Distorted audio on an old transistor radio cassette

[Richard9025]

So , to understand what type of distortion my unit has , I made some recordings (the radio being connected to a 9V battery) , at low/med/high volume , while being on the same frequency. (posted them on google drive because here I can't post mp3s)
LOW volume :
https://drive.google.com/file/d/0B0cJwE4DzQywVDE5eXRZQmNndVk/view?usp=drivesdk
MEDIUM volume :
https://drive.google.com/file/d/0B0cJwE4DzQywNzNZc3RqRjhNcms/view?usp=drivesdk
HIGH volume :
https://drive.google.com/file/d/0B0cJwE4DzQywZ2hmZ2VsWC1BanM/view?usp=drivesdk

 

[73's de Edd]

Sir Richard 9025 . . . . . . . .

Last "easy fix" probability, listed in most likely cause to effect.
Significant decline of capacitance due to age and time related
electrolyte dry out:
C35
C14
C15
C22
C23
C36
In the audio amplifier section.

73's de Edd

Add on, after just hearing the audio samples . . . .
Definite progressive audio flat topping and splatter distortion.
Am I permitted to say, those capacitors, YET another time ?


( Second ensuing post forward. . . .reserved for Sir Constantines . . .
" I told ya' so. "

 

[Richard9025]

C35 - 500uF
C36 - 2000uF
C14 - 1uF
C23 - 50uF
C22 - 50uF
C15 - 200uF
Almost all the caps on the board a8 have 10V maximum voltage
Currently I have an 470uF 16V cap that should replace capacitor C35 . An 220uF 10V cap that should replace C15 and an 4,7uF 25V that also should replace C14 .
So , can these capacitors that I currently have replace the original caps ? I know they are a bit off but will they work ?

 

[73's de Edd]

Yes those values should work just fine with you then only needing to find two 47 or 50 µF for two of the capacitors and then that large 2000 . Consider even using as low as 470-500 for it, just in evaluation.
DO YOU REALIZE that since this set is using a large PCB and not a microsize one like you might encounter in a pocket radio size PCB, that it is very easy to just shunt a replacement / test capacitor across the old, still connected capacitor.
That also lets you listen to the sound to evaluate, as you shunt across the replacement test capacitor.
Sometimes it is just one diminished capacitor that is causing the majority of the distorted sound effect.

73's de Edd

 

[Richard9025]

that it is very easy to just shunt a replacement / test capacitor across the old, still connected capacitor.

I'm not very familiar with the term "shunt" , I heard of it but I don't know how to shunt a capacitor ,
like , to connect in parallel a test/replacement capacitor with the original cap , still connected to the board , or only the pozitive pin on the test cap with the pozitive pin (soldered on the board) of the original capacitor ?
Can you please describe the technique ?

 

[73's de Edd]

Splay the + and - wires of the capacitor apart, so that you can then press your red wire of the test capacitor against the red + solder blob of the units capacitor.
Then you swing the - lead such that the same mating can be done with the black- solder blob of the units capacitor.

 

[Richard9025]

Like this :

Do I need to use almost the same value (uF) of the original capacitor , or any value ±500uF for a test capacitor ?

 

[73's de Edd]

YOU GOT IT !
and yes:
You
Do I need to use almost the same value (uF) of the original capacitor.
Except for that / those 2000 ufd units, you can try a 470.

73s de Edd

 

[Richard9025]

Thank you , I will test the caps tomorrow , when I'll have some free time

 

[Richard9025]

I shunted a 470uF 16V across the original C35 (500uF 10V) and, the moment I touched the legs , I got a popping sound , then , if I hold them , the radio goes mute , not a single sound/hum from that speaker , if I remove the test capacitor , I get the normal , definite progressive audio flat topping and splatter distortion . Maybe C35 works ?
I'll test C15

 

[73's de Edd]

Possibly your ,used, substitute capacitor could be shorted or very leaky.
Do you have another one to try, or even possibly another one, that is even
down around the 330 or 220 microfarads range .
Also, be absolutely certain in matching LIKE polarities to each other.
Almost universal marking protocol is for the negative lead identification being
a nearby stripe.
Also, possibly you made a mistake in the relating of the caps wire leads to its SAME solder blobs on the OTHER side of the board.

My . . .Timely Technological Tip
for this, is to place a RED laser pointer beam, right up against and on top of the suspected solder blob, move laterally aside just enough to illuminate a bit of the PCB also.
Freeze on your position and then confirm, on the other side, that you are actually finding yourself to be on the correct connection.

73's de Edd.

 

[Richard9025]

So , I shunted 2 220uFs and 2 470uFs across c35 and they all did the same thing , a pop and then nothing . Also , I shunted 2 220s across c15 and , they sometimes mute the radio , sometimes do nothing , no popping .
Maybe they are all bad (test caps) even if they look like new and aren't that old .
I used the correct polarity (a line with - on the test cap -> negative , and on the board there is marked + to know what's the pozitive blob) and I didn't mismatched the blobs (I checked with different methods)

 

[73's de Edd]

I cant see the odds of 2 220s and 2 330s all being bad.
Possibly the radio was then trying to work normal then, but might have been confronted with that extra power pull pulling down a marginal 9 V battery beyond its capability,

Considering that this was a battery powered test vice AC power.
If AC powered the 2000 ufd main filter(s) maybe weak and not have enough filtering reserve.

Your capacitive values attempted at C15 are FAR FAR FAR too excessive.

 

[Richard9025]

Yes , I tested it while it was hooked up to a 9V battery , so to test it with it connected to ac power ?

 

[73's de Edd]

You might try it .

 

[Richard9025]

You might try it .

Ok , I'll test it now , hope I don't cook this thing

 

[Richard9025]

It does the exact same thing when connected to ac

 

[73's de Edd]

Does this unit have other on board source capability . . .like FM radio or is it being a tape player also.
Either of those two other sources present a different audio input signal ..

 

[Richard9025]

Yes , it has a tape player , and also an fm band (the old band , 65,8 - 74 MHz , that here , in Romania , where fm is only broadcasted on 88 - 108 Mhz , it receives "images" of the stations on 88-108)
More info on this unit
http://www.radiomuseum.org/r/vef_sigma_260.html

 

[Richard9025]

shunted a 4,7uF across C14 , and then , it goes mute BUT if I wait 5 seconds (until C14 gets charged) and then shunt , it does nothing , C15 does the same effect , C35 , after waiting , does a lot of popping , but also some distorted sound . C36 , when shunting an 470 across it , does nothing .