Capacitor aging question - Ceramics and Electrolytics

[Boogie]

Ive got an old transistor radio (a Radio Shack Flavoradio) which i bought as not working. I thought maybe the speaker was blown so i plugged it into headphones and it works fine. I opened it up and measured the speaker and its blown (infinite resistance).

Then, I thought it would be a good idea, while i was there, to make sure there was some voltage across the speaker terminals (in case the jack was corroded or something and was disconnecting the speaker even when not plugged in)

And the speaker has 9V DC (the battery voltage) across it, and so does the headphone output.

So i guess this means im lucky i didnt fry my headphones?

Is it a fair comment that the most likely problem is aged/leaky capacitors? which are passing DC that they should not be passing?

If this is correct, is it more likely to be an electrolytic or a ceramic? i would have thought an electrolytic.

There are 5 small electrolytics in the radio, it would not be much of a hassle to change them all.

How likely is that to solve the problem?

[Boogie]

the speaker is (was) 8 ohms, and the DC resistance of my headphones is 60 ohms.

there is a schematic which is so small i can only read it with a jewellers monacle, and some of it is illegible, but most of it is readable

[Boogie]

there are two transistors in the final section of the radio that look like a push-pull arrangement. the middle of the output of the push-pull goes to an electrolytic and then the other side of that to the actual output of the radio (ie straight to the hot of the speaker and headphone jack).. i.e. the electrolytic is in series with the output directly between the amp finals and the speaker

so im guessing that is the problem? at least maybe......... seems likely with my limited knowledge of these things

the three "large" electrolytics are all labelled 47uF on the schematic, but in the actual radio are only 33uF (10V). if im changing them which value would be better? (perhaps the bigger the better if they are for filtering etc)

like this (this isnt the actual schematic but similar)... im talking about C1 on this one

http://www.hobbyprojects.com/junctio...s/comppush.gif



the two resistors coming out of the transistors (R3 and R4 on this schematic) are both 10 ohms.

[pdf64]

Yes, if you've got the repair bug then consider replacing C1 as that blocks dc to the speaker, any value around 47uF would be fine as these things aren't close tolerance.
Lower values will increase the frequency of the low end roll-off, but there's unlikely to be an audible difference through the small speaker.
A higher voltage rating may have resulted in a longer lifespan, so there may be a benefit in using a 16 or 25V replacement. A modern 25V one will probably fit in the same space due to technology improvements.

It's strange that there's 9V (full battery voltage?) there; normally with that type of output stage, it would be about half the battery voltage.
However replacing all the electrolytics may resolve that.
There may be other problems though.
Pete

[Jazz P Bass]

My guess is the voltage disappears when you hook up a load. (ie: terminate the capacitor)

[Boogie]

Thanks, yeah i was thinking about the voltage too. Sometimes there wasnt the full 9V, like maybe 7-8V, depending on the volume control setting, some of the range it was 9V.

I was thinking that if the transistor E-C circuit is low enough resistance, and the resistor is only 10 ohms, and since thats all there is between the battery and the output, and my multimeter is an analog FET input with megaohms, then it would be seeing almost all the supply voltage.

But when the speaker is attached which is (was) 8 ohms , it would see maybe half the 9V , or a bit less, if the speaker is 8ohms out of 18+the transistor's resistance.

I was also wondering if perhaps this was normal behaviour for the radio (since its old and was cheap even in its day, and the "headphone" output is mono and really an earphone output, and maybe they were high impedance and wouldnt care about the DC)? but then i thought shouldnt a cap (thats working properly) in series with the output block all DC passing through it?

Thats a good idea about bigger voltage caps, i'll get a bigger voltage like 25v as priority over a bigger value like 47uF, as big as i can find at Jaycar that will physically fit into the space