Panasonic radio plays only one side

[Behemot]

Got this radio where only one side plays. The amplifier section seems pretty simple, there is basically LA4705N responsible for everything. Anybody has some experience with that? Is it the main suspect?

[stj]

Re: Panasonic radio plays only one side

link the left & right input pins to get your answer.

[Behemot]

Re: Panasonic radio plays only one side

[stj]

Re: Panasonic radio plays only one side

if it's a stereo chip and you link the left & right input pins, you can see if you get sound from both speakers.

otherwise, feed some sound through it and scope the inputs to the amp section.

[stj]

Re: Panasonic radio plays only one side

link pins 2 & 7

[budm]

Re: Panasonic radio plays only one side

Using capacitor so you will not cause Voltage offset.

[Behemot]

Re: Panasonic radio plays only one side

Using capacitor so you will not cause Voltage offset.

Some ceramic?

link pins 2 & 7

But why inputs? Why not outputs?

[stj]

Re: Panasonic radio plays only one side

it's a btl amp, designed for bridging - if you link the outputs you blow the chip because they are out of phase.

it wouldnt prove anything anyway.
by shorting the inputs to the amp, you are finding if the channel is missing before, or inside/after the amp chip.

[momaka]

Re: Panasonic radio plays only one side

Using capacitor so you will not cause Voltage offset.

Or you can link the pins on the input coupling capacitors, not on the side that goes to the chip (i.e. pins 2 and 7) but their other side. That way you won't need to any caps to test the circuit and it works just the same .

[budm]

Re: Panasonic radio plays only one side

If the input side of the caps are connected to the pots then it will be fine but if they are connected to the active devices such as the Collector of the Transistors (or the Emitter as in Emitter follower, or output of the OP-AMPs) then that can be the problem since you will tie the Collector of the Left and right Transistors together. Better find out how they are being used first before doing any direct connection.

[popeye]

Re: Panasonic radio plays only one side

Got this radio where only one side plays. The amplifier section seems pretty simple, there is basically LA4705N responsible for everything. Anybody has some experience with that? Is it the main suspect?

Hi I take it this is a stereo amp then you dont even need a schematic because all you need is in front of you check between the componant measurements
from one side of the out put to the other working one if your fault is not there check you are getting the correct inputs if in puts are ok then problem is with the ic if not you know what you should be getting from the working side so compare.

[Behemot]

Re: Panasonic radio plays only one side

It does the same for all settings (radio, CD player) that's why I think input is OK as it goes from different sources with same result.

The headphone output usually goes through amplifier or not? If not I can try some and in such case I guess I can call input OK too?

[momaka]

Re: Panasonic radio plays only one side

If the input side of the caps are connected to the pots then it will be fine but if they are connected to the active devices such as the Collector of the Transistors (or the Emitter as in Emitter follower, or output of the OP-AMPs) then that can be the problem since you will tie the Collector of the Left and right Transistors together.

Most of the time, they are connected to an electronic switch that toggles which source (i.e. radio, tape, or CD) go to the amp. After that, there is usually a resistor, the amp input coupling caps, and the amp itself. At least that's what I have seen in most "modern" (90s +) units.

But I agree. It is better to check what is connected to what before blindly jumpering stuff.

The headphone output usually goes through amplifier or not? If not I can try some and in such case I guess I can call input OK too?

In many cases it does.

So yes, check with headphones. If you do get sound from both channels with headphones, it could be the headphone jack switching contacts are messed up or just dirty.

The way the output on most boomboxes and "mini" stereo systems is done is you have the outputs from the power amp going to the headphone jack. The headphone jack has switching contacts. If you plug in headphones, the output goes to the headphones and mutes the speakers. If you unplug the headphones, the contacts in the headphone jack switch to route the output to the speakers. Inexpensive stereo PC speakers also do it this way.

[stj]

Re: Panasonic radio plays only one side

look at the schematic in the datasheet.

[budm]

Re: Panasonic radio plays only one side

You have the scope so that will be so simple to trace where the signal gets lost at, GOOD AC Volt meter will do as well.

[stj]

Re: Panasonic radio plays only one side

is it fixed yet?

i could fix this while waiting for the kettle to boil.

[Behemot]

Re: Panasonic radio plays only one side

You got teleport device?

[momaka]

Re: Panasonic radio plays only one side

is it fixed yet?

i could fix this while waiting for the kettle to boil.

Not unless it is a "modified" kettle by PhotonicInduction

https://www.youtube.com/watch?v=dDLw1Rx_cAI

[stj]

Re: Panasonic radio plays only one side

seriously, probe pins 2 & 7 for audio.
if it's on both, check between the ic and speakers - and check speaker coils.
no issues? then the chip is bad.

missing audio on one of the pins first checked? then trace it back.

[Behemot]

Re: Panasonic radio plays only one side

I am just moving from shared shop to my own room so I already have 2/3 of my stuff moved and 1/3 is messed everywhere around. Don't even have room for the radio on my table, it is quite huge and I don't want to mess with my O-scope ATM. Thanks for the input so far, I'll try when I am moved completelly. I am exhausted every day last week…