Old silent chap

[Richard9025]

Hi
About a week ago , I got a vintage romanian radio , from '89 named Gloria 4 rp 1524 made by TEHNOTON
http://www.radiomuseum.org/r/tehnoton_gloria_4_rp_1524.html
Now , some little history about this radio
First day that I got it , it was working on all the bands , good audio , except some problems (30min of usage that day)
- all the time there was a strong hum (only when connected to the wall socket)
- the on/off switch wasn't working (volume potentiometer can also turn on/off the radio)
- the light switch wasn't working
About these problems I can tell you that someone disconnected the lights from the circuit and bypassed the power switch
Second day it was all the same , except that on the SW2 band it had little distortions on weak stations . (1-2hr of usage that day)
Third day - the problematic day - , verrryyy strong distortions on all the signals (and all bands) , i couldn't understand anything (weak&strong stations) and , after 5 minutes of staying on , the volume slowly started to get lower and lower until , after 2 minutes it got completely silent , it only maked a "bum" (<half of a second) everytime i turned it on . (30 min of usage that day )
Now , it doesn't even make that bum , completely silent (AC and batteries) ... (but there is voltage in it)
So , I thought that an electrolityc capacitor in the af block and replaced the bigger ones
Schematic with the replaced capacitors (in red squares) (somewhat translated from romanian to english)
https://goo.gl/photos/hQmER8gAx3pLxZaW9
Also , with a 0,1uf poliester cap , an rca cable and some wires , I injected the audio from the intersection of r326 and r327 into a tv av input and nothing , no signal from the origin of the am detected singal . (the problem isn't in the af block)
I'll make some photos of the patient after I get a good camera

 

[duke37]

If there is distortion, then the audio stage is wrong. You seem to be working in the right place. I assume you got the replacement capacitors the right way round.
Measure the current draw of the set and measure the voltages in the audio stages.
The symptoms would suggest a transistor gone leaky due to wrong bias.

 

[Richard9025]

If there is distortion

there was distortion , now completely silent
I'll measure the current draw and the voltages .
And I'll make some photos .

I assume you got the replacement capacitors the right way round

checked 10 times and checked now , they are all the right way (also there are + and - signs on the pcb to tell u how u put the cap)

 

[Richard9025]

Anywhere I put the + probe of the multimeter I get 8,5V .... ?!
with the - probe connected to the - of the 9v battery

maybe that's why I get no sound when I connect the rca&0,1uf cap thing , I only get a very low buzz,the same buzz that appears when I touch the lead of the 0,1uF cap

 

[Richard9025]

Still problems with measuring voltage ...
For C413
if I put the negative probe of the multimeter at C413's negative lead and the pozitive probe at the pozitive lead of C413 I get 0V .
if I put the negative probe of the multimeter at the negative terminal of the battery and the pozitive probe at the pozitive lead of C413 I get 8,7V .
if I put the pozitive probe at the + terminal of the battery and the negative probe at the - terminal of the 9v battery I get 8,7V .
Now I'm measuring the current draw
photos on the way ...
Any suggestions ?

 

[Richard9025]

The miraculos current draw of this radio is 0 mA

 

[duke37]

The most important voltages are T10 to T15, start with T12 and T15. Try the + lead of C417.
All voltages relative to battery negative.

 

[73's de Edd]

Sir Richard9025 . . . . .

Once you find your " floating connection " that you have created within the power supply and have it then supplying power.
If no sound is then present, you might reverse yout signal tracing technique with the RCA connector and then place the TV end of the RCA jack into the TV/VCR audio OUTPUT instead if its input.
You then will be reverting to signal injection, if you place that introduced audio output signal into the high side / top end of the voulume control.

You should be able to test the whole audio string, along for any presence of distortion.

As per your query of signal gound for that RCA connector at the radio end . . . . that is any of the MANY black BAR ground connections that ohm out 0 to the battery commons negative.

