Indentifying an old exploded cap.

[Crystaleyes]

Hi all.

A friend asked me to help remove the mains hum from an old 60's or 70's portable record player.
The brand is Motoplay, which is an obscure Brazilain brand.

Anyway. One of the caps seemingly decided to launch itself into outer space and exploded in the process.

It appears to have been a ceramic disc with coloured stripes, and from the bits I could find and see, it appeared to be either violet or silver on top, followed by some brown, then some orange.

It definitely had silver, brown orange going down, but the brown could have been the overall capacitor colour and the that the orange and silver had some space between them.
It just doesn't fit into what I know regarding capacitor colour coding.

So the question is, does anyone have any idea as to how to identify it's value?

[stj]

show where it was wired

[Crystaleyes]

Originally Posted by stj

show where it was wired

The first pic is the board. It is fed from the optional battery supply and consists of 4 transistors, 5 electrolytics, a transformer, 3 ceramic caps and about a dozen resistors.

The second photo shows the holes where the cap came from. Just above the left side of the blue electrolytics.

The third photo is just a close up of those holes (in the middle of pic).

I should add that after changing a few of the electrolytics, the horrific noise from before has gone and the system plays not too bad, considering.
It still has a mains hum, but there is earthing system in this building, nor in most of the buildings here in Brazil.

So bearing that in mind, it isn't a 'critical' component but it 'has' a purpose and would be good to get something in its place.

[R_J]

How can we tell where it connected without a picture of the trace side of the board?

[petehall347]

looks like a thermistor .

[R_J]

Hard to tell without knowing where it is located in the actual circuit.

[PeteS in CA]

Originally Posted by petehall347

looks like a thermistor .

That was my thinking as well.

I'd suggest staring at the PWA and drawing a schematic showing how the part was used (yes, I've done this in times past with flyback topology P/Ss, and it can be very tedious).

[Crystaleyes]

Gentlemen, please be so kind as to give a lady time to respond...

The trace side of the board has some kind of putty or resin, which can be seen in the photos and the blown cap came from the two empty holes.

[R_J]

It appears to be across that 68Ω resistor, so it could be a thermistor used for temperature compensation? or just a bypass cap, .1 or .01 µf
Can you check the parts resistance? that might give a clue.
The resistor (and other component) seems to be connected between the (drive) transformers center tap and supply voltage. I suspect the two outer pins of the transformer each connect to one of the output transistors (base?)

[Crystaleyes]

Originally Posted by R_J

It appears to be across that 68Ω resistor, so it could be a thermistor used for temperature compensation? or just a bypass cap, .1 or .01 µf
Can you check the parts resistance? that might give a clue.
The resistor (and other component) seems to be connected between the output transformers center tap and supply voltage. I suspect the two outter pins of the transformer each connect to one of the output transistors.

It is across that 68Ω resisitor, and it measures around 170 ohms

The picture below is part of the casing which had blown off, with the silver band being above the orange

[R_J]

My guess is a thermistor, warm it up a bit and recheck the resistance. This is the best I can figure out because of the glue etc.

It may be a similar circuit to this (I said similar) in the ampf1 you can see a manual bias adjustment.
newer thermistors color bands end with the tolerance color (silver 10%) at the top it may have been 130Ω (ntc?)

[Crystaleyes]

Originally Posted by R_J

My guess is a thermistor, warm it up a bit and recheck the resistance. This is the best I can figure out because of the glue etc.

It may be a similar circuit to this (I said similar) in the ampf1 you can see a manual bias adjustment.

Looks like you are correct as the resistance goes down with heat applied.

I never knew that thermistors were also colour coded, so that is another lesson learnt.
In fact I have found a picture online just to prove the point (below).

The resistance now reads around 255 - 260Ω which is too high...? The 68Ω in parallel must give a hint, as electrical current always takes the path of least resistance.

Here is where having some knowledge of circuit design would be helpful.


EDIT:

Looking further online there is a 220R thermistor which is 'Red', Brown, Silver - So is it too mistaken to imagine that the 'Orange', Brown and Silver, is in fact 330R?

[R_J]

It may have been brown, orange, brown, silver 130Ω 10% ntc likely not too critical, it changes the resistance across the 68Ω to slightly vary the output transistors bias when the temperature changes

[Crystaleyes]

Originally Posted by R_J

It may have been brown, orange, brown, silver 130Ω 10% ntc likely not too critical, it changes the resistance across the 68Ω to slightly vary the output transistors bias when the temperature changes

Yeah. I can see that now.

Well done fella for cracking this one. And it's been another valuable lesson for me.

Thanks to all who chipped in.

[Crystaleyes]

So... turns out that 130Ω is not available in any of the shops here.

There are 50Ω or 1kΩ

What would happen if there were an 82Ω resistor placed in series with a 50Ω thermistor?

Or would it require the 68Ω resistor value to be changed?


Or am I completely missing the point?

[petehall347]

there are 120 ohm on that ebay but i didn't look at the temperature curve .....being as yours seems to be working you could figure it out with a thermometer and heat source .

[Crystaleyes]

Just to update...

I actually found the exact replacement after searching online for the colour code of these. Can't remember now, but I think I found it in an old philips receiver.

Anyway, it got sorted.

Thanks all for the help and suggestions