Kenwood KR-A4070 Sound issue

[tech2014]

The sound is very low. You can crank the volume up fully and not a lot of sound. I am not very familiar with audio circuits. Can someone look at the schematic and tell me where to look. I do have a 70MHz oscilloscope. Any help would be greatly appreciated?

[budm]

Re: Kenwood KR-A4070 Sound issue

Did try cleaning the controls and switches first?
This unit uses lots of electronic switch ICs (IC 102, 104) for routing the audio signals which do go bad.
Since you are getting low sound form both channels I would then look at the circuits that are common to both channel by checking to make sure the power supply Voltages are OK, also make sure all the boards mounting screws are tight and not corroded which will cause bad Ground connection to the chassis.
You can also look at the audio signals on CN101 RIN, ROUT, LIN, LOUT.
The schematic is kind of bad due to multiple pages you need to link them together.

[tech2014]

Re: Kenwood KR-A4070 Sound issue

thanks budm. I did find that one of the connectors says -30V, but when I measured it I only got -25v. I agree the schematics suck.

[budm]

Re: Kenwood KR-A4070 Sound issue

I think the -30V is used for the FL (Fluorescent Display), I need to look at the SCH.

[tech2014]

Re: Kenwood KR-A4070 Sound issue

I input audio to the CD input port and I do get Left and Right audio going into IC104 and out of IC102.
I get nothing at CN101. What does IC106 do? Its says UP/DOWN.

[budm]

Re: Kenwood KR-A4070 Sound issue

I input audio to the CD input port and I do get Left and Right audio going into IC104 and out of IC102.
I get nothing at CN101. What does IC106 do? Its says UP/DOWN.

Volume Control motor drive.

[tech2014]

Re: Kenwood KR-A4070 Sound issue

thanks. I am suspecting IC104 since I get nothing at CN101.

[budm]

Re: Kenwood KR-A4070 Sound issue

If you look at PDF page 18, CN101 PIN 3 is the LEFT channel power amp input, pin 6 is the Right channel power amp input.
Q101, Q102 are the MUTE transistors for the power amp inputs which should be turned off in normal operation (not in MUTE mode).
So what audio signal do you see at the CN101?

[tech2014]

Re: Kenwood KR-A4070 Sound issue

I can't see anything at all.

[budm]

Re: Kenwood KR-A4070 Sound issue

So you do see audio on pin 21 INPUT (L) of IC104 but no output on pin 19 (L)?
BTW, the diagram shows IC104 PIN19, 20, 21 are tied together directly!, SAME FOR 8, 9, and 10 for the Right channel. Is that how they are connected?

[tech2014]

Re: Kenwood KR-A4070 Sound issue

I did see -1.6V on the base of Q101 and Q102. Are they supposed to be 0 when in mute or is this normal. Im done for the day. I will get back to it tomorrow and get back to you on my findings. Thanks for the help, learning lots today.

[tech2014]

Re: Kenwood KR-A4070 Sound issue

Found something strange. I think the input selector IC may be shot. When testing the other inputs, like the CD/TAPE/PHONO, I had the audio connected to the TAPE 1/REC input, and I was getting normal sound. Problem is I can change the input to TUNER or CD, and still get sound. Should only be sound coming out of the input selected. Since there was no audio connected to the CD or PHONO inputs, but I get normal sounding audio coming out, would this point to the input selector. I even get sound out of the TUNER even though there is no antenna connected.

I also noticed that if I get low sound on the CD input when there is audio signal, if I toggle between inputs, and select CD, I can get the sound back to normal again.

[tech2014]

Re: Kenwood KR-A4070 Sound issue

I'm confused. I input the sine wave and trace it to CN101. I get L and R IN but no L and R OUT. I get normal volume out of the speakers

[budm]

Re: Kenwood KR-A4070 Sound issue

'Problem is I can change the input to TUNER or CD, and still get sound. Should only be sound coming out of the input selected. ' That is correct, I wonder if it is stuck in one of the input mode, you may have to try feeding each input to see which one will give the normal sound level.
You also verify that the TAPE MONITOR is not activated, right?

[tech2014]

Re: Kenwood KR-A4070 Sound issue

I have CD input selected using the sine wave as a signal. For some reason, no matter what input I select, I have a signal on pin 3 and pin 26 of both IC102 and IC104.

I connected the signal to TAPE 2 MONITOR, I get signal out of REC but not PLAY. Same on TAPE 1

[tech2014]

Re: Kenwood KR-A4070 Sound issue

So could it be IC102 and IC104?

[tech2014]

Re: Kenwood KR-A4070 Sound issue

Pin 3 and 26 of IC102 are always on no matter what input is selected.

[tech2014]

Re: Kenwood KR-A4070 Sound issue

I found bad solder on IC, resoldered and works now.