Resistor exploded in my monitor

[sandord]

Today I soldered a new power switch in my Commodore 1084S-D2 monitor.

When I turned it on, there was a loud bang, rather large flash and a small mushroom cloud rose above R106 (2W, 1 Ohm).



I checked the fuse but oddly enough, it wasn't blown.

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As you can see, there are two blast holes. Also, C111 (50V, 1uF) looked bulged.

Here's a picture with the two parts indicated (I replaced both parts before I took the picture).

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I still have to measure the parts that are in the near vincinity to check whether they are still okay.
I think the blue resistor right above the cap looks a bit scorched, haven't measured it yet though.

I've checked the connections of the switch and nothing seems wrong.

I recapped the monitor a few months ago and it ran just fine afterwards. I think I used it a few weeks ago without problems.

But now I'm afraid to turn the monitor on, as the blast event will probably just repeat itself, blowing up the replacement parts too.

Does anyone have an idea what could be going on?

[PaulJ]

I'd look on the schematic and see if you can ascertain what the resisters function is.

[sandord]

R106 (A) sits between the mains rectifier (X) and the voltage regulator on the pin (B) that connects to the emitter (C) of one of the internal transistors (TR3) of the regulator. This same pin runs through the transformer (D) which in turn connects (E) to the rectifier (F).

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I measured 1.3 Ohm over pins 2 and 4 of the transformer.
I measured 0.3 Ohm over pins 8 and 10 of the regulator.

Does that sound ok?

My knowledge of electronics is quite limited so I don't exactly know what I'm saying here

[exxos]

You need to unsolder the chip and measure 8 & 10. First guess is the transistor has gone short and caused DC to the transformer burning up the resistor, pretty much like a fuse.

[sandord]

You need to unsolder the chip and measure 8 & 10. First guess is the transistor has gone short and caused DC to the transformer burning up the resistor, pretty much like a fuse.

I can't unsolder it right now but when measuring the same pins on another (identical) monitor, it turns out that there should be infinite resistance on the pins.

I'll desolder the regulator tomorrow, my guess is as well that it's shorting.

Then, the next question is: did some other problem lead to the regulator to blow up or did it just die of old age...

[PaulJ]

Ok, I'd look at C114 to be shorted and the regular. Since you measured .3 ohms across pins 8/10 of the reg. its your number one suspect if you took it out of circuit. If not c114 is in parallel with pin 8/10 and could be shorted giving that reading. Transformers are tuff but unless there is visible damage I would give it the benefit of doubt right now. If you have not taken the regulator and cap out and measured them its a toss up. The resister was hit so hard it didn't event have time to smoke, just blew open so it does appear to be a bad short.

[sandord]

Ok, I'd look at C114 to be shorted and the regular. Since you measured .3 ohms across pins 8/10 of the reg. its your number one suspect if you took it out of circuit. If not c114 is in parallel with pin 8/10 and could be shorted giving that reading. Transformers are tuff but unless there is visible damage I would give it the benefit of doubt right now. If you have not taken the regulator and cap out and measured them its a toss up. The resister was hit so hard it didn't event have time to smoke, just blew open so it does appear to be a bad short.

The regulator is definately broken, I took it out and measured 0.1 Ohm across pins 8 and 10. I measured C114 and it doesn't short.

I took the regulator from my other 1084S (which was in a non-working state) and that regulator has infinite resitance across pins 8 and 10.

My plan is now to:

- put the working regulator from my other monitor into this one (the other monitor wasn't working anyway, will try to repair it later after ordering a new regulator for it)
- make sure the parts near the regulator are ok
- disconnect all modules
- turn on the display
- connect modules one by one (with the unit powered off of course)

[exxos]

The regulator is definately broken, I took it out and measured 0.1 Ohm across pins 8 and 10.

Mystery solved then. Either old age, mains surge or overheated.

EDIT:
Looking at the mains filtering side, seems to have a lot of it, so could be ruled out as a surge I guess. Unless mains went to 500V for a few seconds or something, that could do it, probably have took out more parts tho

EDIT2:

Looking at the small heatsink, if you could bolt something on making it "larger" then it will increase the life of the IC if it can run "cooler".

[sandord]

Mystery solved then. Either old age, mains surge or overheated.

Yup, at least I hope it didn't happen because of some other problem (could this have been caused by a short in the LOPT for example?)

EDIT:
Looking at the mains filtering side, seems to have a lot of it, so could be ruled out as a surge I guess. Unless mains went to 500V for a few seconds or something, that could do it, probably have took out more parts tho

Mains is pretty clean here in The Netherlands, never had anything as destructive as that happen here (light bulbs certainly would've lit up twice as bright, not something that goes unnoticed ).

EDIT2:

Looking at the small heatsink, if you could bolt something on making it "larger" then it will increase the life of the IC if it can run "cooler".

Good idea! There's an unused hole in it that is large enough for a bolt or something.

[sandord]

Apparently, R109 (100 Ohm) was blown as well. I already wrote it looked a bit scorched. Well, when touching it, the outer layer partially fell off. I replaced it as well.

Inductor L103 seems okay. I'm a but unsure about caps C110 and C130, I don't know how to check whether they're okay.

Transformers are tuff but unless there is visible damage I would give it the benefit of doubt right now.

Thanks for the advice. As I don't measure a short over pins 2 and 4, I think that winding must be ok then. Although technically, it could short with the other windings. Let me measure those...