthest right lead on the green transistor and R39, which is three resistors above R37

[Myles]

Could you get a pic of the board flipped over?

[Myles]

Also, what is the power at when it hits the resistor?

[LIL EVO]

http://lilevo.com/elec/100_1417.jpg

The arrows point to the two leads

One lead measures 10.6V, the other measures 9.0V

On another unit I have that works properly, one lead measures 10.6, the other 10.3

[BoosTT]

I would guess that the resistor that was in there from the start was a 1% tourance or better. The one you are replacing it with is likely 5% or higher. If the resistance is lower=more current=more heat. I bet your 1ohm resistor is really somewhere around 0.7 ohms.

Sucks that 1% tourance resistors are not cheap.

[LIL EVO]

The working unit has a new resistor, 1/2 watt, 1ohm (from ebay) and works fine. Does not get hot at all. The same resistor in this unit burns up instantly.

The only difference in the two units is that the caps leaked on the second unit and messed up the other stuff.

[BoosTT]

Are you sure that you put a 1ohm in there? Its hard to see the colors, but I looks like it may be something else.

Also- the new diodes could be higher quality and have a much lower voltage drop across them. 0.7v is the standard. Might want to check that out.

digikey.com is a good place to order electronics from.

[Myles]

I would start comparing voltage through out the line to see if there is any major differences. May help isolate a bad component.

[LIL EVO]

1ohm, 1/2 watt, 5%.

Problem isn't the resistor, it's something else before the resistor. I just don't know what it could be

[Myles]

What side is the 10.6v on

[LIL EVO]

The side closest to the edge of the board

[Myles]

Did you check/replace any resistors from that line?

[LIL EVO]

I checked some of them and they had identical values to the functioning unit

[Myles]

Wouldn't makes sense if the problem was down the line... Yeah this one is a interesting one.

[BoosTT]

Wouldn't makes sense if the problem was down the line... Yeah this one is a interesting one.

Yes the problem could be down the line. It could be a pull up or pull down (sinking vs. sourcing) circuit. Most likely it's sinking the display.

[indyzmike]

When you are measuring component values are you lifting one lead from the board?