SOLVED!! Old radio repair, cant find any obvious fault.

Thread Starter

smyth602

Joined Jan 8, 2017
50
They are relatively quiet but the speaker doesn't have a volume control itself, just high or low gain inputs. on high gain they are quiet but clear. Moving the volume control on the radio from half to full didnt make any obvious difference to the volume level but there was some noise from the speaker as it was moving...rustling would be a good description.

Unfortunately as i was trying to retune to identify the stations, the red wire from the volume pot came off at the chip-area end so i'll need to try and find where it came from to reattach it. the wires on this thing are so damn delicate!

I heard one station give an ident so i know it is 1215 (kHz I assume?)
 

AlbertHall

Joined Jun 4, 2014
12,343
Can I recommend that you make a drawing (or possibly a good photo might do the job) showing where each wire goes. It can save a lot of effort later.
 

Thread Starter

smyth602

Joined Jan 8, 2017
50
managed to repair things. the other station transmits on both 693 and 909 so i'm unsure what i'm picking up.

The volume control has no effect on the output other than it vanishing close to switch-off. I have noticed that of you switch the radio off then back on, the sound does not come back until you switch to FM and then back again.

i found another station which broadcasts on both 585 and 810.

I am in scotland and yes, bedtime is approaching, but we are not there yet!
 

MrChips

Joined Oct 2, 2009
30,705
You already have some good photos of the board. Check back on your original posts to find where the wires are connected.

585 and 810kHz is BBC Radio Scotland. You should be able to pick up 1545kHz in Edinburgh, 1152kHz in Glasgow.

So we know the radio section is working.

The audio section doesn't look healthy.
Pin 13 - output to the speaker should be about 4V.
Pin 16 - AGC/AFC - automatic gain control/automatic frequency control should be between 1-3V.

Pat yourself on the back! You've done well for one night.
Maybe it's time to give it a rest and have a fresh look in the morning.
 

Thread Starter

smyth602

Joined Jan 8, 2017
50
You already have some good photos of the board. Check back on your original posts to find where the wires are connected.

585 and 810kHz is BBC Radio Scotland. You should be able to pick up 1545kHz in Edinburgh, 1152kHz in Glasgow.

So we know the radio section is working.

The audio section doesn't look healthy.
Pin 13 - output to the speaker should be about 4V.
Pin 16 - AGC/AFC - automatic gain control/automatic frequency control should be between 1-3V.

Pat yourself on the back! You've done well for one night.
Maybe it's time to give it a rest and have a fresh look in the morning.
Yes I was picking up BBC radio Scotland, BBC 5 live and absolute radio. Wasn't sure if the station names would be relevant so I left them out.

The other half seems to agree with you and is reminding me I have work tomorrow so I think you are right and I will call it a night.

I work just along from an electronics shop (maplin). Should I pick anything up tomorrow at lunch that might help tomorrow evening?

Thanks for all the help so far on this folks :)
 

MrChips

Joined Oct 2, 2009
30,705
I cannot think of anything at this point.

If you are going to replace the IC, TDA1083 you will need a solder sucker and some solder wick.
Before you go there, it might help to reverse engineer the board and draw out what we see by looking at the components on the top side and tracing the tracks on the bottom side.

We need to find why pin-16 is at 7.7V instead of about 2V.

Have a good night!
 

Thread Starter

smyth602

Joined Jan 8, 2017
50
Gentlemen, I have returned. I picked up a 10uF capacitor today as requestedby MrChips and some slightly thicker gauge wire to replace any that break from here on in
 
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MrChips

Joined Oct 2, 2009
30,705
There are three wires, orange, black and red, connecting the volume control to the underside of the board.
I cannot see clearly where they are connected at either end.

You said that it was difficult to disassemble the radio and that you are fairly certain that no one else had a go at it.
Look at the solder joints. Do the joints look like original factory joints?
Is it likely that someone already moved those wires around?
The connections of the orange, black and red wires appear odd to me as if they have been moved.

As for the 10μF capacitor. What is the voltage rating of the cap (short for capacitor)? Solder the cap in series with your red wire on the input to the boom box. Negative lead on the cap (with the minus sign) is soldered to the red wire of your 3.5mm cable.
Positive lead on the cap is your probe.

1) Solder the positive lead on the cap to pin-9 of IC, as you did yesterday. Notice any difference?

2) Solder the positive lead on the cap to pin-8 of the IC. Notice any difference?

Unsolder the three wires (orange, black, red) going to the volume control.

Repeat test 2) above.