73s de Edd

 

[Richard9025]

FINALLY A SIGN OF LIFE !!
I wasn't hooking the battery as I should , I mismatched the negative terminal
So , now it produces some sound , but I need to stick my ear to the speaker to hear it
And it is VERY distorted !!! (when hooked up to a battery ) (remember , I replaced some caps)
It is almost the same distortion as the older radio that I posted here , Edd knows how it sounds
But it is verrrryyy weak and poor ...
I'll check the transistors that are in the audio stage (with my dvm on diode check mode) and then I'll check if there is a signal in the origin of the detected am signal . (these are common transistors , not some russian far-away unknown ones) .
I'll keep you updated .

 

[Bluejets]

If you have an old amp or small transistor radio or similar, you could make yourself a basic RF/AF signal tracer.
Might make life a bit easier.
Failing that, here is a link to an easy one to build yourself.

https://xtronic.org/circuit/audio/circuit-audio-rf-signal-investigator-lm386/

 

[Richard9025]

Photos (sry for terrible photos , I'll make more in daylight , not under a lamp
also link to google foto album https://goo.gl/photos/TmAUhWjjnxMV3V648 (better quality)

Spoiler: photos

I noticed that under the transformer , the plastic is melted , who knows what happened there
Also see how someone disconnected the bulb and the switch for it from the circuit

pozitive battery terminal goes to point 40 (through the white wire that is sticked with the brown wire , photo above) , and the negative battery terminal goes to point 43
And , also see that there are missing parts in the power supply section and the schematic (D9 , D7 diodes)

https://xtronic.org/circuit/audio/circuit-audio-rf-signal-investigator-lm386/

Very interesting , I can't find the 1n34 germanium diode , any equivalent ? , and , how do u turn this amp on/off ? no power switch ? (not that I can add one next to the battery terminal )

 

[Bluejets]

1N34a on Ebay

 

[73's de Edd]

Sir Richard . . . . .

As far as the presence of the AM detector diode, and if you are finding R326 R 327 R 325 and C324, I feel sure that the a.m. detector diode is hidden within the housing of L310 and that all is well with it.
As far as the wire to the dial scale lamp, I might suspicion that it is not going be a momentary contact action switch and was always being left ON and running down the battery . He clipped its power wire.
I am not where I can consult the units schematic now, however as per your MAIN power switch, I believe that you are going to find it has been incorporated with the bank of GANGED pushbutton switches that switch between bands and functions . When you press in a pushbutton, it will remain engaged until you push another and the last one pressed in will pop out. . . . . one position, possibly Phono ? , is going to be the POWER OFF position for the radio .
I will reaffirm later, after being able to consult its schematic, in the interim see if any fine print associated with one of the push buttons might designate an OFF position . . . .albeit. . . .printed on in another language.

73's de Edd

 

[Richard9025]

When you press in a pushbutton, it will remain engaged until you push another and the last one pressed in will pop out. . . . . one position, possibly Phono ?

The last one pressed in will pop out.
The power off switch of this radio is incorporated in the volume's potentiometer.
To turn it off u just need to rotate the volume's potentiometer to the left until it makes a "clack!".
To turn it on , rotate to the right until it makes again that clack and then u adjust the volume (it is a potentiometer with a switch).
BUT , somebody bypassed it (with that wire junction that he made) , and stays on all time ...
Tommorow I'll test the signal and all the transistors .

 

[Richard9025]

All transistors in the af+power supply stage ,
With my dvm in diode check mode :

I double-checked all values , so real-guarantee is advised , and , emitter of T15 surely is open , I checked it 4 times .
Also don't trust my multimeter , it is ~200mV off ...

 

[Richard9025]

RADIO FIXED !!
I replaced T13 with a BC 558 B
T14 with a BD 137-16
and
T15 with a BD 137-16
Now it sounds better than ever with these new caps , loud and clear !
Many thanks to Edd and Duke !!!