National Panasonic RQ4350 1970s Boombox no left channel - UK Vintage Radio Repair and Restoration Discussion Forum

marty2be2000 · 16th Jan 2022, 10:11 am

I have owned this boombox for 2 years now, and it has been faultless until a few weeks ago.

I was listening one day and noticed that there was no audio from the left channel when listening to the radio, it was playing ok to start with.

Symptoms at the time
From plugging into the AC supply, the sound would come out of the left channel , but over a minute or so the sound would get quieter until it had faded away.

Symptoms as of today
No audio at all from left side from power up

Observations
Makes no difference if the unit is powered by AC or batteries
Makes no difference if listening through speakers or headphones

If you go into record mode on the cassette recorder, both left and right VU meters are working ok.

I have taken the front of the system, and located the amplifier section. There are no obvious signs of issues, all the capacitors look externally ok.

I cannot locate a schematic, but this russian youtube video https://www.youtube.com/watch?v=JdjY...%D1%80%D0%B433
Shows the amplifier section for a short period from around 2 mins 17 to 2 mins 24.

From the video there are two vertical discrete sections on the board.

The left stack is the right channel. If I touch the small cap at the top I can induce hum that comes out the right speaker.
The right stack is the left channel, if I touch the same capacitor there I get nothing from the left speaker.

At this stage I suspect capacitor failure, I just need to work out where I should be looking. I think the audio input occurs at the top of the board and flows down the board, coming out of the speaker connections which are on the bottom of the board.

16th Jan 2022, 10:11 am	#1
marty2be2000 Triode Join Date: Dec 2017 Location: Stevenage, Hertfordshire, UK. Posts: 27	National Panasonic RQ4350 1970s Boombox no left channel I have owned this boombox for 2 years now, and it has been faultless until a few weeks ago. I was listening one day and noticed that there was no audio from the left channel when listening to the radio, it was playing ok to start with. Symptoms at the time From plugging into the AC supply, the sound would come out of the left channel , but over a minute or so the sound would get quieter until it had faded away. Symptoms as of today No audio at all from left side from power up Observations Makes no difference if the unit is powered by AC or batteries Makes no difference if listening through speakers or headphones If you go into record mode on the cassette recorder, both left and right VU meters are working ok. I have taken the front of the system, and located the amplifier section. There are no obvious signs of issues, all the capacitors look externally ok. I cannot locate a schematic, but this russian youtube video https://www.youtube.com/watch?v=JdjY...%D1%80%D0%B433 Shows the amplifier section for a short period from around 2 mins 17 to 2 mins 24. From the video there are two vertical discrete sections on the board. The left stack is the right channel. If I touch the small cap at the top I can induce hum that comes out the right speaker. The right stack is the left channel, if I touch the same capacitor there I get nothing from the left speaker. At this stage I suspect capacitor failure, I just need to work out where I should be looking. I think the audio input occurs at the top of the board and flows down the board, coming out of the speaker connections which are on the bottom of the board.