My good old radio is broken.

[Antraciet]

A transistor radio, mid '70 i think stopped working 
Tha day before, when i turned it on, it had a few secunds time to play. Which was weird, but ok, once working, it played well again.
The next day, when i turned it on : nothing. no sound; no cracks, just nothing.

I checked the trasformer, i tried also with batteries, i checked the loadspeaker, the connections. I see nothing that is burned, no caps leaking, no wires that are broke.

What can i check next ? I watched lots of youtube videos about radio repair, and i noticed that lots has problems with the caps. But tere are so many in my radio. Check them all ?
Any ideas ?
Some pics;

[SeanB]

First check the power socket switch that changes over between battery and mains. It often goes open circuit or high resistance. Check there is voltage to the main board and that the main power switch is not open circuit ( very common) to eliminate the simlest and most common contact problems.

Next is there any noise from the speaker? Place a screwdriver on the volume control wiper with it in the mid position and see if you get hum from the speaker ( running on batteries of course) and also check the headphone socket is not open on the little switch that disconnects the internal speaker . Plug in some cheap headphones and try the hum test again.

If you get hum then the fault is further back. Then you have to trace back, where you probably will come to a band switch, check if AM works, or that you get a noise on switching bands. Ifd so then you will probably find it is only faulty on one band, thus cutting out 2/3 of the radio.

If AM or FM then most likely is a faulty oscillator transistor in the RF side, the FM one is in the wax just above the FM tuning capacitor. Do not bend those coils or disturb them at all, they are that way and set in the beeswax for tuning and are very hard to adjust correctly when disturbed.

However if the fault finding is in the RF sections you will need some test equipment, like a Rf signal generator that can give you an IF signal of 10.7MHz and 455kHz for the FM and AM IF stages respecively, and some care in fault finding.

In my experience though the most common faults are switches and pots going open circuit. Then a failing oscillator transistor and a dry joint on the assorted iron cored transistors. Capacitors here have to be almost open circuit to show as a failure, and then the most common fault is hum.

[Antraciet]

In my experience though the most common faults are switches and pots going open circuit.

HAHA Sean !
Many thanks for your help.
Yes, something wrong with the pot meter. I made a bridge (see pic, red line) and the radio is working. I measured the pot before, and it seems fine, but yeach,....
I just tried today on good luck  80 % of the time when i am busy in electronics i am not sure what i am doing, hehe

So, a new pot ? But i can't see the type. I measure 880 k or something, on the net i see it as 'japanese pots' 10k : http://www.elektronicawereld.nl/categorie/onderdelen_weerstanden/potmeters_speciaal/product/4094/japanse_potmeter_10k/details
It is a pot with switch.

[SeanB]

They can be hard to get, probably the best is to add an external switch to it instead. Place it in the top of the case where it will be easier to operate.

[Antraciet]

They can be hard to get, probably the best is to add an external switch to it instead. Place it in the top of the case where it will be easier to operate.

Yes ok, i can simple use the AC/DC switch, that is no problem.
But what can i do about the volume ? When i make a bridge, it is working on 100 % And i don't understand, because when i measure the Potentiometer, it seems to work on both sides : max. 8.65 Kohm.

and when i replace it, i can buy another with a switch,...
This one will do ? :http://www.gotron.be/componenten/potentiometer/potm-jap/potentiometer-mono-10-kohm-logaritmisch.html
Is not with a switch on/off, but i ask when i am in the shop.

EDIT : potentiometer is half working,.. when i shake on it, is working, sometimes not. I desolder the pins, but that is not the fault. So a newone,...

(allways fun to repair my own stuff with my knowledge (very very basic) of electronics  )

[SeanB]

I saw one brand new today for $3, in a complete brand new Everready branded radio I bought on impulse. Sad thing is the PM3 batteries it uses are $5 each. Will do a simple mod to have an old wall wart in place of the battery, much cheaper and much longer lasting. hopefully the 10v Crapolytics inside will not blow like the last one on 11V, but I never changed them anyway, the 100uF capacitor was bypassing the power, and the 470uF 50v one I used to simply keep the joins apart did the job.

[Antraciet]

And i ordered a 'japanese pot' 10K

[SeanB]

Well done, easy to change those. Only the one nut underneath and the 5 wires to solder.

[Antraciet]

Well done, easy to change those. Only the one nut underneath and the 5 wires to solder.

Ok Radio repaired, works fine now.
But i have another one. A Sony 7F-74L. That one is from my father. Lots of pics on the net, build 1966 i think.

I hear only noise (on batteries, on elec.: i hear the '50hz' noise) .
What can be wrong on this radio ?

[SeanB]

Plug in a set of headphones and there is no noise? You will be buying a few 100uF 4v surface mount electrolytic capacitors to replace a whole lot on the main board that have failed. A rather common Sony fail that, I have a radio with the same problem.