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.

I tried cleaning away a lot of the goo but nothing changed.

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

Do you mean the three thin wires coming from near the chip and all going to the part with a ferrite core? Is this a transformer of some kind to amplify the signal from the aerial?

 

Yes, those wires. That ferrite core 'transformer' is the aerial.

 

If that is the aerial then what is the extending metal part that sticks out the top of the radio and is connected to elsewhere on the board that I would refer to as the aerial?

 

If that is the aerial then what is the extending metal part that sticks out the top of the radio and is connected to elsewhere on the board that I would refer to as the aerial?

The ferrite rod is the AM aerial, the extending bit is the FM aerial.

 

Gotcha.

So does anyone have any more ideas short of replacing the chip itself?

 

Do you mean the three thin wires coming from near the chip and all going to the part with a ferrite core? Is this a transformer of some kind to amplify the signal from the aerial?

Yes, it is a transformer of sorts. It detects the magnetic wave from the AM broadcasts and transforms it to an electrical signal.
It is also a tuned resonant LC circuit that is tuned to the desired AM radio station. The inductor L is the coil wrapped around the ferrite rod and the C is the variable capacitor. The capactor is adjusted so that the LC combination resonates at the same frequency as the AM broadcast signal.

If that is the aerial then what is the extending metal part that sticks out the top of the radio and is connected to elsewhere on the board that I would refer to as the aerial?

The extendable metal rod is the antenna for FM stations. This picks up the electrical wave, not the magnetic wave.

 

Yes, it is a transformer of sorts. It detects the magnetic wave from the AM broadcasts and transforms it to an electrical signal.
It is also a tuned resonant LC circuit that is tuned to the desired AM radio station. The inductor L is the coil wrapped around the ferrite rod and the C is the variable capacitor. The capactor is adjusted so that the LC combination resonates at the same frequency as the AM broadcast signal.


The extendable metal rod is the antenna for FM stations. This picks up the electrical wave, not the magnetic wave.

I think that makes sense, thanks. My background is in electrical distribution, not electronics so this is a steep learning curve.

 

The inductor L is the coil wrapped around the ferrite rod and the C is the variable capacitor. The capactor is adjusted so that the LC combination resonates at the same frequency as the AM broadcast signal.

So would this be the variable capacitor? The large cube?

 

Yup - or rather four separate variable capacitors all operated by a single spindle. Two for FM and two for AM. One of each pair tunes the aerial circuit and the other tunes the local oscillator. Each of the four also has a trimmer capacitor - the four screwheads on the top - to match up the four sections tuning ranges.

 

so if im understanding things correctly so far...

Red is the variable capacitor
Blue is the volume control
Purple are the aerials, with the one closest to the chip on the diagram being the AM one?

 

Yes, except for the red bits. Those are RF transformers of one sort or another. C6 and C14 are the FM tuning and C5 and C13 are the trimmers for those two. C19 and C21 are the AM tuning and C8 and C22 are the trimmers. Note the dotted lines between each pair of the main capacitors which indicates that they are ganged together.

 

Yes, except for the red bits. Those are RF transformers of one sort or another. C6 and C14 are the FM tuning and C5 and C13 are the trimmers for those two. C19 and C21 are the AM tuning and C8 and C22 are the trimmers. Note the dotted lines between each pair of the main capacitors which indicates that they are ganged together.

yeah ok that makes sense.

 

Hello,

When you take a closer look, C4 is tuning the input circuit for FM and C14 is tuning the local oscillator for FM.
The difference signal between the tuning frequency and oscillator frequency is fed into the IC.
For AM C19 is tuning the ferrite antenna and C21 is tuning the local oscillator for AM.

Bertus

 

BTW, don't touch any of the trimmer caps in the variable capacitor or any of the tuning slugs (the screw-driver slots on the top of those metal cans). If you do the radio will be completely detuned.

If you already have or suspect that they have been tampered with please be sure to let us know.

 

BTW, don't touch any of the trimmer caps in the variable capacitor or any of the tuning slugs (the screw-driver slots on the top of those metal cans). If you do the radio will be completely detuned.

If you already have or suspect that they have been tampered with please be sure to let us know.

Not touched any of them, the most i have tried is reflowing some suspect-looking joints and reattaching the capacitor and speaker which i took off yesterday.

 

The way we used to fix AM radios without fancy equipment like oscilloscopes etc. requires two pieces of equipment - a signal injector and a signal tracer. You might already have these two laying around your house.

An audio signal injector is any music source. Do you have any kind of MP3 player or music source?
An audio tracer is a simple audio amplifier. Do you have any kind of PC powered speakers?

A PC with powered speakers plugged into the headphone jack is actually both pieces together.

 

Yeah i have plenty of audio sources and powered speakers. Or do ypu mean speakers powered purely by the headphone jack?

 

Yeah i have plenty of audio sources and powered speakers. Or do ypu mean speakers powered purely by the headphone jack?

I mean speakers that need a power source, batteries or wall-wart (power adapter).

Ok, if you can post photos of what you have we can see if they are suitable.

We will use these to test your AM/FM radio.

Firstly we need to test the audio source (injector) and the audio amplifier (tracer) together to make sure they work.
That is, you need access to the signal from the source (maybe a 3.5mm phone plug) and be able to send it to the audio amplifier.

Something like the following would come in handy.

Once you have your test set up ready (injector and tracer) you're set to go.
You will inject the signal at the input of the volume control and try to find where the signal goes.

(There are other ways of approaching this.)

That's just for starters.

 

don't touch any of the trimmer caps in the variable capacitor or any of the tuning slugs (the screw-driver slots on the top of those metal cans). If you do the radio will be completely detuned.

I got a radio for repair and the owner said he had found several loose screws and had tightened them but it hadn't fixed it. All the tuning slugs and compression trimmers were screwed down tight

 

I got a radio for repair and the owner said he had found several loose screws and had tightened them but it hadn't fixed it. All the tuning slugs and compression trimmers were screwed down tight

I wouldn't be surprised. I suspect something like that. Perhaps not by the present owner, the TS.

With the above tests we can check out the audio section and maybe more.