I recently picked up an old promotional radio made by Wurth.



I knew it wasn't working when I bought it but i liked the look of it and thought it would look good on my work bench. The more I thought about it, though, the more I wanted to get it working so I managed to open the case and extract the guts.



My knowledge of electronics is pretty basic but from what I can see, there is nothing obviously wrong with it. No visible damage to components, solder joints look good enough (I reflowed a few suspect looking ones to be sure) and the off/vol switch is passing full battery voltage on the switch side of things, but when you switch it on all you get is a very brief crackle from the speaker and then silence. Not even static.

The radio is based on a TDA1083 chip so I guess that could be faulty, I downloaded a data sheet for it but figuring this all out seems beyond my abilities for now. I've attached a bunch of photos so you can see what I'm blabbering on about.

Anyone got any suggestions where I might start looking?

 

Hello,

What happens when you move the give bridge (it is the on/off switch):



Does the crack repeat?
if so this part might be dirty.

Bertus

 

Thanks for replying so fast. Yes the crack repeats. I have tried cleaning the switch with an electrical contact cleaner and some fine sandpaper but no change.

 

What do you hear when you switch AM-FM?
Turn the volume all the way up and put the speaker to your ear. Do you hear any noise at all? If so is it different between AM and FM?

 

Hello,

What happens when you change the AM/FM switch?
In AM I would expect some noise.

Bertus

 

There seems to be a lot of goo around pins 3-8 of the TDA1083 chip. If the goo has deteriorated with age it may be providing a conductive path between the pins.

 

Absolutely no difference between AM and FM, nothing from the speaker aside from a very slight crackle when switching between the two. The same crackle can be heard if your finger is on the switch in either position and just wiggling it slightly without actually moving it enough to switch it. Similar to what you might hear if you move the volume control on an old walkman etc when nothing is playing.

 

The antenna wires are very delicate. Has one been damaged or become disconnected?

 

It sounds (!) like the problem is in the audio part of the circuit.
If you have a multimeter, measure the DC voltage on all the pins of the IC with the negative meter lead on the negative battery connection.

 

I'll measure these just now. Stupid question first though. I can see that the pins are numbered on the data sheet, but how does that correlate to real life?

 

I'll measure these just now. Stupid question first though. I can see that the pins are numbered on the data sheet, but how does that correlate to real life?

There's a rectangular dent on one end of the top of the IC. The pins are numbered anticlockwise round the chip from there.

 

battery voltage was 8.3, dropping to about 8.03 by the time i finished. the board is drawing 30mA from the battery.

so pin 1 is bottom left in this picture?

 

so pin 1 is bottom left in this picture?

Correct.

Pin 12, the audio amplifier output, which I would expect to be around half the supply voltage (~4V) is actually at 0.85V.
It might be caused by the capacitor from pin 12 to the speaker has become very leaky, or the circuitry on pin 9 or 10.

Pin 16 is expected to be between 1.4V and 2.1V but is actually at 4V.
I have no idea why this might be

 

the orange wire from pin 9 goes to the volume pot. the dial was at full volume when i made the measurements. could that have affected things?

i can try replacing the capacitor from pin 12 too. i assume my local electronics store can provide.

 

The volume pot will short pin 9 to ground at minimum volume so it should affect that voltage but I don't know what voltage it should be. The example circuit in the datasheet has a capacitor from pin 10 to ground and if that was leaky it could affect things.
The chip has a 'supply stabilisation' circuit and if that was faulty it could mess up the whole chip function.

 

I desoldered the capacitor and speaker from the board - the speaker connections didn't look very good so i will redo them, and i will source a replacement capacitor tomorrow. Wish me luck!

Through the wonder of the internet, I've found that the Maplin beside my work has one 100uF 10V electrolytic capacitor in stock for the grand total of £0.19. This should be a direct replacement

 

Before you replace it, measure the voltage on pin 12 with the capacitor removed. If it's still around the same voltage (0.85V) then the capacitor is innocent.

 

0.84V without the capacitor

Damn.

 

Pin 16 is expected to be between 1.4V and 2.1V but is actually at 4V.

The pic shows it at 7.67V.

 

The pic shows it at 7.67V.

Yes, my mistake. It's still not the expected 1.4V to 2.1V.