Go around the IC pins and measure the voltages under two conditions:

a) while receiving a radio station
b) while not receiving a radio signal
 

Thread Starter

smyth602

Joined Jan 8, 2017
50
it isn't impossible that someone has been in it before, I just know that to me the label seemed far too fragile to try and peel off, so I slit it with a scalpel instead. Quite a few of the solder joints seemed poor to my inexpert eye, so either someone has been in it before me or perhaps it was just poorly assembled at the factory? The very fact that the unit has survived so long in good cosmetic condition makes me think that perhaps it never worked to begin with, so was forgotten about on a shelf. the second hand shop I found it in had a workbench at the back that I think the guy used to make repairs to stuff before he sold it. He seemed unaware if it worked or not so I don't think he had tried to fix it.

It is a 50v cap - the only 10uF they had in stock.

Putting it in series on pin 9 makes no discernible difference.

Putting it on pin 8 results in no sound from it at all.

Leaving the speaker on pin 8 and removing the 3 wires to the volume potentiometer makes no difference except the roughness heard through the speaker when moving the volume control has vanished and the crackle/pop heard when turning the radio off/on is much more gentle.

I will start testing pin voltages now, however I have a question first - The radio seems to be very power-hungry. Since last night's testing and in approx an hour or so of messing about with it today, the (fresh at 10pm-ish last night) battery has dropped from 9.6V to 7.7 at the moment. A quich check with the multimeter shows the unit drawing approx 25-30mA so I wouldn't have expected the battery to drop so fast. Am i better testing the voltages with a fresh battery, or with a bench top 9v supply to ensure a steady supply voltage?
 

MrChips

Joined Oct 2, 2009
30,705
Putting it in series on pin 9 makes no discernible difference.

Putting it on pin 8 results in no sound from it at all.
Pin-8 is the audio output from the radio section.
Pin-9 is the audio input to the speaker amplifier.
Hence your findings make no sense at all. I would expect maximum audio signal to be present at pin-8.
Signal is attenuated and controlled by the volume control before going to pin-9.

BTW, if you ever want to entertain replacing the IC, TDA1083 is equivalent to NTE1624. Either should cost just a few quid.
 

Thread Starter

smyth602

Joined Jan 8, 2017
50
Well in that case I'm probably more confused than when we started. Maybe I messed something up - I will have another look tomorrow evening.
 

Thread Starter

smyth602

Joined Jan 8, 2017
50
okay so with a fresh mind I checked things again and indeed everything i said last night is correct. No sound at all on pin 8, only pin 9. I also procured a 9V wall wart so i can stop burning through batteries. It actually seems to be running at about 10.5V but to be honest a very fresh PP3 will sit at this so i'm not overly concerned.

I'll post up the chip voltages after i re-measure them.
 

Thread Starter

smyth602

Joined Jan 8, 2017
50
Here we go.

Receiving:
  1. 3.00
  2. 3.00
  3. 0.00
  4. 10.36
  5. 10.36
  6. 2.49
  7. 2.49
  8. 0.93
  9. 0.41
  10. 1.60
  11. 0.00
  12. 0.85
  13. 10.34
  14. 10.34
  15. 10.34
  16. 4.12

Not receiving has all the same values except pins 4 & 5 which were at 10.34V

Probably worth mentioning is that when the multimeter probe was on pin 6, the output volume increased considerably. The same happened if I held either end of the AM antenna core. I would guess that reception seemed to increase dramatically? One of the antenna wires is attached to pin 6 which I assume is relevant.
 

MrChips

Joined Oct 2, 2009
30,705
So far all tests suggest that the IC needs to be replaced.
The anomaly with pins 8 and 9 is still unexplained.
(You are absolutely certain that you have identified pins 8 and 9 correctly while looking at the underside of the board?)

One final test you can perform is to test the audio amp section independently of the radio section.
Unsolder the red, orange and black wires at the underside of the board.
Solder one lead of your 10μF capacitor to pin-9 (where the orange wire was as in your photo). At this time the polarity of the cap will not matter much.
Connect your mp3 player to the other lead of the capacitor. Connect the GND shield as before.
Listen for any sound out of the radio's loudspeaker.

Edit: post a photo of the underside with the three wires removed.
 

Thread Starter

smyth602

Joined Jan 8, 2017
50
Pin 8 is top left in the photo I posted of the pin voltages?

I've tried testing the audio amp as you suggest and with the mp3 player on max volume, there is some sound from the radio's own speaker. It is very clipped and intermittent but it is sound at least. I tested the speaker by driving it directly from the mp3 player and the speaker works fine.

I've ordered a replacement tda1083 chip so we shall see what we can do when that arrives.
 

AlbertHall

Joined Jun 4, 2014
12,343
I've ordered a replacement tda1083 chip so we shall see what we can do when that arrives.
If you are not experienced at removing multi-pin devices from a PCB then I recommend that you use clippers to cut the IC legs as close to the chip as you can. Then you can easily remove the individual legs. You are replacing the IC anyway so the fact that it is wrecked by this is of no matter, but it will make it easy to avoid damage to the PCB.
 
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